I received a couple very good questions via PM. I think that route is good as it provides opportunity to consider and deliberate without posting garbage. So, please PM questions if you think they are not "prime time" ready. How I arrived here has been a much longer road than I would have ever expected.
My intent in this thread to to elaborate my decision making process and how I came to the conclusions I did. Oftentimes, I hold an opinion which is contrary to the common sense. Sometimes, I'm even correct. To an outsider, someone not wandering around inside my head, my actions seem crazy. Sometimes, they're correct.
Keith Trostle sent me the follow note. I am aware of much of the past history of stunt design. I'm an engineering scientist guy, I watch even though I'm not participating because cool ideas happen in many places.
I cannot attach two of the NACA reports L-355 and L-380 which form a large part of the technical information used in the development of the offset hinge flap idea because they are too large. It's actually not new in any way. It simply hasn't been applied to a CLPA airplane before to my knowledge. Or if it has been it was discounted. PM me with your email address and I'll send you the reports.
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Mark,
I am looking forward to your progress reports as you work on your new project.
I viewed with interest your illustration of your wing/flap arrangement in your 11:41 post just now. It appears that the hinge line on that flap is well behind the LE of that flap. This results in a significant "bump" on the airfoil at the flap LE when it deflects from neutral. I have experience with only a slight "bump" with flap deflection and for me, it causes significant reduction in turn performance. I allluded to this in a previous communication with you.
This is related to the physical airfoil tests Al Rabe did with a number of airfoils on the hood of his car years ago where he took my airfoil and intentionally positioned the hinge so that the LE of the flap would rise above the TE of the wing at the flap hinge line. He measured that the airfoil with the deflected flap did not have the lift performance of a more "normal" stunt airfoil/flap arrangement. He did this so that the deflected flap would tend to "seal" the gap at the flap hinge line. In my humble opinion, the thought of sealing the gap is good. However that "bump" further disrupts the airflow which is probably already turbulent at that position of the airfoil and cause it to separate before the airflow passes over the rest of the flap, causing a significant reduction in the effectiveness of the deflected flap. In my experience, I have demonstrated proof that such a bump causes a significant reduction in the performance of the deflected flaps.
If you want me to go into detail on this, I will be happy to explain.
Keith Trostle
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Thanks Keith
There is an answer to that and it lies within the expected operating range and the curvature of the parent airfoil. The picture posted has the flap in close to the maximum deflection. However that isn't where the expected operating range will be. The bump is an issue but can be mitigated to a degree by contour. There are several NACA reports dealing with different nose profiles of the flap design which project in to the airstream. Actually it is on control surfaces but that pretty much what the flap is. In some recent modern aerobatic airplanes "the bump" is a feature that is tuned to help create suction on the forward portion of the offset hinge flap. The flap (aileron) is actually made thicker than the wing and "the bump" is present even when the flap is in trailing neutral. This helps reduce the hinge moment. Look up the Extreme Decathlon which is actually very noticeable.
When we consider the aircraft operating conditions the design parameters can be determined. For the CLPA airplane making tight 6-10 G maneuvers, the design operating point is between Cl =1.3-1.5 which is less than the Cl max even on the current NACA 0020 sections. To the individual in Cl, it seems, all are focused upon Cl Max which really infrequently occurs in operation. That's one of the reasons I did the AOA testing to figure out where that airfoil is actually operating. Turns out that in the squares to be somewhere around AOA of 7 degrees and Cl of 1.3 ish. I think I posted a NACA just like that condition. It’s the OH8’s which reach the highest AOA angles.
So the design point definition has to be understood. I'll talk about that some in my next post. It begins with an analysis of the aircraft trajectory. If you watch a combat plane or non flapped airplane do the pattern, which many are capable of including my profile non flapped prototype of this airplane, they don't present well especially in the exits of the squares. This is due to the need to achieve the 1.3 Cl point which a non flapped airplane has to pitch inward of the path tangent by 15-17 degrees. A flapped model using a 20% flap deflected 25 degrees needs 4-7 degrees AOA. Math model and flight test substantiated. This means a flapped model has a much smaller pitch movement to hit the flight path trajectory on the exit, therefore presenting better.
I find that I, as the meat servo feedback, tend to oscillate a slight bit with that. Pilot Induced Oscillation, PIO. More experienced meat servos find it much more rapidly. I'll likely never be that guy. I'll get close and then a squirrel will wander by and I'm off. Like I said, I like the technical challenge. So from this I derive a design requirement. In order to make an improvement to the maneuvering presentation of the PA model, the combination of flaps and pitch rate need to be able to generate the required Cl of 1.3-1.5 with zero AOA in order to maintain fuselage attitude to the flight path tangent.
The 20% flap on the NACA 0020 and relatives simply won’t do that. A 25% flap might marginally do it and the 40% flap will do it readily. The reason why these airfoil flap combinations won’t do it is because the flap has to be deflected in excess of 25 degrees which promotes separation and must be pitched beyond 0 AOA up to around 7 AOA. Around 30 % flap ratio the angle of deflection reduces and a 35% flap will achieve the 0 AOA Cl of 1.3 with about 16 degrees flap deflection. Reset the flap to this position and the bump isn’t as pronounced.
Now if I were to take the standard approach to this maneuvering requirement and look simply at the flapped airfoil, do the standard AOA sweep analysis, I would find that the Cl Max is lower somewhat. My question to you is why is Cl Max important when it isn’t an operating condition? Yes we could get there but not under normal operating parameters.
Interestingly for some of these sections doing the AOA sweep I would find that the Cl curve is non linear and at times the Cl decreases with reduced AOA as a result of separation occurring on the underside of the airfoil. From that I pose a similar question, does that mean we shouldn’t use that airfoil – flap combination on our airplanes? The answer better be no because some of the sections are in use and do in fact behave that way but, due to the operating zone in use, that range is never used.
We tend to be focused upon Cl to the point of obsession and disregard the drag portion which steals our precious energy. If I go back to the operating point of Cl 1.3 and I compare the larger lower angle deflected flap to the smaller higher angle flap the drag is significantly less with the larger flap. Granted there is some unknown as a result of the “bump” which likely reduces some of that benefit but the bump has other benefits.
There is significant drawback to using a large flap which is the resulting hinge moment. There’s lots of ways of evaluating this but typically we use a pressure distribution which is rectangular then tapering from the hinge to the TE and then integrate the moments about the hinge line. The short hand way of doing this is assume all of the flap lift acts through the 25% MAC of the flap. Not perfect but this 10 man aerodynamic engineering team either. Moving the hinge line from the extreme LE to the 25% would nearly remove all of the hinge moment. This is what my spade testing was about as spades do the same thing pretty much and they cause some interesting commentary.
There is a trend in some modern aerobatic airplanes using the offset hinge to actually increase the thickness of the flap behind the wing cutout resulting in the flap being thicker than the wing in this region. The idea of this is that the airflow velocity increase will reduce the pressure on the projecting portion and help lift the surface. This in turn helps reduce the hinge moment. In the NACA reports, they wind tunnel tested lots of different nose contours and many help quite a bit. The elliptical curve I have drawn is a place holder kinda. This wing and airplane are being built to be readily modified and different flap configurations can be readily substituted including moving the hinge point.
If we look to the NACA reports, L-301, L-355 and L-380 an interesting thing pops out when they evaluated the shape of the flap nose overhang. This result from L-355; “The lift effectiveness of the aerodynamically balanced flap was increased slightly over that of a plain flap when a blunt or medium flap nose was used on the balanced flap”, the medium nose referring to a contour similar to a symmetrical airfoil section. This is partially why I chose the elliptical profile of the flap. The report further concludes; “The medium nose on the flap gave the highest values of lift at positive angles of attack and flap deflection with the larger gap tested”.
So, there’s set of compromises taking place in all of this and it is true that sometimes the “bump” will cause a reduction in Cl max. This, I hope, has been mitigated by limiting the deflection to a region where it isn’t an issue. The Log crank will functionally end up limiting the flap deflection to around 20 degrees. The luffing bar bellcrank should help dampen the PIO.
Along my road, I ran various sections through my airfoil modeling software’s. I primarily use two, Visual foil which works well for mass crunching and Java foil which can handle sharp curvature variations. The airfoil tools use a polynomial curve fit and visual foil tends to crash when I model a thin flap airfoil.
I evaluated the thick parent sections like what Al’s airfoils are, albeit without the super thin flap. And they work fairly well but not as well as my NACE or the Eppler section I plan on using. The reason they don’t perform as well is likely why the “bump” was seen to problematic. These sections tend to have a long thick zone stretching back quite far in to the profile. The result of this is that here becomes a rather steep adverse pressure gradient in the last portion of the airfoil. The boundary layer encounters a steep rise in pressure. If a perturbation occurs in this zone, separation is likely to occur resulting in a lowering of Cl max. Both the NACE and Eppler have shallower pressure gradients in this zone and consequently should be able to weather the perturbation there.
Keep in mind that model based analysis is just that and requires verification. My experience with these models is fairly positive. If I had to place a confidence level on the tools I would use 80%-85%. I’ve had results which plain didn’t work especially in the lower Rn ranges, lower than where we operate.
Topic: New Build - Frankenlaser CLPA (Read 2947 times)
