Design > Engineering board

Twisted wing panel with correction tab - loss of lift?

(1/3) > >>

Dennis Toth:
I acquired a ship that had the outboard wing panel with a built-in twist up. I added a tab/wedge under the outboard wing to level the wing. This worked but the ship seemed to be doing rather large loops (65 - 70deg ish). I was wondering if the area with the tab/wedge was just coming along for the ride and not contributing to carrying any weight through the maneuvers?

Best,   DennisT

Dave Hull:
Here's my simple take:

--The drag goes up due to the inefficient airfoil; and
--The maximum available lift goes down due to the inefficient airfoil and the spanwise elements fighting each other. In other words, the zero lift line is not constant spanwise.

I believe a simple way to visualize this is to take sections thru the wing every few inches. Forget what it is supposed to look like, just draw how it is, and then align the sections with the waterline of the plane at zero lift. At small angles, some of the sections with the warp will not be in a favorable position to provide any lift. Depending on how it is warped, this may be upright or inverted. To really visualize this, you need a good diagram of an asymmetrical wing section that shows the chord line and also the zero lift line. The AOA should be measured from the zero lift line. A few years back, there was a diagram in Model Aviation that accompanied an article that was trying to explain this--but it didn't have enough detail to get much right. In other words, good enough to explain to your 10 year old as long as you didn't expect him to understand because the theory was missing. I suspect that you'd find the best diagram and explanation in Abbott and Von Doenhoff, The Theory of Wing Sections, but I don't have it in front of me right now.

In your example, I would expect that inside loops would be softer for sure, and probably outsides as well. You didn't say how much of the span was affected by the warp, and if you placed the wedge over the entire affected area. I would have tried that approach--after I gave up trying to straighten the wing.

Wing twist may be included on full size aircraft for stability. It's how a plank flying wing achieves pitch stability beyond using a reflexed airfoil that attempts to limit the movement of the center of pressure with angle of attack.  It is also common to use washout in the wingtip to keep the tip from stalling first and creating snaproll problems. (They also may use a turbulator on the inboard section to force it to stall first for the same reason.)

The Divot

Howard Rush:
I was wondering how to visualize the effect of a trim tab on a warped wing.  I think one could come pretty close by doing something similar to the stuff I posted here on wing asymmetry awhile back.  It would reduce max lift, but the most annoying effect would be the wandering around of the spanwise center of pressure with angle of attack.  At best, you’d always have the wrong amount of tip weight.

This is an example of the engineering principle that other people’s disciplines are linear.   Hence EEs put trim tabs on their stunt planes and Bob Carver and Norm McFadden rolled their eyes at my assertion that op amps are perfect.

As for reflexed airfoils stabilizing flying wings, yes, I’d always read that, including in textbooks, but it’s based on an unnecessarily ambiguous definition of stability.  A reflexed airfoil gives a positive Cm, allowing a flying wing to be trimmed at a forward CG, but it don’t do @#$% for dCm/d alpha.

Dave Hull:
"This is an example of the engineering principle that other people’s disciplines are linear."

Boy, that one really hit the mark. One of my favorites was when an EE tried to explain to me (a mechanical engineer) how ball bearings work because he was worried that I didn't appreciate proper sizing and selection, etc. He wasn't really sure what the difference between a shield and a seal was. He was an excellent high voltage power supply designer, though....

Dave. you can make a trim tab work but it will have two effects that are "unbecoming" to a good flying stunt ship.

First, it will be speed sensitive. That means it's effect will change throughout a manuever.

Second, it will change the neutral point for the control forces. For example: if you put a downward tab on either flap, as the lines go slack the elevator will want to move to the down position. This will manifest itself as a tracking issue on a windy day when you may have light line tension in certain maneuvers.



[0] Message Index

[#] Next page

Go to full version