Guys,
Has anyone come across information that relates the "into the wind" increase in drag on the airframe vs. that on the prop? What I am getting at is in high wind conditions the ship will come from the downwind side of the circle and hit the wind usually resulting in a slight slowdown till it passes the upwind point then will start to accelerate. The question is when the wind hits there is a sudden increase in airflow over the airframe which increases the drag, it also pushes against the prop windmilling it a bit but it is like the prop is coming from a dive and braking the ship, which one has a greater influence and could the motor be trimmed to pull through this by sensing the load?

Best,       DennisT

That's an interesting topic.  How do you discern going upwind when you want the prop to add power from going downhill when you want the prop to reduce power?   I have an idea, but I haven't tried it yet.

Do you want the prop to add power going upwind?  Is it better to have the plane flying at a constant ground speed, or is it better to have the plane flying at a constant air speed?  Flying at a constant airspeed would mean that the plane would have to decelerate coming into the wind, not accelerate.

(Note: I'm not being a smarta** here -- I really don't know, and I'm curious as to the answer).

There is also wind drag on the lines, which I think may be more important than wind drag on the airplane.

There is also wind drag on the lines, which I think may be more important than wind drag on the airplane.

That's interesting, too.  Going into the wind has a different effect on lines, hence airplane yaw angle, than speeding up on the downhill part of the wingover.

I think maybe a constant ground speed, but maybe not.  When we make a system to do constant ground speed, we'll probably find out that's not what we want.  Stunt has a requirements issue.

    I think it's about halfway between constant ground speed and constant airspeed. Anyone who was at the 2003 NATs on Friday can see what the effects of constant airspeed can be. Gieske's airplane was 49 oz with a 65, ran along at a constant (low) airspeed, and couldn't make it around the upwind side. Mine was a little closer bit still way too slow going upwind. Mine would have made it if it was about 1/4 sec faster. Just flying level, I would have had a better chance with a Nobler and a Fox than with the regular airplane. Of course, it wouldn't have done many maneuvers, and I would have made it fine if I could have gotten to all of them.

   You can see the same "too much speed stability" in less extreme cases, too  - particularly the top legs of the square 8. Its pretty easy to make them too stable with tuned pipe engines and very low pitch. I think it's a compromise between short-term stability (to keep it going in the middle of and out of corners)  and long term speed variation (you need some That's one reason we moved away from the original piped systems, where we ran pretty big 2-blades at 3.25" of pitch at 12,000 in the air - it was amazing in good air, like super-slow-mo, and had no tendency to whip up. They also tended to run out of gas punching back into the wind - it would always make it but you had to make extreme timing corrections to get the tops and bottoms of equal length. I think you need a compromise between the short-term and long-term airspeed stability. You need lots of stability for getting through the middle and exits of corners and not nearly so much for punching back into the wind in places like the square 8 and overheads.

    Brett

Interesting thoughts have been presented. The apparent speeding up, and slowing down of our tethered modelos, is not due to anything other than the apparent airspeed being affected by wind speed relative to a fixed point on the ground..

In free air, flying without contraints, as in full sized flight, the airspeed is dependent on the prop pitch, engine rpm, and the state of trim of the aircraft. When balanced out, the 4 forces affecting flight are balanced out. Trimmed to fly level, with the speed and pitch constant, the aircraft is left only affected with the wind speed, and direction.

One of the early skills taught flying full sized craft is a "Turn around a point". It teaches the new pilot about the effects of the wind in the air to maintaining a turn around a specific point on the ground. You quickly learn that the radius of the turn is constantly changing while you attempt to keep your wing tip attached to a point on the ground.

When flying into a head wind, the airspeed indicator still registers the same speed, but ground speed will be, the indicated air speed, less the actual speed of the head wind. one of the skills a new pilot must learn is how to vector these forces. Without these skills, Navigation can be tricky.

An example would be, an indicated air speed of 200 mph. An actual head wind of 60 mph, you would wind up with a ground speed of 140 mph. Your aircrfaft is trimmed to fly at 200 mph, and as far as it is concerned nothing has changed. You sitting inside, will notice nothing, unless you try to turn around a point. Oh yes, if you were flying to destination 200 miles up wind, you will still be 80 miles out after an hours flying. I hope you put enough gas in the tanks to fly about 1 1/2 hours for that 1 hour flight.

If the airspeed did slow down. going into the wind, the plane would loose a proportional amount of lift, and the plane would have to be re-trimmed to maintain level flight.
 
Since our models are tethered to the ground, in a circular flight patterrn, going up wind, and down wind once each revolution. it appears that the model is speeding up going down wind, and slowing up going into the wind. Since we are not floating in the "river of air" our model is flying in, it appears to be so. Now, if we we're floating in the air, we would not notice this effect at all, relative to ourselves, as we would be affected by the same wind.


So the difference we observe while flying our planes, is not airspeed, but relative ground speed when observed from a stationary, fixed, point on the un-moving ground. This gives rise to the thought that our models are flying at a more constant speed, relative to the air. 

For me its a question of airframe drag. Not weight or power avaliable. Bobs bear would have been the traditional thicker wing section - Dave has solved that issue of flying into wind with a thinner wing.

  No, PJ, the airplane in question was a Billy-style medium airfoil very close to if not identical to David's (and incidentally very close to the Imitiation and just about every other airplane we flew before the first-generation piped designs). It's only thinner compared to the sort of Trivial Pursuit and Infinity-style wings. Ted, David, and I came to the conclusion that they were probably thicker than we needed quite a while ago.  David's airplane has a Thunderbolt airfoil.

    I mentioned the power and weight because the power setup and light weight tend to enhance the speed stability  - negligible momentum and no "carry". Drag is certainly part of it since the drag and weight determine the ballistic coefficient but the engine setup is critical, too, as it can greatly enhance the effect. That's *why* the Trivial Pursuit and Infinity are unnecessarily thick -we were still stuck in the 4-2 break design concept and that's how you dealt with larger engines at the time. We underestimated how effective the engine and prop could be in controlling the speed. It actually still has some desirable characteristics and it makes the performance much less dependent on perfect engine runs, and you would have a tough time finding anything the flies a lot better. 

 Mine was only slightly better than Gieseke's, because although it had a much thicker airfoil (with slightly higher parasitic drag) it was also much heavier, which gave it a somewhat better ballistic coefficient. But the engine was still running ram air intake and with that and the overall setup of the engine, it killed any momentum coming around into the wind very effectively. And it was faster to begin with. It was great in the maneuvers but had a lot of problems getting around upwind.

   Brett

   

Guys,
It sounds like we need a combination of low drag thinner airfoil for level flight with a flap design to give higher lift then we used in the past and a little extra weight (at the CG). Could slightly larger flaps visa-vee Al Rabe with a thinner section then he used coupled with modern power help get through the tough wind?

    Al's airfoils are not consequentially different, thickness-wise, to the Imitation type, although the shape is drastically different. Al used his own personal definition of thickness that did not include the flap chord, which is not the conventional way to do it. Once you take that into account you can see that most of us were using something in that ballpark before piped engines came along, maybe 21-22%. The Infinity, by comparison, is 26%, measured the normal way, and is *much* thicker than Al's.

   I also don't necessarily think you want more flaps. If nothing else, the hinge moments will greatly increase. By far, the biggest problem in flying piped airplanes in extremely heavy wind is muscling it through the corners. If anything I would do just the opposite - cut the flap area to the bone, if wind flying was the only consideration. With piped airplanes you are going to have *no* problem running out of lift with any reasonable airfoil and any flyable weight. Check out Jim Aron's 585 square inch 64 oz airplane with a very thin airfoil (but a good one with a pretty blunt LE). Oh, and a piped 65.

    But designing for conditions like the 2003 NATs is not something I think is worth pursuing. It's just such a "corner case" that it's certainly not worth  compromising it for anything else. Regular airplanes got through it if they were set up and flown correctly - I made a needle mistake on the second flight but I got through just fine on the first flight with only marginally better conditions. We now suspend flying at the NATs at far lower wind speeds. And one of the ways Ted and I got through to the end so easily in 2006 was because our airplanes handled the conditions better than most and Ted, at least, was hoping for more rough weather because he thought it gave us an advantage. I wouldn't have gone that far but it sure didn't hurt anything.

    Brett

 

By far, the biggest problem in flying piped airplanes in extremely heavy wind is muscling it through the corners.   

I don't see the problem.  Do you, PJ?

to use the slightly thinner airfoil (say 16% measured the normal way) with slightly larger flaps (increase about 5% ish)

This will bring you to low airfoil drag in level flight and high arfoil drag in corners, exactly something you do now want. You have to optimize airfoil for high load (high AoA, deflected flaps) and then carefully design model to fly corners at its optimal point. I did that and got model which makes windy day always an advantage. If you then limit hinge moment, you have good plane for turbullence.

Would line groupers work on a stunt model? The team race bods used them to keep the lines together and reduce drag, but I think they've been banned in competition?

Cheers

Not sure if they would work with a stunter. The object is usually to reduce line drag, which they do significantly, but this relies on the two lines being one behind the other over their whole length. If twisted together you'd get a wave profile all along the lines, the nodes would certainly be one behind the other, but at max. amplitude you'd get a profile twice the line dia.
As Brett says, after the first loop the're twisted anyway, so it's a situation we already fly in all the time.

Neat thread!  I've been out of touch for a while.

Some quick thoughts.

I expect the only effect on "airspeed" is the result of engine changes that come with loading and unloading, because...

In level flight you don't "suddenly" have a tail wind or headwind.  Because we are flying tethered in a circle the wind is constantly changing, going from a maximum headwind at one split second and a maximum tail wind in a slightly shorter split second.  Every other point of the lap is some predictable fraction of those two maximum conditions.  These external forces balance out in simple level flight around the tether.  We essentially never experience what is being talked about, i.e. a sudden change from a head wind of X MPH on one side of the circle and a tail wind of X MPH 180 degrees later.

The other factor which one would think could effect airspeed (but doesn't much) would be inertial effects as the mass in motion wants to continue at the same rate despite the change to a headwind or tailwind.  Inertial stuff like that is the result of speed across the ground, not through the air.  (this is also why lines bow more upwind and less downwind even though the speed through the air mass is essentially constant--a function of rotational peed relative to the tether(pilot) and centrifugal/centripetal forces).  Again because the ground speed of the airplane is constantly changing (not "suddenly" changing) the inertial effects balance out...for every instance of acceleration there  is a balancing instance of deceleration).  If this wasn't true the lap times would constantly increase or decrease...which they don't.

Looked at another way: If the four pound stunter was flying in dead air at 55MPH (Ground Speed and Air Speed)and "all of a sudden" was struck by a twenty MPH headwind it wouldn't instantaneously slow to 35 MPH GS (Ground Speed) and remain at 55 MPH AS (Air Speed).  Because of inertia (mass in motion) GS would remain at 55 for a split second and gradually slow to 35 as the inertial dissipates.  Airspeed would momentarily jump to 75 and reduce as inertia ebbed.  The opposite effects would result from a "sudden" tail wind of 20 MPH.  In a steady state wind with a tethered plane, however that "sudden" increase/decrease in the flow of the airmass relative to the airplane never occurs.  (By the way, a sudden change in wind speed or direction is the "definition" of "Wind Shear" and is a significantly different animal where as the air mass itself is altered,,,not the path of the airplane through it.)  

The airspeed, therefore, will be relatively constant (not considering powertrain changes previously discussed) in level flight although line tension will vary a lot because ground speed "will" be changing.  This is what Brett was talking about.  The airplane thinks it's going just as fast and doesn't understand why the pilot is whining about anything!

Gotta think some more and Shareen wants help putting away the Christmas stuff.

Ted

p.s.  When you start to talk about maneuvering in winds the effects of inertia become a much bigger deal and when combined with rapidly changing lift produced by the wing and tail as the pitch attitude relative to the ambient airflow changes the whole thing becomes a lot more complex and things such as "wind up" can occur  (or it's less talked about reverse phenomenon, "wind down", should you seriously miss the wind with reference to your maneuver).  I'm not sure I even want to go there.

All aircraft fly in the air so drag is consistant but the perception of change when flying our CL planes in a circle is the issue.  Crosswind is constantly changing but airspeed remains the same, ground speed changes. I believe that drag remains constant relative to the air that we are flying through just as speed does.
Interesting subject though.

Howard, I said nothingt about inertia, I was addressing the issue of drag.  The percieved drag change would be relative to the ground bound pilot going round and round in a circle, constantly changing relationship to the prevailing winds.  Ofcourse the plane loads and unloads according to where it is in relationship to the cross wind compponent. Perhaps I was too quick on the trigger.          :-)

It is an interesting topic to think about, Howard.        :-)

TED WROTE
"Looked at another way: If the four pound stunter was flying in dead air at 55MPH (Ground Speed and Air Speed)and "all of a sudden" was struck by a twenty MPH headwind it wouldn't instantaneously slow to 35 MPH GS (Ground Speed) and remain at 55 MPH AS (Air Speed).  Because of inertia (mass in motion) GS would remain at 55 for a split second and gradually slow to 35 as the inertial dissipates.  Airspeed would momentarily jump to 75 and reduce as inertia ebbed.  The opposite effects would result from a "sudden" tail wind of 20 MPH.  In a steady state wind with a tethered plane, however that "sudden" increase/decrease in the flow of the airmass relative to the airplane never occurs.  (By the way, a sudden change in wind speed or direction is the "definition" of "Wind Shear" and is a significantly different animal where as the air mass itself is altered,,,not the path of the airplane through it.)  "

HI Ted

This is also why the engines, even ones on pressure go rich and lean, the inertia of the ship driving into upwind air, unloads the engine and as it speeds the prop and helps the engine to go rich, the opposite occurs downwind

Randy

Yep, I can hear my engine change speed.  I don't understand why.  

I was hoping that you would fill all that stuff in with sines and cosines and such.  

   Actually, I think I have an idea how to do it, with some guesses at the drag coefficient. What I don't have is the engine/prop effects but it might be good enough to get a feel for it, if not exactly accurate. I keep hinting at Frank Williams to fire up his wind tunnel with a running engine in it and a thrust stand and dyno, but so far he hasn't bit. Maybe if the TTs are in Houston we can set it up. Assuming of course I end up flying airplane AT ALL this year.

     Brett