Dennis and Ted,
Great discussion you have going here. Please indulge me as I share some thoughts that have been buzzing around in my head ever since I took aerodynamic courses at Park’s Air College about 25 years ago.
I’ve been modeling since I was 7 or so and have been involved in stunt since the late 70’s although family and career have slowed my involvement over the last 10 years. Your discussion here has rekindled many thoughts and ideas that I have wanted to share and discuss since reading my first Stunt News I the early 80s. I just never took the time to put them down in written form and now these forums (and spell check) have taken away my last bastion of excuse!
A common trend I have seen throughout my modeling life is that we are very effective at analyzing what our airplanes are doing but not as effective at determining why they behave as they do. Now, I am not the leading expert in aerodynamics and stunt ships but I do want to share some of my thoughts on flaps, what they do, what they don’t do and why. I hope that this discussion will correct some misconceptions and be beneficial to all in trimming and design.
Dennis: First the premise: When a wing produces lift it sheds vortexes (mini-tornados) of air off the wing tips. The vortex is relatively high energy air with curling, highly 3 dimensional flow. If you have a control surface like a full-span flap or aileron in the vortex then you have to turn that vortex when you deflect the control surface, but the vortex resists getting turned!
Dennis, I agree with everything except one statement: the vortex does not have to be turned. These vortexes induce drag and they rob lift by decreasing the effective wingspan, but they do not apply any forces that resist pitch. Take a look and a video of an F-16 or other high performance fighter performing extreme high alpha (high angle of attack-AOA) maneuvers. Many have smoke generators on the wingtips that exaggerate the visual effects of the vortices. These vortices flow straight away from the wingtips at varying angles to the cord line depending on the AOA. There is no curve to them indicating a resistance to pitch or rotation. In essence, air that is already behind the wind can not exert a force on that wing. A deflected flap will change the pressure differential between the top and bottom of the wing (the source of the vortex) and increase the magnitude of the vortex. The vortex will also decrease the effectiveness of the flap but I do not see there being a resistance to flap movement. Only a minute percentage of the flap’s effective area would be affected.
Steven: Deflected flaps lower wing's AOA. If flaps are fixed near the tips, that portion of the wing will be effectively stalled in a turn thus decreasing roll axis stability.
Steven: You are on the right track but think you remember this backwards.
Let’s go over a few flap basics.
Pitch angel: This is the angle between the flight path and the longitudinal axis of the airplane (the thrust line if you will).
Angle of attack – this is the angle between the relative wind –RW-(the flight path of the airplane) and the chord line of the wing.
Chord Line: This is a line drawn from the center of the leading edge profile (the LA center line on our symmetrical stunt wings) and the trailing edge of the wing. With the flaps in neutral, this line is the same as the center line of the airfoil as seen on the plans. With the flaps deflected 45 degrees down, it is still defined by a straight line between these two points, but now the line is angle down. Go to your plans and draw the flap fully deflected. Then draw a line from the forward most point on the LE to the TE of the flap. This is the chord line for full flap extension. Flap deflection increases the camber of the wing by lowering the TE.
Assuming that the fuselage remains at the same angle (the relative wind does not change) deflecting the flap increases the angle between the RW and chord line, thus increasing the AOA. This is how the flaps increase lift.
There are only 2 ways to get a given wing to produce more lift. You can increase the speed or increase the angle of attack. As a rule we try to avoid significant increases in speed during our maneuvers.
AOA can be increased be increasing the pitch angle and by increasing the camber of the wing without increasing the pitch. Notice how your flapless OTS design can be landed in a very nose high attitude after the engine dies? As airspeed slows, you have to increase the AOA to maintain the same amount of lift. We do this by increasing pitch. On a flapped airplane, we increase camber by defecting a flap, also increasing the AOA. In a square corner, we actually do both, increase both the pitch and the camber.
With the above being said, any flapless portion of the wing outboard of the flaps will have the same AOA as the flapped portion only when the flaps are neutral. If the flaps are deflected any amount, the flapless tip will always have a lower AOA. This may be beneficial as I will cover below.
Dennis: I wish Paul W would check in, I would like to have him expand on his conclusions - what is described could easily be construed as a mini-stalling behavior caused by the now more ferocious corner. I experienced that even with the Oriental, where when the corner was improved the bird started to stall - necessitating a correction to the wing leading edge shape which solved the issue.
There is no doubt that we can force or planes into an accelerated stall without too much difficulty with predictable results – the flight path will be unpredictable! The question I have is what part the wing is actually stalled. Is it the root, the tip or the whole wing? This is an aspect of flap design that I am surprised to have never seen addressed.
Any wing will always stall at the same AOA regardless of the airspeed. This is called the critical angle of attack. Going back to our flapless OTS example, it does not matter if you stall the plane in level flight with the engine dead, 5 feet above the ground and barely moving on the upwind side of the circle or if you stall it in a blinding corner at over 60 mph. The wing stalls both times at the same AOA.
On a flapped plane, every time you change the flap deflection you change the camber and in effect have “new” wing. However, for any given flap deflection, there is an associated critical AOA that never changes regardless of the airspeed.
Full size aircraft such as the Cessna singles are built with significant amounts of wash out in the wings. The wing is actually twisted slightly from the outboard end of the flaps to the tip. The LE is lower at the tip and the TE is higher. The result is lower chord line at the tip compared to the root. In a stall, the root with a higher angle of attack stalls first, with all the associated warning signs while the outer portion of the wing remains un-stalled. Designers spend a lot of time on this design feature to impart gentle, predictable stall characteristics into the plane.
A stunter with a longer fixed flap at the tip may have the same result. The tip with its lower AOA remains flying and producing lift while the flapped portion is stalled.
Here is the part of flap design I have never heard addressed. Assuming a wing has the same root and tip airfoils, and the flap chord is a constant percentage of the wing chord at any station along the span of the wing, then the AOA changes the same amount along the span of the wing for a given flap deflection.
If a Twister wing had a constant chord flap (say 2 inches from root to tip) then at 30 degrees of deflection, the AOA would be the same at the tip as at the root. But you say hey, the Twister has tapered flaps – approx 3” at the root and 1” at the tip. What does that do to our AOA? Let’s go back to our profile plan view. On the center rib, draw the root of the flap (3” chord) deflected 30 degrees and then draw the chord line as we discussed earlier. Now draw the tip of the flap (1” chord) and again draw the tip chord line. Because the flap tip chord is a smaller percentage of the wing tip chord the resulting AOA of the tip is less than the AOA at the wing root. In theory, the tip should remain un-stalled and lift producing while the root stalls first. Actually the stall would commence at the root and progress outward towards the tip as the critical AOA was exceeded progressively from root to tip.
On a tapered wing, it the flap chord decreased as a percentage of the wing chord from root to tip we would get the same effect. If the reverse happened - the flap chord increased in percentage of the wing chord from root to tip - then the tips would stall first resulting in potentially severe hinging and banging. If the flap chord is a constant percentage, the theoretically the intire wing woulod stall at once. Ted, as I recall you have used a slightly different airfoil at the tip than the root on some of your designs. I believe the high point of the airfoil was moved forward a few percentage points at the tip. Many factors such as this would affect the critical angle of attack for a given section along the span. However, the premise still holds that excessive flap chord towards the tip may lead to the tips stalling before the root and manifesting itself in udesireable hinging and banging.
I wonder how many poorly cornering ships have been abandoned in the past because the flap percentages actually cause the tips to stall first instead of the root? I have no idea. AS I said, I have never heard this aspect of design addressed.
Dennis: Short fat flaps versus long skinny ones - whew, now THAR's the rub! Here's another one - do you think the flaps are there for LIFT or for DRAG? I think the answer is mainly for lift - but the drag does not necessarily hurt either! For the reasons I laid out I will never consider using FULL span flaps. I have wondered about using relatively narrow flaps with a LOT of throw - maybe 3:2 versus elevator (insted of the "normal" 2:3 or 1:1) to increase the drag component. If using wide flaps I would want to restrict the deflection - and probably get a GREAT lift component. Wide flaps and a lot of throw sounds like too much control effort.
More flap trivia: The first 20 degrees or so of travel flaps increase lift a lot with only a slight increase in drag. From 20-45 degrees, lift increase very slightly with a huge increase in drag.
Remember that flaps increase lift by increasing the AOA through a change in camber. Let’s revisit our Twister-type wing. If we installed 1/2” chord flaps along the whole wing and the tota; chord was say 12 inches, 20 degrees of deflection would result in only a slight change in the chord line and result in little additional lift. If we deflected them 45 degrees there would still be only a slight change in the chord line (again with little lift) but a whole lot of drag!
Now let’s take the a wing and install flaps that are 50% of the wing chord -six inches of wing and six inches of flap - and deflect them 20 degrees. Again draw the chord line and we see a much greater change in its angle. With a larger flap chord we will see a higher angle of attack and more lift for a given angle of deflection. Drag will increase but not as much as lift increases.
These two examples are meant to show the extremes to make the point. As the flap chord decreases as a percentage of the wing chord to zero, the flaps would become less and less efffective. Inversely as the flap chord increases other negative aspects such as control forces and structural limitations would overshadow and improvements is lift. I also do not have any idea what effect changes in flpa chord have on pitching moment - it may be positive or negaitve. Unfortuneatly, whey designers make a significant change to flap chord, other characteristics of the model are changed as well and resulting changes to flight characteristics, either good or bad, may not be the reslult of just the flap change but of other changes in combination. The only way to know the true changes are to change only one aspect at a time, such as you did on your Oriental project.
So Dennis, your thoughts on narrow vs wide flaps are correct. Now I hope you understand why.
There are times we could use more drag but the example I just gave reveals that the drag we think is helping is actually working against us when we don’t want it and is AWOL when we need it. Our planes tend to wind up in consecutive round maneuvers when control deflections are low and flap induced drag is low. Bottom line is the drag is not much help here. The flaps create their most drag at high deflection – AKA square corners – at a time when we likely want to preserve energy for the next 17 corners of the square eight or the excruciating climb to the top of the hourglass. I don’t know what we can do about it but I thought it worth mentioning.
Ted: My guess is that the vortex is the result of all of the lift up to that point of the wing span. The vortex is the result of high pressure air on the bottom of the wing trying to get to the low pressure on the top of the wing. The "vortex" is, I believe, the result of the natural spanwise flow of the air (air doesn't go straight parallel to the fuselage but tends to move towards the tips). Because of that spanwise movement the "low pressure" the air is seeking is back towards the fuse. The fact that the airplane is constantly moving forward combined with this "inward" movement makes the spiral. I can't quantify or send you to a source off the top of my head but I think it is logical to expect the vortex at any point of the span to be the result of the air pressure change up to that point.
Ted, I would talk about this if it was not 1 am. I’ll catch ya later on this one.
Ted: The fact that the sailplane pitches up when flaps are extended is the result of a different aerodynamic reality that takes place when flaps are deflected. Lowering flaps changes not only the camber of the airfoil but also its angular relationship to the stabilizing surface. The net effect is increased decalage between the two. Thus, when you drop the flaps, the angle of attack of the wing increases proportionately to the flap extension while that of the tail does not. The tail is, in effect, applying "up" elevator to the airframe itself and the nose will pitch up in response.
If this were true, then why don’t our stunter’s flaps, which are a whole lot bigger and effective as a percentage of the wing’s surface, have the same positive pitching moment when deflected? Heck, we are even helping with a half of a 25% stab standing up at 30 degrees in the prop blast. The situation you describe above happens in all airplanes when the flaps are extended yet the result is not the same.
I believe that this phenomenon in sailplanes is due to the physical relationship between the location of the horizontal stabilizer, its area and the resulting airflow changes from the trailing edge of the wing with flaps extended and how that air flows over the stabilizer.
I know that in a Piper Arrow with the stab slightly higher than the wing, there is a significant pitch change with full flap extension but very little pitch trim change. On a C-170B however, the thing about wants to stand on its tail when the first notch of flaps are applied. It takes a whole lot of nose down trim to correct this. However, when the 2nd and 3rd notch (30 and 40 degrees) is added, there is very little pitch up and little additional down trim in needed.
Dennis: Implicit in that is when re-trimmed with the flaps deployed, the fuselage assumes a nose-down inclination versus the no-flap trim condition. I guess I always thought the pitching moment would have a more powerful effect - but then the effect from the stab (the up elevator you described) probably overpowers the flap PM, especially on a long-tailed sailplane...
Dennis, remember what the flaps do…they increase the angle of attack. When you put the flaps down on a sail plane, you are not trying to produce more lift, but rather produce the same amount of lift at a lower airspeed. Because the flaps increase the AOA by changing the camber and moving chord line, the pitch of the fuselage will decrease (nose low) while the AOA remains relatively high. If you fly any airplane with flaps up and then flaps down at the same airspeed, the resultant pitch attitude will always be higher with the flaps up. This is not a function of pitching moment or tail volume. Remeber we increase AOA by increasing pitch or increasing camber.
Conclusion:
Back to the issue of flaps and stunters and fixed tips and narrow and wide and deflections. As Dr. David Manor, PHD told us over and over again, there is no free lunch. We use flaps because our planes fly better with them, but they also bring some baggage that makes our lives difficult.
Long skinny full span flaps probable are not the best answer. Neither are very short, high chord flaps. The optimum is somewhere in the twain. The aero lab at Parks had a 36 inch wind tunnel capable of speeds over 120 mph. A 60% size model of a modern stunt ship tested at 120 mph would have had approximately the same Reynolds’ number as our actual models. I wish I could have built a test model with removable flaps, stabilizer and elevator and done tests on lift over drag, pitching moments and drag for various flap spans, chords and elevator/flap ratios. That could have answered a lot of questions. Instead we plod along with very educated guesses based on experience and trial and error. There are countless other factors not addressed that have an impact on our model's flight characteristics that have not been addressed for brevity and simplicity. When comparing two models with different flap configurations, we must consider that the changes in flight characteristic may not be due soley to the change in flaps, especially if other changes have been made as well to the stab/elevator, tip shape, weight, etc.
One more consideration on comparing two examples of the same model with different flap configurations. Micheal Seilig did a tremendous amount of research in the 90s on low speed airfoils for RC gliders. He cautioned that the inability to accurately and consistently recreate/duplicate a given airfoil or design parameter using traditional building techniques could have a significant impact on the resulting flight characteristics. A characteristic we attribute to a new flap configuration may be due to some other change introduced unitentionaly during the building phase.
We have seen many different configurations be successful, from Bob Barron’s flapless Humbug to Windy’s and L Jay’s huge flaps of the 80’s to the more traditional designs seen today. Full span or not, large chord or narrow, all can be made to work.
Thanks for letting stick my nose into this discussion. I hope I have not stepped on any toes. As I said, we observe very accurately. I hope I have given some information to help take some of the error out of our trials.
Wow, this got long. Thanks for reading.
Bruce
edited for clarity
Topic: Dennis Adamisin: Clipped flaps...a discussion??? (Read 18106 times)