Guys,
I am interested in hearing what are the basic stunt ship design "numbers", which I assume all go back to the Nobler. I'd like to here what different designer/modifiers think each one affects and what impact is expected by changing them, how much change is needed to see an impact.

For example, tail moment length - generally measured flap hinge line to elevator hinge line, affects stability and rate of turn; shortening will increase the rate of turn which tightens corners but also reduce the stability by reducing the tail volume coef.; lengthening slows rate of turn, opens corners but adds smoothness. Changes by >5% significant.

   I think the question is a little too broad to be answered easily - but I would strongly recommend the Imitation article where all the various factors are extensively discussed. I would also note that the airplane itself is about as good as they come - just about everything we have done around here since then has been trying to reproduce the performance of the Imitation with a full fuselage. About the only thing that has consequentially changed since then is the thin low-aspect ratio tail.
 
    If you don't believe me, check David's WC airplane - aerodynamically it's *extremely similar* to the Imitation. Of course it has a piped 75 instead of a wimpy little ST46 but otherwise pretty close.

     Brett

Is that the article " Designing the imitation ? "

Ive read that - Very concise.

Yep.   I cannot remember off hand how many issues of Model Aviation the whole article spanned, at least two, maybe three.

I have the original magazines, but actually printed out all of the articles just to have them in one place! LOL!!

I have put off building an Imitation for close to 20 years or more, simply because I can't decide whether to build it as the "profile" or make it a full fuselage like Brett did once.

Now I am leaning to building it as the article was written but using an OS .40VF and pipe! 

And I only have one set of cores.......... LOL!!

Big Bear

here is my number --ONE!

I am interested in hearing what are the basic stunt ship design "numbers", which I assume all go back to the Nobler. I'd like to here what different designer/modifiers think each one affects and what impact is expected by changing them, how much change is needed to see an impact.

For example, tail moment length - generally measured flap hinge line to elevator hinge line, affects stability and rate of turn; shortening will increase the rate of turn which tightens corners but also reduce the stability by reducing the tail volume coef.; lengthening slows rate of turn, opens corners but adds smoothness. Changes by >5% significant.

Generally but probably not properly: aerodynamics guys are almost certainly going to want to measure this stuff from the mean aerodynamic center (MAC) of the wing, not the (fairly arbitrary) flap hinge line.  The only airplane population I see that measures things from LE to LE, or hingeline to hingeline, etc., is control line, and control line stunt specifically.  The rule of thumb when you're marching to a different drummer: either you're smarter than the entire rest of the world, you're dumber but you think you're smarter, or you're making a mountain out of a molehill.  Only one of the choices is the correct one*.

I'm going to go read me the Imitation article, at least if I have MA going back to '79 (I think I do).

* I march to a different drummer, but it's 'cause I'm smarter.  I think .

I agree that the proper measurement is from the MAC of each surface but for our purpose the H to H is an easy way to keep track and check the effect of moment changes. We co the same with the nose moment measuring it from the LE of the wing.

...The Intimidation article did indicate that changing nose moment length had no "feelable" impact, I would have thought that as you increase the nose moment it would slow the turn as the weight is further away from the CG but it seems that if the CG is in the same place (and the tail weight is the same) the moment for say a lighter engine with 1/2" longer nose moment is the same as a heavy engine with a corresponding shorter moment.

I've posted my hinge-hinge opinion in the other thread. While it may not always be noticable, still as you surmised, two models that are indentical, except for engine weight and placement will accelerate into turns at different rates. I think assertions to the contrary come from experiences with planes that are not otherwise identical after significant changes of this kind. While they may balance the same with lighter engines placed further forward and heavier engines moved aft from the c.g., the engine/prop/tank contributions to the the moment of inertia, which governs angular acceleration, are different. The moment of inertia varies as the square of the distance from the c.g., while the nose "moment" contribution varies linearly with both the distance the mass.

For example, nose moment contributions of a 6-oz Fox placed 9 inches ahead of the c.g. is 54 in-oz. That is the same as a 9-oz engine placed 6 inches forward. However the moment of inertia contributed by the Fox is a constant times 6 x 9² = 486k, while the 9-oz engine's contribution is k x 9 x 6² = 324k. These are quite different, with the heavier engine actually enjoying a turn entry/exit advantage - unless you want the pitch change retarded or a greater handle deflection.

Also, as pointed out elsewhere, not measuring the nose moment from the c.g. has caused some published planes to have greater nose moments (and even greater inertial moments) than intended, due to use of shaft extentions on lighter engines like the Fox. Published efforts to reduce nose heaviness have actually increased it. Fortunately, we usually only address balance, and that usually can only be designed in with each heavy item in a certain place. If you need to compute how long to make the nose in order for the c.g. to be placed correctly though, then you need to use the distance from these items to the c.g. and nowhere else.

SK

Serge,

The Imitation was really a pretty good pragmatic testbed for stuff like this.  If you'll recall, I made spacers to fit between the firewall and the RC engine mount specifically to allow lighter engines to be moved further from the CG than heavier ones.  The net result was an airplane that flew very competitively with a variety of engines of very different weights.  The difference in response from one to the other seemed mostly based on the performance of the engine (lighter engines were generally less powerful than their more robust brethren).

Having said that, however, there is no question in my mind that my favorite combination was with no spacers and the Enya .46 4S on the front end.  This was the last engine I personally flew the airplane with and it was an absolute blast.  You could fly patterns of whatever size you wanted with it, including ~60 degree verticals and hourglasses.  Response was very predictable and the turns would stop wherever you desired with no "hop".  Perhaps your Moment of Inertia at work, yes?

The Enya was the only engine that I used that didn't require a spacer to get the CG in the "desirable" range.

FWIW, the airplane I won the Walker Cup with in 1986 (the Excitation V) was designed specifically for the Enya with a nose shortened to account for the greater weight but, for reasons not worth expanding on here, I ultimately went back to a Tiger .46 (plus several oz of lead in the nose to account for the lighter weight--passing through an OS .46 FSR on the way) for the win in Lake Charles.  The nose of that airplane was about an inch and a half shorter than its predecessor which was designed for the Tiger from the get-go.  It was obviously a very good airplane with the short nose.

Don't know if any of the above "proves" anything other than proper trimming makes a variety of configurations do pretty good tricks.

Ted

p.s.  Here's a question for you guys that can quantify such stuff.  Is there anything to be gained by investigating the arm between the Aerodynamic Centers of the wing and tail with respect to the pitching moment developed by deflecting the flaps?   I know from real world experiments (adding flaps to a well used Dick Mathis Coyote) that the pitching moment is very real and retards pitch response compared to the identical airplane without the flaps.

I bought the 1969 MAN magazines several years ago. Last night I asked my computer guru(my wife) to show me how to use a spread sheet. Of course her answer was you have to have some info to put in it. The '69 Man mags have the critical dimensions for several of the all time stunters so thats what I used for data. Three different Noblers, Magnum, Super Master and the Stingray. Its amazing that in many ways these planes are very similar. My question is, If these critical dimensions were "averaged" for , lets say 20 airframes of similar size do think the "offspring" would be as good? Also curious about some of the more recent 35-40 size airframes, which would you include in this experiment?

Thanks
Steve

If you averaged the colors of 20 successful modern cars, would you have come up with a good car?

or if the color is red.

The four airframes( Impact, Trival Pursuit, SV-11 and Patternmaster), do the all have a common engine size, or maybe wing area fairly close? I don't want to compare .40 size to .60 size, you know that apple to oranges thing.

Steve

Can someone provide me with the following measurements for the Trivial Pursuit, Impact and SV-11:
Span
Area
Weight(range)
Nose moment
Tail moment
  Is there any other dimension that might be critical?

Thanks Steve

Damien,
 definitely need tail volume

Thanks Steve

  Thanks Randy
Anyone want to add or change anything ?

Steve

A few thousand, maybe.  Despite 60 years of stunt tradition, I don't think this approach will lead anywhere.

Your right Howard it won't. Stunt ships are sooo highly developed for a particular individual needs, that, there is no getting around having to roll your proverbial sleeves up and learn the fundamentals.

You could average ALL of the best stunt ships current or past stunt build it and still puke on the results, simply because you don't like it. That is what’s' so neat about this sport as opposed to say F3A. You can't just buy a good result.

There is a certain genius about this sport that almost forces you to figure out just exactly what it is you need. You are forced into engineering freedom whether you like it or not LOL.

That said, I admire Stuntguy13’s attempt to get to grips with what’s up with these things. Not everybody has an extensive stunt library that they can readily access. It is important to remember that many years ago, a hell of a lot of guys jumped on the Nobler “numbers” until they figured out just what they wanted.

To answer your initial question specifically, I would advise Stuntguy13 to order a bunch of stunt plans and study them, if you want to get to grips quickly with the "numbers". Just remember, ultimately, you will have to learn the “why” behind the numbers or you will not know what to change!!

  So I suppose the real challenge is to get to where you are flying a specific airframe to the best of YOUR ability.Then look at what part of the pattern you're not happy with and make changes to your setup or airframe. Is it safe to assume that every change for the better in one area could negatively affect another manuever in the pattern? Well this little exercise in spreadsheets just turned into the quest for the Holy Grail.
   So is anyone flying an absolutely STOCK airframe in expert, or does everyone tweak some dimensions to their personal flying style? I'd be curious to measure all the Impacts or Sv11, Trivial Pursuits to see how much they vary from each other.

Steve

Guys,
I hear what is being said and I agree with Bill that straight and of the correct weight is 98% of the battle if you are flying an existing proven design. However, part of the "fun" in stunt is feeding our ego by thinking we can find something the other guy doesn't have. So enter the "kit basher" or the original designer. Stunt is one of the last great "cut and try" events and "tweeking" is the tool. So the question that has not been addressed is "cause and effect" of a change. Let's say you take the one of the three ships that Brett spoke of (the Impact, TP, SV 11) as a base design.

Lets also say that most "tweaks" will be to the common easy to change components - nose moment; tail moment; tail volume; wing tip shape, flap% of wing area; flap length (full or partial) and control ratio's. There are other areas that the original designer might change - airfoil thickness and area, as well as shape but for now lets do the simple to get to ones.

So far there has be one area that has been discussed and that is the nose moment length - for the most part as long as it is adjusted to maintain the same CG location the general agreement is it has minimal impact (no pun intended).


What we need is the "rest of the story". So how about it Brett, Ted and Randy, what would be the generally expected reaction to changing each of the other components?

Best,      DennisT

( haven't tried any overheads so for me level flight)

You should -- a wingover is easier than a loop, if the plane maintains line tension and the engine doesn't quit.

Just sneak up on it -- start by climbing up to 45 degrees and back down over the course of 1/2 circle.  Then try 50 or 55 degrees.  Then 60, etc.  Each one is a bit more challenging for you and the plane, but you're never committing to that straight overhead until you've already figured out what's going to happen lower down.  If you find that at 75 degrees or whatever that the controls are getting uncomfortably soft -- stop trying wingovers for that flight.  Just noodle around for the rest of the flight, then adjust your leadouts when you land and try again.

(If the engine does quit when you're overhead run backwards like hell while gently applying up elevator.  This will often save the plane, and if it doesn't you'll at least look like you were trying).

Lets also say that most "tweaks" will be to the common easy to change components - nose moment; tail moment; tail volume; wing tip shape, flap% of wing area; flap length (full or partial) and control ratio's. There are other areas that the original designer might change - airfoil thickness and area, as well as shape but for now lets do the simple to get to ones.

As I said before, these "numbers" aren't of much value.  You can measure them, but they don't mean much.  The width of a car door, for example, might be some indication of wheelbase, but not necessarily.  Nevertheless, I'll try to give you qualitative answers both in aero talk and in what phenomena to expect.  Qualitative answers is all you'll get from anybody except Serge, because we either are too lazy to give quantitative answers or we don't know how.

"Nose moment" is unknown in the aeronautical literature, but I'll guess that stunt fliers define it as the distance from the propeller to somewhere else on the airplane.  I may have found its origin: a simplification of moment of inertia that Wild Bill made in an article on stunt design.  This has a bunch of effects, each of which should be covered separately.  You ain't changing just one variable.  

1. The farther forward the engine (or any other mass) moves in the airplane, the farther forward the CG will be if nothing is done aft of the CG to balance it.  Moving the CG forward increases static stability in pitch and yaw.  Moving the CG forward requires moving the leadouts forward to keep the same yaw angle relative to a tangent on the circle.  Moving the CG forward will reduce the effects of bad stuff in your airplane: control friction, control slop, other causes of hunting.  Moving the CG forward could make it easier to fly level on a calm day, but on a windy day the airplane will climb going upwind and dive going downwind.  Moving the CG forward will require more elevator deflection for a given turn radius-- maybe more than is available.   Moving the CG forward will increase control surface hinge moment for a given turn radius.  Increased hinge moment makes the airplane harder to fly accurately if there is turbulence or if the airplane is mistrimmed: differences in line tension result in more varied control surface deflection.  

2. The farther forward the engine (or any other mass) moves away from the CG (including moving the engine forward and adding weight to the tail to balance it), the higher the moment of inertia is about the affected axes.  Moving the engine forward affects the pitch and yaw axes.  In pitch, the higher the moment of inertia, the lower the static stability.  Pitch damping will be reduced.  The higher the moment of inertia, the more elevator it takes to get the airplane rotating and the more it takes in the opposite direction to stop the rotation.  The higher the moment of inertia, the harder it is to avoid pitch and yaw oscillations.  

3.  Moving the propeller forward does not increase the gyroscopic effect of the rotating crankshaft and propeller.  

4. Moving the propeller forward increases the effects of the propeller being at an angle to the airflow.  

5. Moving the propeller forward gives a tad of effective engine offset.  

6.  Moving the engine mass forward changes the resonant frequency of the nose, and can exacerbate or quell vibration modes.  Ax me about B-17 stunter engine resonances, a subject my ignorance of which was exposed multiple times.


"Tail moment" is unknown in the aeronautical literature, but I'll guess that stunt fliers define it as the distance between flap and elevator hinge lines, maybe at the airplane center if one is swept.  It's sort of a race among the aerodynamic, elasticity, and mass effects of the tail length.  I actually wrote stuff on this here on this very forum, but nobody read it.  This isn't surprising, because hardly anybody read aerodynamic stuff I got paid a lot of money to write.  Here is a combination of a couple of posts:

1. The effect of downwash: the more lift, the more downwash.  Downwash is destabilizing: you put in a little up elevator, and the downwash acts as even more up elevator. Longer tails make this better.

2. Pitching moment due to pitch rate.  This is caused by the change of angle of attack on the tail because of the air being round in a loop, rather than flat in level flight and maybe from the change in direction of the wind on the tail from airplane rotation.  The longer the tail, the more negative pitching moment due to pitch rate is, hence the more stable the airplane is.  Longer tails make this better.

3. Pitching moment due to rate of change of angle of attack.  The tail contribution to this is maybe (according to Etkin) due to the time between when the wing starts making downwash and the downwash gets to the tail.  I would reckon that this effect would make the airplane worse as tail length increases, but I don't know.  

4. Contribution of pressure distribution on the tail to pressure distribution on the wing.  An elevator hinged at the trailing edge of the wing acts as a flap going the wrong way, limiting the lift capability of the wing.  Longer tails make this better, but may not have to be very long to make this evil go away.  Maybe that's why the later, balanced-elevator Fierce Arrows fly better than the original.

5. Tail lift fighting wing lift.  The longer the tail, the less it has to push down to rotate the airplane to the requisite angle of attack, hence the higher net airplane lift capability.  Also, given the stabilizing effects of a longer tail, a longer tail lets you get away with a farther aft CG, hence even less force required of the tail.  Longer tails make this better.

6. Structure and pushrod weight.  Weight goes up fast as tails get longer, particularly if the airplane is designed to withstand indignities such as hitting the ground.  Pushrods get fatter to maintain stiffness.  Longer tails make this worse.

7. Pitch moment of inertial (barbell effect).  Longer tails make this worse, although the leverage of longer tails helps them get the airplane rotating.

8. Ground handling (fitting into cars and shipping boxes). Longer tails make this worse.

9. Reduction in dynamic pressure at the tail due to wing "wake".  Serge read something that Martin Simons wrote saying this is a big deal.  It's not.  Here is something quantitative: http://naca.central.cranfield.ac.uk/reports/1939/naca-report-648.pdf  

10. Sensitivity to CG position.  The stabilizing effect of a longer (and larger) tail let you get away with a wider CG range.  Longer tails make this better.

I had some of this stuff in mind when I came up with the Nemesis II combat plane as a kid.  It had a way-longer tail than its contemporaries.  I think the main things I had in mind were items 1., 4., 5.,  and 10. above.  I found 1. in a library.  I knew about 4. and 10. from a flying wing I made, which sucked.

I persisted in having longer-tailed combat planes than most.  A Dane was teasing me about my Snort having a long, vulnerable tail at the 1990 world champs.  He said he would cut it off.  I thought that would be difficult for him to do with the Snort behind his plane nibbling his streamer.  



"Tail volume" is an actual aeronautical term, but doesn't include some easy-to-include stuff that contributes to its aerodynamic effect-- wing and tail lift curve slopes that are functions of wing and tail aspect ratio, sweep, and taper-- nor some hard-to-include stuff such as 1. 4. and 9. in "tail moment" above.  Its effects are about the same as for 5., 7., and 10. in "tail moment" above, but it also gives credit for tail area.



Wing tip shape doesn't do much.  Flite Streak tips might work a little better in lift and drag, but I'll bet you can't feel the difference.  They will adversely affect rolling moment due to sideslip, the effect of which is to narrow the region of the circle in which your maneuvers will come out right in the wind.




The more flap span / wing span you have,

1. The tighter the plane will turn, or the more margin you have at a given turn radius, assuming that there's enough elevator.

2. The less hinge moment you'll get for a given turn radius.  See the deliterious effect of hinge moment in "nose moment" above.

3. The more prone the wing tips will be to stalling.

4. The potential for perversions from discontinuity at the end of the flap changes.  It's most at maybe 80% span, but I don't know.

5. The more poop you'll need from the elevator.

6. Wing tip vortices and their effects for a given turn radius will not change.

7. The heavier your flap and wing structure will need to be (a simplification: ask PW to amplify it).




Flap chord  / wing chord is tougher (I separated your area thing into span and chord).  The more flap chord / wing chord you have,

1. The tighter the plane will turn, or the more margin you have at a given turn radius, assuming that there's enough elevator.  There is an upper limit to how much of this benefit you'll get before it's swamped by the negative stuff.

2. The more hinge moment you'll get for a given turn radius.  See the deliterious effect of hinge moment in "nose moment" above.

3. The potential for perversions from discontinuity at the end of the flap increases.

4. The more poop you'll need from the elevator.

5. Wing tip vortices and their effects for a given turn radius will not change.

6. The heavier your flap and wing structure will need to be (a simplification: ask PW to amplify it).



Control ratios of interest might be the flap / elevator ratio and the line displacement / control surface deflection ratio.  You can calculate both (and linearize them for your airplane, if that's what you want) from my Excel-VBA program.  Line tension difference due to hinge moment decreases with line displacement / control surface deflection ratio.  Rich Porter and Paul Walker have thought about this, to some benefit for at least one of them.  Increasing flap / elevator ratio:

1. Causes the airplane to rotate about a point farther back.

2. Will give less pitching moment for a given elevator deflection.  This will reduce maneuvering capability and eventually cause you to run out of elevator authority, limiting maneuvering capability

3. Is what you need to do when the CG moves aft.  

Flap / elevator ratio need not be constant.  See Igor's mechanism and the Beringers' elastic flaps, noting that both have been on the World Champs podium.  

Thanks guys

As usual when I get answers to questions I end up with more questions. I'm going to let the answers soak in before asking any more

Steve

If you're an AMA member you can look at the "Imitation" article online -- Ted put a chart in there that seemed to cover just about everything that might impact the performance of the plane.

Go to the AMA website, sign in, find the "Model Aviation" archives, and search on "Imitation" -- you'll find the articles, and be able to read them.

True,,
I was merely trying to show there were more factors involved than the basic list, mine was neither exhaustive, complete, nor accurate apparently.
Your point is taken,, and the use of the word phrenology is most appropriate,,
Lacking proper mathematical background, my "design work " is primarily comparative rather than calculated
That's why I am glad we have you math guys around!
NOt sure why my post ended up so far down, I thought I had posted way early this morning, before you got yours in, how peculiar,, Yours of course is far more accurate so I withdraw my humble comments,, lol,,

Interesting how these general discussions of Stunt Design which happen periodically on this forum often overlook the in your face obvious. How do the "numbers" of the FAI winning planes compare with the typical conclusions some tend to reach with confidence. Sharks and the Berringer iterations are no evolved Impact, Imitation or Gieseke Nobler. Windy flew Patternmaster derivatives into top 5 until recently. Not that long ago. Contemporary to the Impacts. Funny also and ironic that a Super Tigre 60 set up by an American won the worlds this past year. Many folks where I fly still find the ole warhorse useful and competitive. Then there is the weather. The Pacific named Pacific for a reason by folks familiar with the oceans East and West. On the East Coast last weekend flying with some darn good fliers and designers in what the calendar calls Spring, there we were again, facing comically changeable wind conditions. No one did a pattern that was useful for perfecting the flying of the shapes. Good for laughs. Good fun. But no practice time that would lead to an incremental gaining of skills.

Bill, I agree with where you're going. It's the excellent fliers that design the excellent planes. Almost always is that way in the history of stunt. Palmer and Aldrich, Bob Hunt, Werwage, Fancher, Windy, Yatzenko brothers, Berringer, Randy Smith and many others. They build planes that suit their feel for the event. I think we have been thinking about this backwards. There is no single ideal design. Different designs have won and placed well at the recent NATs. Same is true for the Worlds. The history of winning stunt birdies shows a lot of variety. The numbers and approaches vary because the styles and preferences of the top fliers vary. The rest of the field picks and chooses from these possibilities. Some of this influenced no doubt by the folks they fly with. Which also makes sense. Since the skill and expertise has been developed via a familiarity with a certain design family. The coterie passes on that knowledge because it's what they know best.

Excellent observation, Dennis.  This underscores comments by Brett and myself over the years that it isn't the design, per se, that makes the airplane and its pilot competitive.  It is the state of trim plus the competency of the powertrain aboard a fundamentally sound design that does the trick.  The best proof of concept I can think of is the wide variety of "significantly" different designs with which Bob Baron was competitive at the highest level.

   Precisely. And almost everything we do by way of design is to *correct problems that you found that could not be trimmed out* of the current design, or to *handle a powerplant or power settings that the current design can't accomodate*.

   Brett

  So from a novice's perspective it would be best to pick a proven design that, 1) you like the look of, 2)the local flyers might have experience with. Then as your skills progress you can look at changes/modifications to better suit your flying habits/ style.

That's what I did, except the changes and modifications didn't work out.

I think the main reason for " share " of information is pretty simple...

Its not all about the equipment you use but the ability to fly it.

All things being equal. trim ect..

I think you all are very lucky.


I'd think I won the lottery if one of those " 3 - 4 " flew my plane and suggested a trim change that made it better. Or made a suggestion to my pattern that made it better..

I'm only asking for one of those - once in a lifetime.. - Couldn't imagine doing it week in week out..

I say well done. 

I'm flat out Jealous... land in like Howard are awe of their amazingnessness

  The only C/L advice I get at my field is to fly R/C. So late last year one of the guys got me flying on a buddy box, now everybody is telling me I should fly Mode 2, not Mode 1.
 Several members offer advice which I gladly consider, several others offer opinions which I ignore.
  So besides Jack Sheeks, any "stunt gurus" close to Indy? Or am I going to have to move?

Steve

  The only C/L advice I get at my field is to fly R/C. So late last year one of the guys got me flying on a buddy box, now everybody is telling me I should fly Mode 2, not Mode 1.
 Several members offer advice which I gladly consider, several others offer opinions which I ignore.
  So besides Jack Sheeks, any "stunt gurus" close to Indy? Or am I going to have to move?

Steve

Hi Steve,

Jim Vornholt (NATS Junior Champ in the early '60s) is there, and I know there are a few more like Clancy Arnold, etc..  The meet, somewhere, fairly regularly I understand.  Are you a PAMPA member?  The Members Guide is a great resource, and Stunt News is a great CLPA "magazine"!

Big Bear