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Aerodynamic Turning Model

Old 10-06-2002, 11:17 AM
  #26  
rmh
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Default Aerodynamic Turning Model

Cajun- I do understand all of the comments - thanks - I am puzzled tho that you you refer to drag created by higher AOA as profile drag - I always refer to that as Induced drag -
Flaps/variable incidence, etc., do keep the deck(fuselage) flying at a lesser AOA- but what I was suggesting is part of an old NASA project which had a free wing - the prop and fuselage moved relative to the wing.
I looked at it - -and tried to think of a practical application - couldn't - and I wondered about using the concept to use vectored thrust -sorta like a Harrier does - as a aid to overcome the slipping losses in a high G turn.
I really do think our models are too small for much fiddling with shifting airfoils - VI etc., but fun to conjecture -- armchair flying is fun--
Old 10-06-2002, 06:23 PM
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Default Aerodynamic Turning Model

I've had this idea for some time. How about a "live hing" full-span aileron. Make the aileron less tortionally rigid as the wing and locate the actuator a close to the root as possible. Deflecting the aileron, or flap, for that matter, in essence, twists the wing thus making the wing appear more elliptical (less induced drag).

Just a thought.....
Old 10-06-2002, 06:44 PM
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Frank Weston did it on his Magic about 10 years ago. His motive was to cure a flutter problem on a long skinny aileron that didn't have enough torsional stiffness to resist flutter. The aileron's effectiveness was almost as good as before the tip was tied to the wing. It cured the flutter problem and tip stall on the down-going aileron too.
Old 10-06-2002, 06:50 PM
  #29  
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Long skinny ailerons -- I used em --once upon a time - they were only worth having using reed sets on models that just poked along.
Counter balanced barn door ailerons (and low aspect ratio ailerons) work better on models -simply because it is easier to make em stiff .
Over the past year I did aileron mods for some large ARFS -aerobatic stuff - which fluttered easily in stock form-
Old 10-07-2002, 01:58 AM
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Default Aerodynamic Turning Model

Ollie...

Would an elliptical wing planform be any benefit in reducing the induced drag for these pylon racers?

Bill
Old 10-07-2002, 03:41 AM
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Dick,

Induced drag goes up essentially when the coefficient of lift increases, which does happen when the AoA of the wing increases. Profile drag of the wing, which does not vary with lift per se, also goes up as the AoA goes up. Profile drag of the fuselage and everything else also increase, generally, as the AoA of everything else increases. When trying to reduce the inevitable increase in drag that occurs during a turn, it is important to realize that the increase is not due entirely to induced drag. For a pylon racer, even the turns are at high enough speed that the induced drag is not very big at all. Not to say that it can be ignored, but one's efforts to manage drag in turns should probably be concentrated on profile drag, which is dominant for every part of a pylon race.

I think best view of a variable camber wing ( or coupled flaperons ) is that it is the feasible implementation of a variable incidence wing, with the added advantage the camber is increased also.

banktoturn
Old 10-07-2002, 08:18 AM
  #32  
rmh
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I do understand the problems with profile drag -and wetted area etc..
But -in our small models -I really think that induced drag is still the real culprit.
I once fretted over intersections -leading edge shapes -etc.. for aerobatic stuff- bottom line - It was all nice purty up stuff -but had little to do with how the model performed.
I raced a little - my stuff worked well - but I was not an ardent racer -
Old 10-07-2002, 08:39 AM
  #33  
Ollie
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Default Aerodynamic Turning Model

Bill,
The induced drag is minimum when the lift distribution is elliptical. In full scale an elliptical planform is one way to get an elliptical lift distribution. It doesn't work as well at model reynolds numbers because the profile drag increases and the lift decreases faster with decreasing reynolds numbers at the tips of models than it does at full scale.

You can achieve a very, very close aproximation of an elliptical lift distribution with a moderately tapered plan form and a touch of washout. See:
http://aero.stanford.edu/WingCalc.html
Input an aspect ratio of 8, a sweep of zero, a taper of .75 and a twist of .5 degrees. Then click on compute and click on the bottom of the graph until the Cl equals 0.388. The coefficient of induced drag will be shown as 0.006 and e, the efficiency factor, will be shown as 0.994. This means that the lift distribution is within 99.4% of an elliptical distribution. You can play around with other aspect ratios, tapers, twists and angles of attack to see how close you can come to an elliptical lift distribution. This on line program can be a great aid to refining the design of a wing. Remember, angle of attack is not incidence!! In this program, the angle of attack and the twist are relative to the zero lift angle(s) of attack of the airfoil(s).

Dick,
The Coefficient of lift increases linearly with angle of attack up to the stall. However the induced drag coefficient increases as the square of the lift coefficient. This means that if the angle of attack increases 41%, the induced drag coefficient doubles and if the angle of attack doubles, the induced drag coefficient quadruples. If the angle of attack tripples, the induced drag coefficient increases nine times.
Old 10-15-2002, 12:55 PM
  #34  
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Ollie - your last paragraph sums it up nicely-
For this reason - I use very thin wings on my aerobats - (12/10%)
They are all very light in wing loading and will brake /stall snap instantly - even at lower AOA.
Of course -for 3D models /fun fly stuff - the big ol fat foils are quire insensitive to AOA- but they are worthless for low speed snap ability.
Old 10-16-2002, 03:29 AM
  #35  
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Originally posted by Ollie
What you need is a variabl camber airfoil (flaps). If flaps are permitted then a relatively thin airfoil with flaps mixed to the elevator will extend the range of coefficients of lift and with the low drag you are looking for. To keep from loosing too much speed in the turns, the flap travel should be limited to about 5 degrees. With careful design of the hinge line and a thin airfoil, the drag won't go up much with so little flap but the increase in lift will be substantial. Go with the MH33 or S6063 and 22% flap width. Use full span flapperons. Don't make the tip chord less than about 75% of the root chord unless you want easy spin entry and snappy snap rolls.
Ollie, Im interested in your tip/root chord ratio comment (<0.75). Why do you suppose very few of the current Q40 & FAI racing models do not have wings like this? Example: here is a Hepperle influenced FAI model, tip = 0.125m, root = 0.280m, tip/root ratio = 0.446. Way less than 0.75. Some of the newer models appear to have longer spans, different airfoils but similar (<0.50) tip/root ratios? I dont get your logic, please explain.
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Old 10-16-2002, 03:49 AM
  #36  
AQ500
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This might be the reason why. This is a chart where you can find the efficiency factor when computing the induced drag. The lower d is, the better.
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Old 10-16-2002, 04:54 AM
  #37  
Ollie
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Q500,
Perhaps you would define "d" for me.

My sole purpose in recommending a minimum taper ratio of 0.75 was to avoid tip stall by providing plenty of stall margin when turning almost as tightly as possible with flaps.

Example 1: At a coefficient of lift of 0.86 (with flaps), the taper ratio of 0.5 yields an induced drag of 0.024 and with a taper ratio of 0.75 the induced drag goes up to 0.029. The wing with a smaller taper ratio has an advantage unless the pilot over does it, tries for too much and, snaprolls. If the piloting is good enough to never snaproll then the more tapered wing is best down to a taper ratio of between 0.3 and 0.4. Such wings will have vicious snap roll tendencies when trying for high lift coefficients by pulling a bit too hard or encountering a gust in the turn.

Example 2: At a coefficient of lift of .4 (without flaps), the taper ratio of 0.5 yields an induced drag of 0.0066 and with a taper ratio of 0.75 the induced drag goes up to 0.0068 which is hardly a significant increase compared to the total drag.

So, the choice of taper ratios boils down to a matter of sqeezing the last bit of drag reduction out of the design versus a design that is more forgiving of pilot error and gusty air. Obviously, highly tapered wings are best for pylon racing by top notch pilots. I should think less tapered wings would be better for combat.
Old 10-16-2002, 12:02 PM
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Default Aerodynamic Turning Model

OK.....It's the induced drag factor. It is equal to zero for an eliptical lift distribution. The span efficiency factor is e=1/(1+d), and the induced drag is proportional to 1/e.

Cdi = Cl^2/(Pi x e x AR)
Old 10-16-2002, 01:12 PM
  #39  
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I see all of the numbers and plots , etc..
Would anyone venture to plot what the actual flite path looks like pulling the minimum radius possible turn?
To clarify:
The actual path vs the theoretical path at say --150 mph at STP condition.
Use a 400sq in 4 lb model with your ideal wing for this type craft .
My gut feeling is that the model actually sinks deeper into the turn than theoretical lift formulas project.
Here is why I ask this.
During the recent TOC- I watched some models (aerobatic types ) pitch 90 degrees and slide forward , SIDEWAYS and sometimes backward which demonstrates that control can be held at attituded completely beyound "calculated" AOA possible.
I am not sharpshooting anyone - just wondering if anyone knows of calculations which can explain why control exists (momentarily) when model is actually proceeding on a course 90 degrees to attitude.
As line super at the event - I was up close and could easily see the path flown in these cases.
The maneuvers were inertial types with models capable of almost instant rotation to departure attitudes.
Old 10-16-2002, 01:30 PM
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Dick,

My assumption would be that the models you watched at TOC were well designed to retain control authority even when flying beyond stall, as is required for those maneuvers. The significant thing about those maneuvers is that they don't require, and don't get, high lift from the wing. During a pylon turn, the situation is different. Speed remains high, and the aircraft is designed to generate very high lift for a fast turn. I expect that the flight path for the pylon planes would not exhibit the 'mush' that you saw. If it does, then too much speed is being lost in the turn. Having said that, the formulas are not perfect in their predictions, but they are pretty good if you apply them where they are valid.

banktoturn
Old 10-16-2002, 01:42 PM
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I understand -- but would not the mush which you say is a speed looser -- actually be a speed enhancer ?
By that , I mean that the model retains more speed in the turn thru an increased radius.
The conjecture being--as the model turns in ever sharper turns - the air speed must change.
N/Y?)
Old 10-16-2002, 02:14 PM
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Dick,

The 'mushing' is a speed loser because it dramatically increases drag. If it were beneficial to increase the turn radius, it would need to be done with minimum drag. It is certainly true that, all other things being equal, an aircraft can maintain a higher airspeed in a large radius turn than a small radius turn. This is for two reasons. The first is simply that tighter turns require more acceleration of the airplane's mass toward the 'axis' of the turn, which consumes more power, which costs airspeed. The second is that the additional lift and AoA needed for a tight turn generate more drag than a wide turn, even if we optimize it. The reason that this doesn't push us toward wide turns is that maximum airspeed is not our goal: fastest completion of the turn is our goal. I don't know what the theoretical optimum solution is for minimum turning time, but my guess is that it is something like: "make the tightest turn you can without incurring an excessive drag penalty". Possibly you would run into structural constraints as the turns get tighter, but my gut tells me that we can make these things so strong that drag ( profile drag ) is enemy #1.

banktoturn
Old 10-16-2002, 02:21 PM
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Yep- I would still love to see a real flight path vs the theory on it -
We always opted for the biggest turn possible -to reduce induced drag.
Like everyone else - there is a trade off found whre the fastest top speed setup is not the best elapsed time setup -
I always had to run ballast to hit min weight.
Old 10-16-2002, 02:34 PM
  #44  
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The TICK,

I just read through this thread again to remind myself how it all started. I think that for your purposes, flaps or flaperons with a fairly conservative maximum throw are the way to go. I don't know whether the 5 degrees suggested by Ollie is the right number or not, but it would be a good starting point. If you get the lift you need from that small a throw, then flaperons are probably OK. If you need more throw, I would be inclined to separate the flaps ( camber flaps, as Martin Simons calls them in his book ) and ailerons to make sure that you maintain good roll control. I would mix the camber flaps with elevator, but have the flaps reach their maximum throw pretty early, so you still have elevator travel after the wing reaches its "high lift configuration". This might need some tuning. On the subject of books, I think that Martin Simons' Model Aircraft Aerodynamics is perfect for this kind of project. He actually addresses this topic of camber flaps specifically, and I don't think you will get the kind of practical information he offers from any of the other titles that have been mentioned here.

I'd love to hear how it goes if you try this out. I can only build one plane every 10 years, so I have to live vicariously through you.

banktoturn
Old 10-16-2002, 02:44 PM
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Dick,

I find this discussion really interesting. I have no experience with pylon racing, and thus no access to practical information, although I have recently learned that the regional pylon races are held near me, in a suburb of Minneapolis, so I will have to check it out next spring.

If you are finding that a wider turn ends up being faster, it may be worth experimenting with the variable camber idea that has been kicked around here. I know you are skeptical of this next part, but I am 95% certain that it is not induced drag you are fighting, but profile drag. You need to get the AoA down. A different airfoil section could help. Higher aspect ratio helps give more lift at small AoA. Variable camber also can get you the high lift for turns at smaller AoA.

Thanks,

banktoturn

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