Author Topic: how to design a plane that flies good in dead air? (Read 19709 times)

Matt Colan · « **on:** August 24, 2010, 05:13:38 PM »

I was flying today, and the air was on the dead side. After I finished the pattern, I began thinking about what sort of things would be designed into a plane to have it fly better in dead air.

After what I've learned from the forum, and other sources, this is what I came up with (for a 60 size stunter):

1) lightweight
2) raked wing tips
3) no full span flaps
4) a little thinner airfoil

Does what I came up with sound about right?

Tim Wescott · « **Reply #1 on:** August 24, 2010, 05:33:09 PM »

Dead air? Do you mean "Not Willamette Valley Oregon air?"

I've been thinking that half-elliptical tips would look mean on a stunter. I'm not sure how you could arrange the flaps so that you didn't get buffeting (and leftover turbulence the next time around) at the intersection of the flap and wing -- perhaps let the flap trail off to zero, and make darn sure you have a gapless hinge line?

RandySmith · « **Reply #2 on:** August 24, 2010, 07:51:45 PM »

Quote from: Matt Colan on August 24, 2010, 05:13:38 PM

I was flying today, and the air was on the dead side. After I finished the pattern, I began thinking about what sort of things would be designed into a plane to have it fly better in dead air.

After what I've learned from the forum, and other sources, this is what I came up with (for a 60 size stunter):

1) lightweight
2) raked wing tips
3) no full span flaps
4) a little thinner airfoil

Does what I came up with sound about right?

You will need a Clean, smaller plane with a small prop, as small as you can setup for, This would be something like my VECTRA with a 10 inch prop

Randy

Howard Rush · « **Reply #3 on:** August 24, 2010, 09:36:12 PM »

Quote from: Matt Colan on August 24, 2010, 05:13:38 PM

I was flying today, and the air was on the dead side. After I finished the pattern, I began thinking about what sort of things would be designed into a plane to have it fly better in dead air.

After what I've learned from the forum, and other sources, this is what I came up with (for a 60 size stunter):

1) lightweight
2) raked wing tips
3) no full span flaps
4) a little thinner airfoil

Does what I came up with sound about right?

No, but the raked tips wouldn't hurt. I think you'd be better off paying attention in physics class, rather than reading stuff on this forum. Randy's advice on the small prop corroborates what I've heard from another successful stunt flyer.

Brett Buck · « **Reply #4 on:** August 25, 2010, 12:40:35 AM »

Quote from: Howard Rush on August 24, 2010, 09:36:12 PM

No, but the raked tips wouldn't hurt. I think you'd be better off paying attention in physics class, rather than reading stuff on this forum. Randy's advice on the small prop corroborates what I've heard from another successful stunt flyer.

That has been my experience as well. Although it was somewhat at odds with with I had suspected. I figured the 50-60 lbs of lift and corresponding drag would be the primary turbulence generator, but Paul (as usual) was right on - it's much more a function of the prop. I think the reason is that the lift/gdrag induced turbulence just doesn't stick around in the track, but instead goes outside the maneuver track, The propwash stays in the track so you can run back through it the next time.

Brett

Randy Powell · « **Reply #5 on:** August 25, 2010, 10:31:54 AM »

I've always been a proponent of use the smallest prop you can get away with. Probably Paul's influence.

Igor Burger · « **Reply #6 on:** August 25, 2010, 10:41:07 AM »

Quote from: Howard Rush on August 24, 2010, 09:36:12 PM

Randy's advice on the small prop corroborates what I've heard from another successful stunt flyer.

Hmmmm ... yes and no. The primary reason for turbullence is wing tip whirlpool. And its reason is spanwise flow comming from lift. It is proportional to induced drag. And it depends on lift coefficient and aspect ratio of the wing. So the way is to minimize wing load to keep low lift coefficient (therefore no flaps, not necessary thick and blunt wing) combined with long span (something like low span load?

)

The prop directly does not change induced drag of wing, but if prop is large, with strong engine, it allows to fly model with higher wing load in tighter maneuvers and it directly creates that turbullence, so it can looks like the prop does it. I do not thing that size matters, and may be also power does not.But what does matter is, if we push engine to battle with excessive drag of model in tight maneuver.

All of that I proofed on my indoors, having flat large wing (no flaps) with small wing load combined with large prop but never having problems in calm air of the gym. Model is under 6oz, span is 32" an prop is 10x4.7

RandySmith · « **Reply #7 on:** August 25, 2010, 12:31:45 PM »

Quote from: RandySmith on August 24, 2010, 07:51:45 PM

You will need a Clean, smaller plane with a small prop, as small as you can setup for, This would be something like my VECTRA with a 10 inch prop

Randy

In addition I would have the lightest plane I could , and it will help greatly flying the plane with a more tail heavy CG, moving the flaps and elevator more is worse for turbulance,
The KATANA tips and wing is also excellent in shape for reduced air disturbance

I have stood right on the edge of the circle of many planes, I remember the huge amount of air Paul's Impact puts out as it passes, I was standing on the edge judging one day at the tha NATs as he came out of an hourglass, the air rush was a real slap in the face as the plane passed. They also make a lot of wake turbulance when they pass you after the motor has shut off.

Ahhh Howard we just need a "University of Toy plane" then maybe these thing could get sorted out :-)

Regards
Randy

John Miller · « **Reply #8 on:** August 25, 2010, 12:45:26 PM »

Consider a High aspect ratio wing, rear tapered tips, minimal flaps, light weight, for real dead air stunting. Most High A.R. designs are described as turning on a dime, until the wind or turbulence goes bananas.

Serge_Krauss · « **Reply #9 on:** August 25, 2010, 02:55:08 PM »

For dead air, aside from thrust, prop efficiency, disc area, etc., the high-A/R wing is absolutely the best thing: more lift with less induced drag means that there is less energy lost in tip vortices. Its disadvantages mostly disappear with the gusts.

I think we sometimes confuse the flap thing with weight. Lighter planes are better - within reason - in still air, since they require less lift and will not encounter gust upsets. There is a limit, though, which I proved in the notorious post that provoked displeasure at its first-month high school algebra symbols. The point is that the deflection of flaps is dependent on aircraft weight, and the lighter plane needs less flap deflection. It's not the flaps, but the lift required that makes the difference. You can get that lift throught higher angles of attack or greater camber (flaps). Both will create the same vortex loss, not splitting hairs on parasite drag from interference.

The equation for induced drag (for elliptical lift distribution - which is 'close enough' here) is * (See Below) Di = C_L²/(pi x AR) or simply KL²/(AR).

C_L = Lift Coefficient; pi = approx. 3.14159...; AR = Aspect Ratio; L = Lift; K = a constant determined by air density, speed, chord or area, etc.

*Edit: Howard (below) is correct: I should have written C_di instead of Di, because I gave the expression for the induced-drag coefficient. This makes the weight and span of equal importance - span loading, as Howard says.

This says what you need: simply, *(Edited for clarity) For constant area, the greater the lift coefficient the greated the induced drag and the greater the aspect ratio, the lower the induced drag. **edited to eliminate an invalid characterization: Of the two, lift required (due to weight, inertial mass) is hardest to handle, while the aspect ratio is easiest to increase. With lower induced drag, you get smaller tip vortices, which means less wake turbulence.

Wing plan form is also important, and tip shape should be optimized within structural limits. Of the standard shapes, the elliptical chord distributions, like on the spitfire, T-Bird, etc., have the least tip losses for a given lift. There are others, pictured in other threads, which are a bit more efficient. If we were concerned here with gusts, their aero centers are advantageous too, being significantly inboard (at 42% of the half-span) of those of the most familiar configurations, thus reducing lateral upsets. Although that's off-topic here, I've included a drawing below to show the taper necessary to have the same inboard lift center as an elliptical wing of the same area and aspect ratio (accuracy limited by the MicroSoft Word tools - not CAD) - just FWIW.

SK

P.S. The slightly thinner air foils are probably better in windy weather, rather than calm.

RandySmith · « **Reply #10 on:** August 25, 2010, 03:22:10 PM »

Quote from: RandySmith on August 24, 2010, 07:51:45 PM

You will need a Clean, smaller plane with a small prop, as small as you can setup for, This would be something like my VECTRA with a 10 inch prop

Randy

The Vectra wing is a high A/R eliptical design, and goes thru dead air pretty clean, this was why I recomended it, The KATANA wing is a tapered type wing but is somewhat high A/R and has tips that are very clean, the Pond Racer and the new design NEMESIS Reno racer use the same type tips that I started using back in early 90s , and is on the Katana, and several designs before that such as the Typhoon.
If I were to be **only*** interested in dead calm air, the A/R would be increased :-)
Randy

Igor Burger · « **Reply #11 on:** August 25, 2010, 03:42:15 PM »

Serge, lower AR at the same span will save you something from cl (because of larger area), so lower AR does not mean automatically more induced drag

That is why I wrote the weight to span matters

Howard Rush · « **Reply #12 on:** August 25, 2010, 04:20:53 PM »

Quote from: Serge_Krauss on August 25, 2010, 02:55:08 PM

The equation for induced drag (for elliptical lift distribution - which is 'close enough' here) is Di = C_L²/(pi x AR) or simply KL²/(AR).

Actually, that isn't close enough. That's the equation for induced drag coefficient.

I think the way to look at this is:

1. Solve for the vortex energy per foot along the airplane's flight path, which you can do with Serge's formula above after you put in the requisite qS, energy being force x distance, and the force being induced drag. That should be a measure of how messed up the air is.

2. Then solve for the airplane's response to wake turbulence. Assuming none of the variables here affect the spatial wake turbulence spectrum, I think that rolling moment due to turbulence is proportional to the wing's lift curve slope x the spanwise position of the aerodynamic center, d₁ or d₂ in Serge's interesting picture above. You should probably include roll damping, which I'm too lazy to look up.

3. Then solve for the airplane's tendency to right itself, the roll stiffness from the control lines. That's proportional to mV²/r x b/2, where m is airplane mass, V is airspeed = ground speed, r is line length, and b is wingspan. Of course when the airplane is rolled by the turbulence, you'd add or subtract lift x the sine of the roll angle relative to lines.

Item 1 x item 2 / item 3 should be a crude measure of the effect of wake turbulence.

You would get a little advantage from Flite Streak tips. If I were to be only interested in dead calm air, I might use Flite Streak tips.

I think you will observe that the reason an Impact makes a lot of wake is because it's turning a hard corner, not because it lacks Flite Streak wingtips.

Howard Rush · « **Reply #13 on:** August 25, 2010, 04:21:36 PM »

Quote from: Igor Burger on August 25, 2010, 03:42:15 PM

Serge, lower AR at the same span will save you something from cl (because of larger area), so lower AR does not mean automatically more induced drag

That is why I wrote the weight to span matters

Yep, it's span loading squared.

Howard Rush · « **Reply #14 on:** August 25, 2010, 04:26:07 PM »

"how to design a plane that flies good in dead air? "

Although physics class is important, don't neglect English.

RandySmith · « **Reply #15 on:** August 25, 2010, 07:56:46 PM »

"I think you will observe that the reason an Impact makes a lot of wake is because it's turning a hard corner, not because it lacks Flite Streak wingtips."

Howard
I found the Impact makes a large wake when doing round loops, as does a SV-11 and most every other large stunter, you can see that pretty easy by observing the wake hitting you from just outside the circle, I don't think the tips had a substantial amount to do with the amount of wake turbulence.
That seems to come from just putting a large stunt ship thru most maneuvers.
I have even seen a pretty large wake from a big stuntship that had the engine off, and was wind flying.
I think many would be amazed by the sheer volume of air moved by a big stuntship

Regards
Randy

Serge_Krauss · « **Reply #16 on:** August 25, 2010, 11:10:56 PM »

Quote from: Igor Burger on August 25, 2010, 03:42:15 PM

Serge, lower AR at the same span will save you something from cl (because of larger area), so lower AR does not mean automatically more induced drag

That is why I wrote the weight to span matters

Igor-

I agree. You can convert that induced drag equation to D_i = KL²/b² = k (W/b)²

Edit: Howard (above) is correct: the expression that leads to this is the one for the coefficient of induced drag. Span loading is a key idea. Another expression I came up with shows induced drag proportional to chord squared. Anyway, I tried to edit my original post, without erasing the basic error; 'hope I succeeded in leaving it truthful, but not unrecognizable and too confusing. I hate it when that happens, and I remember several years ago quickly writing out the same mistake and wondering why some other simple fact didn't come out. She-e-e-e-e-sh!

I think I'm agreeing with both you and Randy (mostly) here. The big disturbance from large stunters comes from their greater weight and inertial mass. I think that tips are less important on models than full sized aircraft, and the smaller they get the less important the tip shape appears to be. That seems supported by the success in indoor "Peanut Scale" models, whose spans are limited, of models like the Fike with very large chords to maximize area. Even with their broad, squared tips, they seem to fly longer. Of course, these have very low wing loadings. As I said though, I'd still use the best tips I can find for the effort I want to put into something. Randy seems to have found them significant in his planes.

Still I was trying not to compare "apples and oranges", but rather wings of the same area, where increasing A/R does increase the span.

SK

Igor Burger · « **Reply #17 on:** August 31, 2010, 04:59:23 AM »

Quote from: Howard Rush on August 25, 2010, 04:26:07 PM

don't neglect English.

I promise .... in my next life

proparc · « **Reply #18 on:** August 31, 2010, 08:57:58 AM »

Believe it not, one of the better dead air planes that I have flown, was actually a profile. The plane was a Shameless, from a Trimble kit. Flew beautifully in calm air. Pretty much fell in line with most of the specs mentioned in this thread.

PJ Rowland · « **Reply #19 on:** September 03, 2010, 11:49:18 PM »

Smaller props arent the answer !!!

The Bomber only had 8 x 6 and the propwash in dead air was INSAIN....

ehhheheh

RandySmith · « **Reply #20 on:** September 05, 2010, 10:35:03 AM »

Quote from: PJ Rowland on September 03, 2010, 11:49:18 PM

Smaller props arent the answer !!!

The Bomber only had 8 x 6 and the propwash in dead air was INSAIN....

ehhheheh

Hi PJ I agree with that completely, the bigger the plane the bigger the wake, that is why I wrote the below. I think any serious attempt at making a low wake stunt ship would be to use the smallest plane possible

""Howard
I found the Impact makes a large wake when doing round loops, as does a SV-11 and most every other large stunter, you can see that pretty easy by observing the wake hitting you from just outside the circle, I don't think the tips had a substantial amount to do with the amount of wake turbulence.
That seems to come from just putting a large stunt ship thru most maneuvers.
I have even seen a pretty large wake from a big stuntship that had the engine off, and was wind flying.
I think many would be amazed by the sheer volume of air moved by a big stuntship""

Brett Buck · « **Reply #21 on:** September 13, 2010, 02:33:00 AM »

Quote from: PJ Rowland on September 03, 2010, 11:49:18 PM

Smaller props arent the answer !!!

The Bomber only had 8 x 6 and the propwash in dead air was INSAIN....

Right. 4 of them, or the equivalent of a 32" prop!

Brett

FLOYD CARTER · « **Reply #22 on:** October 09, 2010, 06:36:55 PM »

I try to avoid standing that close to the circle where I can detect wake turbulence!

Floyd

Matt Colan · « **Reply #23 on:** October 10, 2010, 08:27:02 AM »

Quote from: FLOYD CARTER on October 09, 2010, 06:36:55 PM

I try to avoid standing that close to the circle where I can detect wake turbulence!

Floyd

I've actually been about 5 feet from Bob Krug's Strega at the Lee contest, and I could feel the wake turbulence when it went by...

peabody · « **Reply #24 on:** November 02, 2010, 06:31:03 PM »

That was VERY dangerous Matt....I generally hide behind a car or other heavy object when the Krugster flies!

PJ Rowland · « **Reply #25 on:** January 13, 2011, 05:23:03 AM »

HAH I know Brett.. part of my point was a little tongue in cheek.

Phil Bare · « **Reply #26 on:** January 20, 2011, 03:27:01 PM »

Quote from: Brett Buck on September 13, 2010, 02:33:00 AM

Right. 4 of them, or the equivalent of a 32" prop!

Brett

Wouldn't prop disc area come in to play?

PJ Rowland · « **Reply #27 on:** January 21, 2011, 01:19:48 AM »

The bomber was weird in dead air.. Even tho we had massive 32" of prop area - it still did get knocked around a fair amount lots of wing bobs and drops.. My point to this is - we are talking about a 110 oz Monster, so Weight I didnt feel was a factor.

For those who thought that more weight would help. - Would be interested to hear the other opinion on 100+ oz models in dead air.

How about controlling the exit wake more effectively?

Ted Fancher · « **Reply #28 on:** January 23, 2011, 11:19:13 AM »

Quote from: PJ Rowland on January 21, 2011, 01:19:48 AM

The bomber was weird in dead air.. Even tho we had massive 32" of prop area - it still did get knocked around a fair amount lots of wing bobs and drops.. My point to this is - we are talking about a 110 oz Monster, so Weight I didnt feel was a factor.

For those who thought that more weight would help. - Would be interested to hear the other opinion on 100+ oz models in dead air.

How about controlling the exit wake more effectively?

Tip vortices (as differentiated from prop wash) are the product of lift and spanwise flow off the tips. They are greatest when lift is greatest. A 110oz "bomber flying the same radius loops as as a 36oz Nobler is pulling the same number of "Gs" but each "G" is multiplied by 110oz vice 36. The wing must produce enough lift to support that G induced weight. Thus the bomber is going to produce massive amounts of lift and spanwise flow compared to the little Nobler. The vortices that result will probably be proportionally bigger yet because of the greater lift required (I don't know the math but I'm sure Serge or Howard will straighten us out). Of course, the mass of the bomber should make it somewhat less susceptible to the turbulence and roll associated with encountering its own wake but unit for unit it would be unreasonable to expect less effect.

I'm not so sure about any prop wash effect. If prop wash were a big deal it would be reasonable to expect it to be an issue in level flight in dead air and I don't recall ever encountering prop wash lap to lap on the deadest of days. Props continue to be a mystery to me with regards to what happens behind the blades. Again (I've raised this before), with the wings acting as huge "stators" shortly behind the props (real shortly in the case of a bomber like yours or Paul's), logic leads me to believe there is little or no "rotating mass of air" aft of the airplane that would cause a disruption any significant period of time after the passage of the beast.

Once an airplane is up to speed and thrust equals drag, how much "air blast" (straight and/or circular) still exists behind the props?

I do know that, when flying multi-engine ships like the DC-6s and 7s of my "youth", adding power--while on approach for instance--would produce what we called "instant lift" to the wing due to the comparatively sudden increased airflow over the wing from prop blast and thrust temporarily exceeded drag. This was one of the things that was "different" when long time prop jockies checked out in jets and had to be taught to be aware of, 1. the much increased spool up time of jet engines before the desired thrust was obtained, and 2. the fact that shoving the thrust levers forward did essentially nothing in terms of lift until the thrust was generated and accelerated the whole airplane. Our stunt ships, of course, are more like jets in this regard in that we've no throttles and we have to wait for drag to decrease as a result of the aircraft slowing before our more or less static thrust can once again exceed drag and the resulting airspeed finds a new "static relationship" of thrust to drag. That's why we want to operate on the back side of the torque curve so increased load (due to drag) causes the engine to "back into" a stronger point on its torque curve and mitigate speed loss due high drag conditions such as maneuvers.

In other words, I don't have a clue whether prop vortices have a thing to do with vortex turbulence for stunt ships.

Ted

PJ Rowland · « **Reply #29 on:** January 23, 2011, 07:30:21 PM »

I can agree with your anology there interms of propwash vs tip vortices. I also agree I have not expereicend any real turbulent wake in level laps. -

" Of course, the mass of the bomber should make it somewhat less susceptible to the turbulence and roll associated with encountering its own wake but unit for unit it would be unreasonable to expect less effect. "

pound for pound it would be the same, becasue there is the same amount of lift needing to overcome mass. As you said. Which I also now agree with !

So is there any effective way to reduce tip vorticies? This seems to be the crux of the issue yes??

Igor Burger · « **Reply #30 on:** January 24, 2011, 01:41:54 AM »

Quote from: Ted Fancher on January 23, 2011, 11:19:13 AM

Tip vortices (as differentiated from prop wash) are the product of lift ...

I will little bit repare that statement and everything will be clearer. Tip vortices (as result of spanwise flow as Ted wrote) are product of lift COEFFICIENT and aspect ratio, not just lift. And that means that it can be minimized. If a model needs some amount of lift in meneuvers (as result of centrifugal force counterballanced by lift), it still can be produced at high, or at low lift coefficients, so the same lift can produce more or less vortices.

So if you want minimize tip vortices in dead air, you will need low lift coeffcient and that needs large wing area, aspect ratio and no flaps (good weight to span ratio as we wrote earlier in thi thread). That is why we use large wings without flaps on our indoors.

Ted Fancher · « **Reply #31 on:** January 24, 2011, 08:57:44 AM »

Quote from: Igor Burger on January 24, 2011, 01:41:54 AM

I will little bit repare that statement and everything will be clearer. Tip vortices (as result of spanwise flow as Ted wrote) are product of lift COEFFICIENT and aspect ratio, not just lift. And that means that it can be minimized. If a model needs some amount of lift in meneuvers (as result of centrifugal force counterballanced by lift), it still can be produced at high, or at low lift coefficients, so the same lift can produce more or less vortices.

So if you want minimize tip vortices in dead air, you will need low lift coeffcient and that needs large wing area, aspect ratio and no flaps (good weight to span ratio as we wrote earlier in thi thread). That is why we use large wings without flaps on our indoors.

Valuable distinctions, Igor. I would suggest, however, that my comments regarding increased G loads demanding more lift from essentially the same airspeed were a real world demonstration of the distinction between lift and the coefficient thereof!

I agree regarding both flaps and aspect ratio. I've published, for instance, the article on the Doctor that addresses the advantages and disadvantages of flaps and reduced wake turbulence is definitely a positive factor in support of the flapless approach to a stunter. Do any of us recall worrying about dead air when we were kids flying Ringmasters and All Americans? I don't think so! Whether it is, ultimately, determinative of whether we should compete for a major championship with a flapless design is a different question.

High aspect ratio is absolutely a plus for reducing tip vortices. In calm air I would always elect to compete with a high aspect ratio stunter. My best ever calm air ship was built back in 1977. It had around a six to one aspect ratio and flew like a dream in dead air. The combination of very modest weight, around 50 oz on ~700 square inches of wing area and the low drag in maneuvers made it very undemanding in terms of horsepower. Although I flew it with modest ST .46s there is no doubt in my mind a good Max .35S would have been adequate.

The downside, of course, was that in winds the low co-efficients and minimal drag increase in maneuvers allowed the ship to accelerate like mad in consecutive maneuvers unless place perfectly with respect to the winds. The problem could have been mitigated with the now common further aft CG and larger tail volumes but it would never have been acceptable in high winds.

The beauty of flying indoors is that you can design and trim for one set of conditions and wash your hands of the whole "adapt to the environment" mystique that has made stunt so demanding when conducted au naturale! You guys are clearly having a great time with that project.

Ted

This addendum was added because my comments regarding the shortcomings of my long winged ship were incomplete. What I left out is worth knowing, so here goes...

In addition to the high Aspect Ratio's low drag buildup with increased speeds and the forward CG which loaded the tail more in maneuvers, the airplane's shortcomings in the wind were also exacerbated by the fact the stab and elevator were just too darned flimsy. As noted, the big airplane was very light. Part of the light weight was the result of the built up stab and elevator constructed of minimal amounts of very light balsa and then covered with Monokote.

While "adequate" under ideal calm conditions where downloads (upload in outsides maneuvers) were modest and constant throughout the maneuver, when the airplane accelerated in winds the Forward CG would require ever more tail authority to maintain the desired turn rate for constant radius maneuvers. As these loads increased the elevators had to be deflected more and, when doing so, caused the stabilizer to twist and "wash out" lowering their angle of attack and, therefore, reducing their effectiveness. Ultimately, it didn't take a lot of wind to cause this spectacular flying calm air ship to do its Jeckel/Hyde thing. It was absolutely possible to run out of tail authority, leaving no option but to let the maneuver grow bigger and bigger or even have to bail out of it.

This was pretty much the start of my lessons in the desirability of large tails (~25% of the wing area), stiff stabs and elevators and a properly located Center of Gravity (very close to 25% of the MAC [yes, including the flaps] for a flapped ship). If there is a "secret" to "modern" stunt aerodynamics that last sentence pretty much covers it.

Matt Colan · « **Reply #32 on:** January 24, 2011, 04:12:26 PM »

Quote from: Ted Fancher on January 24, 2011, 08:57:44 AM

Valuable distinctions, Igor. I would suggest, however, that my comments regarding increased G loads demanding more lift from essentially the same airspeed were a real world demonstration of the distinction between lift and the coefficient thereof!

I agree regarding both flaps and aspect ratio. I've published, for instance, the article on the Doctor that addresses the advantages and disadvantages of flaps and reduced wake turbulence is definitely a positive factor in support of the flapless approach to a stunter. Do any of us recall worrying about dead air when we were kids flying Ringmasters and All Americans? I don't think so! Whether it is, ultimately, determinative of whether we should compete for a major championship with a flapless design is a different question.

High aspect ratio is absolutely a plus for reducing tip vortices. In calm air I would always elect to compete with a high aspect ratio stunter. My best ever calm air ship was built back in 1977. It had around a six to one aspect ratio and flew like a dream in dead air. The combination of very modest weight, around 50 oz on ~700 square inches of wing area and the low drag in maneuvers made it very undemanding in terms of horsepower. Although I flew it with modest ST .46s there is no doubt in my mind a good Max .35S would have been adequate.

The downside, of course, was that in winds the low co-efficients and minimal drag increase in maneuvers allowed the ship to accelerate like mad in consecutive maneuvers unless place perfectly with respect to the winds. The problem could have been mitigated with the now common further aft CG and larger tail volumes but it would never have been acceptable in high winds.

The beauty of flying indoors is that you can design and trim for one set of conditions and wash your hands of the whole "adapt to the environment" mystique that has made stunt so demanding when conducted au naturale! You guys are clearly having a great time with that project.

Ted

Ted, if you built that same airplane, and redesigned it for a piped engine, and everything else the typical modern stunter has, would it be a better flier in the wind, or still be hard to fly?

Ted Fancher · « **Reply #33 on:** January 24, 2011, 11:12:22 PM »

Quote from: Matt Colan on January 24, 2011, 04:12:26 PM

Ted, if you built that same airplane, and redesigned it for a piped engine, and everything else the typical modern stunter has, would it be a better flier in the wind, or still be hard to fly?

HI Matt,

It could be made to work better, yes. A properly set up tuned pipe running a low pitched prop would fight the wind up. In addition, a larger tail which allowed a more aft CG would mitigate the tendency for the controls to load up so that you run out of elevator to keep the maneuver sizes correct (and, not incidentally, run out of elevator and altitude at the bottom of consecutive loops!)

The underlying problem would still remain, however; everything you would change would ultimately only be to reduce the effects of the added energy of the wind and the minimal induced drag associated with higher aspect ratios. There is a reason that the consistently top stunt ships have hovered around a five to one AR. They've gotten a tiny bit longer winged in some cases with the tuned pipes that helped control wind up but nothing outlandish. David's 2008 WC Champion is the highest AR of any recent winner of a major meet of which I'm aware. It has a Billy P-47 wing but without the big flaps and with the T.P. style wing tips. The result is something in the vicinity of a 5.7 AR (IIRC, I measured it back in Hungary and my memory is very good but not very long).

Just to illustrate the problem; for a number of years here in the San Francisco Bay Area we had an "older "combat flier who was a huge advocate of high aspect ratio combat ships. These things were competitively fast in level flight but really stood out when they started to turn. No apparent slow down at all during multiple consecutive loops. He loved to show the things off flying solo. Twice, however, in only modest winds I saw him literally blow the wings off of perfectly sound models that just went faster and faster in the loops done dead downwind. The same energy that allows stunt guys to "wind fly" endlessly after the engine quits exists just as much when the engine is running. If you get the maneuvers in the right relationship to the winds the energy of the winds and the powertrain team up to make life "difficult" for the PA pilot.

IOW, I wouldn't advocate designing a high AR ship to compete in all the conditions you're apt to encounter at contests. This is sort of where Jose Modesto is coming from with this questions about different wings for different conditions with his composite airplanes. His thinking is on the money. I don't, however, have a clue how the regulatory questions will be answered.

One last comment about the effects of AR and lift coefficient on airplane performance. Howard Hughes magnificent "Hughes Racer", which resides at the Smithsonian, held both closed course (actually, a speed trap, rather than a race course setup as I recall) and long range (coast to coast) speed records for a period of time. Hughes made two different sets of wings for the two different tasks. A short span for the short flat out runs and a higher AR set for the long range record. Lower parasitic drag at very high speeds the goal of the shorter wings (less frontal area and a higher airspeed requiring a lower angle of attack for short bursts of speed) and longer flight segments at slower but more efficient speeds with the longer wings allowing greater distances for a given amount of fuel consumed.

OUch! That'll teach you to get me started.

Good to hear from you, Matt.

Ted

afml · « **Reply #34 on:** January 25, 2011, 06:19:58 AM »

From Ted: "I've published, for instance, the article on the Doctor that addresses the advantages and disadvantages of flaps and reduced wake turbulence is definitely a positive factor in support of the flapless approach to a stunter." "Whether it is, ultimately, determinative of whether we should compete for a major championship with a flapless design is a different question."

Here's one from the late Bob Baron!

Howard Hughes Racer..This one??

Just re-visited the A&S Museum and the extension at Dulles this past fall.
AWESOME! Took MANY pics fer sure!

"Tight Lines!"

Wes

Ted Fancher · « **Reply #35 on:** January 27, 2011, 02:43:23 PM »

Quote from: afml on January 25, 2011, 06:19:58 AM

From Ted: "I've published, for instance, the article on the Doctor that addresses the advantages and disadvantages of flaps and reduced wake turbulence is definitely a positive factor in support of the flapless approach to a stunter." "Whether it is, ultimately, determinative of whether we should compete for a major championship with a flapless design is a different question."

Here's one from the late Bob Baron!

Howard Hughes Racer..This one??

Just re-visited the A&S Museum and the extension at Dulles this past fall.
AWESOME! Took MANY pics fer sure!

"Tight Lines!"

Wes

Bob successfully campaigned more "unusual" designs than any other flier of which I'm aware. This is a classic example.

Yup, that's the bird. It's got my vote for the most beautifully macho airplane in the Smithsonian collection1

Ted

Ted Fancher · « **Reply #36 on:** February 07, 2011, 06:34:13 PM »

I originally posted this comment as an edit to an earlier post. Turns out it doesn't show up as "new" info, so I've cut and pasted it here in case anyone still wants to spend some time on the subject.

This addendum was added because my comments regarding the shortcomings of my long winged ship were incomplete. What I left out is worth knowing, so here goes...

In addition to the high Aspect Ratio's low drag buildup with increased speeds and the forward CG which loaded the tail more in maneuvers, the airplane's shortcomings in the wind were also exacerbated by the fact the stab and elevator were just too darned flimsy. As noted, the big airplane was very light. Part of the light weight was the result of the built up stab and elevator constructed of minimal amounts of very light balsa and then covered with Monokote.

While "adequate" under ideal calm conditions where downloads (upload in outsides maneuvers) were modest and constant throughout the maneuver, when the airplane accelerated in winds the Forward CG would require ever more tail authority to maintain the desired turn rate for constant radius maneuvers. As these loads increased the elevators had to be deflected more and, when doing so, caused the stabilizer to twist and "wash out" lowering their angle of attack and, therefore, reducing their effectiveness. Ultimately, it didn't take a lot of wind to cause this spectacular flying calm air ship to do its Jeckel/Hyde thing. It was absolutely possible to run out of tail authority, leaving no option but to let the maneuver grow bigger and bigger or even have to bail out of it.

This was pretty much the start of my lessons in the desirability of large tails (~25% of the wing area), stiff stabs and elevators and a properly located Center of Gravity (very close to 25% of the MAC [yes, including the flaps] for a flapped ship). If there is a "secret" to "modern" stunt aerodynamics that last sentence pretty much covers it.

Ted Fancher

Matt Colan · « **Reply #37 on:** February 07, 2011, 06:56:59 PM »

Quote from: Ted Fancher on February 07, 2011, 06:34:13 PM

I originally posted this comment as an edit to an earlier post. Turns out it doesn't show up as "new" info, so I've cut and pasted it here in case anyone still wants to spend some time on the subject.

This addendum was added because my comments regarding the shortcomings of my long winged ship were incomplete. What I left out is worth knowing, so here goes...

In addition to the high Aspect Ratio's low drag buildup with increased speeds and the forward CG which loaded the tail more in maneuvers, the airplane's shortcomings in the wind were also exacerbated by the fact the stab and elevator were just too darned flimsy. As noted, the big airplane was very light. Part of the light weight was the result of the built up stab and elevator constructed of minimal amounts of very light balsa and then covered with Monokote.

While "adequate" under ideal calm conditions where downloads (upload in outsides maneuvers) were modest and constant throughout the maneuver, when the airplane accelerated in winds the Forward CG would require ever more tail authority to maintain the desired turn rate for constant radius maneuvers. As these loads increased the elevators had to be deflected more and, when doing so, caused the stabilizer to twist and "wash out" lowering their angle of attack and, therefore, reducing their effectiveness. Ultimately, it didn't take a lot of wind to cause this spectacular flying calm air ship to do its Jeckel/Hyde thing. It was absolutely possible to run out of tail authority, leaving no option but to let the maneuver grow bigger and bigger or even have to bail out of it.

This was pretty much the start of my lessons in the desirability of large tails (~25% of the wing area), stiff stabs and elevators and a properly located Center of Gravity (very close to 25% of the MAC [yes, including the flaps] for a flapped ship). If there is a "secret" to "modern" stunt aerodynamics that last sentence pretty much covers it.

Ted Fancher

And Ted,

I'm sure covering that high aspect ratio plane in monokote didn't help matters at all. I would think if the aspect ratio was high enough, the wings could start flexing, did it?

Brett Buck · « **Reply #38 on:** February 07, 2011, 10:00:37 PM »

Quote from: Matt Colan on February 07, 2011, 06:56:59 PM

I'm sure covering that high aspect ratio plane in monokote didn't help matters at all. I would think if the aspect ratio was high enough, the wings could start flexing, did it?

The wings flex A LOT in the corners! No matter how you build them. It's just a matter of how much, and what happens to the shape when it flexes. Some types of construction are better than others, of course, and a high aspect ratio with monokote is probably on the "worse" end of the scale.

One of the big advantages of having a lot of power (essentially all you could possibly use) is that you can afford to beef up the structure to limit the deflection. You couldn't do that back when Ted's airplane was built. Or actually you could, but it would be so heavy that you lost more performance from that than you ever got from making it stiff.

Used to be, that building the lightest airplane was everyone's goal, and if you didn't, you were a hack. Now it means that you aren't taking full advantage of the power you have available.

Brett

PJ Rowland · « **Reply #39 on:** February 11, 2011, 10:10:10 PM »

Used to be, that building the lightest airplane was everyone's goal, and if you didn't, you were a hack. Now it means that you aren't taking full advantage of the power you have available.

Brett - I dont understand what you mean by this statement - can you elaborate? Im reading it : " building the lightest airplane was everyone's goal, and if you didn't, you aren't taking full advantage of the power you have available.

Is that what you mean? and if so is the rule of 62 - 64 oz still valid for a regualr sized 650 - 680 sized ship

Ted Fancher · « **Reply #40 on:** February 11, 2011, 10:49:43 PM »

Quote from: Matt Colan on February 07, 2011, 06:56:59 PM

And Ted,

I'm sure covering that high aspect ratio plane in monokote didn't help matters at all. I would think if the aspect ratio was high enough, the wings could start flexing, did it?

Hi Matt,

Actually, the wing wasn't all that much of a problem. It was a foam core (Bubba Hunt) wing that was cut with integral flaps that were then cut out after sheeting the complete wing. It was the first (maybe the only) wing Bob cut that way. It was covered with monokote but was still much more rigid than had it been built up and covered. It was really the tail that was flexing and causing the problems. I might have a picture I can attach of that plane.

Ted

Brett Buck · « **Reply #41 on:** February 11, 2011, 11:48:36 PM »

Quote from: PJ Rowland on February 11, 2011, 10:10:10 PM

Used to be, that building the lightest airplane was everyone's goal, and if you didn't, you were a hack. Now it means that you aren't taking full advantage of the power you have available.

Brett - I dont understand what you mean by this statement - can you elaborate? Im reading it : " building the lightest airplane was everyone's goal, and if you didn't, you aren't taking full advantage of the power you have available.

A restatement- Building the lightest airplane you can means you are not taking full advantage of the power you have available. In other words, building as light as you can is not a very useful goal.

Brett

John Sunderland · « **Reply #42 on:** February 12, 2011, 01:40:51 AM »

While i love the Hughes racer..for ease of making a slick racer into a stunt ship...I like the Laird Turner Meteor...still on the drawing board too!

PJ Rowland · « **Reply #43 on:** February 12, 2011, 06:57:45 AM »

So what is a weight goal ? If sub 50 oz isnt ideal? 62 - 64 oz ?

Im interested to understand why more weight gives access to power. ?

Ted Fancher · « **Reply #44 on:** February 13, 2011, 09:45:14 PM »

Quote from: PJ Rowland on February 12, 2011, 06:57:45 AM

So what is a weight goal ? If sub 50 oz isnt ideal? 62 - 64 oz ?

Im interested to understand why more weight gives access to power. ?

How hard could you run a "corn cob" Pratt and Whitney 4360 in a Cessna 150?

Pretty much everything bigger than a piped .40 has been detuned in one fashion or another to fly the same sized planes in the same sized circles. There are some positive arguments in favor of doing so but, when push comes to shove, the power to fly 5.2 second laps with a 62 oz stunter on 66' or so of lines is pretty much the same whether it comes from .4 cubic inches or .75 or more. The .40 is running slightly above its "sweet spot" and the .75 is throttled well below. The .40 VF in an Infinity or Impact is about as bulletproof a combination as there is. If somebody passes a rule giving us the ability to fly competition with 1100 square inch stunters on 100' lines the equation will change. Until then we're fiddling around the edges.

At least that's my opinion.

Ted

Tim Wescott · « **Reply #45 on:** February 13, 2011, 09:56:02 PM »

Quote from: Ted Fancher on February 13, 2011, 09:45:14 PM

How hard could you run a "corn cob" Pratt and Whitney 4360 in a Cessna 150?

Oh, I wish I were a cartoonist. I'm not sure if I'd draw a 4360 with a smushed Cessna underneath, or a Cessna with a 4360 installed such that it extends back through the cabin (for balance, don'cha know), and a landing gear beefed up with bed springs and old Ford axles.

Cessna 150 dry weight: 1111 lbs (per Wikipedia).
P&W 4360 dry weight: 3870 lbs (again, Wikipedia).

Ratio: about 3.5:1.

Ted Fancher · « **Reply #46 on:** February 15, 2011, 10:50:34 PM »

Quote from: Tim Wescott on February 13, 2011, 09:56:02 PM

Oh, I wish I were a cartoonist. I'm not sure if I'd draw a 4360 with a smushed Cessna underneath, or a Cessna with a 4360 installed such that it extends back through the cabin (for balance, don'cha know), and a landing gear beefed up with bed springs and old Ford axles.

Cessna 150 dry weight: 1111 lbs (per Wikipedia).
P&W 4360 dry weight: 3870 lbs (again, Wikipedia).

Ratio: about 3.5:1.

Thanks, Tim. I rest my case.

Ted

RandySmith · « **Reply #47 on:** February 15, 2011, 11:19:10 PM »

Quote from: Ted Fancher on February 15, 2011, 10:50:34 PM

Thanks, Tim. I rest my case.

Ted

Geeze we gotta make a lighter P&W 4360

:-)

Randy

Ted Fancher · « **Reply #48 on:** February 16, 2011, 04:23:34 PM »

Quote from: Tim Wescott on February 13, 2011, 09:56:02 PM

Oh, I wish I were a cartoonist. I'm not sure if I'd draw a 4360 with a smushed Cessna underneath, or a Cessna with a 4360 installed such that it extends back through the cabin (for balance, don'cha know), and a landing gear beefed up with bed springs and old Ford axles.

Cessna 150 dry weight: 1111 lbs (per Wikipedia).
P&W 4360 dry weight: 3870 lbs (again, Wikipedia).

Ratio: about 3.5:1.

Schultzie could whip up a cartoon of that in a flash! Couldn't you, Don!

Steve Fitton · « **Reply #49 on:** February 16, 2011, 06:13:38 PM »

Well, an R-4360 might not work well, but how about nice modern power like a PT6A?

Ted Fancher · « **Reply #50 on:** February 18, 2011, 10:23:24 PM »

Quote from: Steve Fitton on February 16, 2011, 06:13:38 PM

Well, an R-4360 might not work well, but how about nice modern power like a PT6A?

Humbug! Woudn't sound right!

Ted

Jim Pollock · « **Reply #51 on:** February 23, 2011, 01:09:27 PM »

There's nothing like standing nearly underneath the finish line at the Reno Air Races with Darryl Greenameyers
Big modified 'Cat coming across the finish line at full throttle on a R-4360 with a B-29 Prop! I'll never ever forget that particular moment in time! Or the sound!

Jim Pollock

Air Ministry . · « **Reply #52 on:** February 24, 2011, 08:16:30 PM »

I think that Bearcat had a R-2800,Sheltons had a 3350 .
A few seafurys ran 4360s , dreadnaught and havenaught !
Critical mass / blind mans bluff is 3350.
A few Yaks run these,maybe ? best is a Curtiss Commando engine,
weight wise.
One Yak 11 ran a 4360 / corncob greenemeyers " Mr Awsome '
in name only . Originally built by someone else. A bit of a hand full
Yaw connected to throttle. Somewhat less than entirely sucessfull .