Airplane stability (engineering notes)

[davidchoate]

David, You never answered My question on biplanes. What effect does the top wing being at a slightly higher angle have. Is it beneficial on a powered aircraft?

[Steve Blanchard]

For rubber powered or free flight models it is normal to set one of the two wings on a biplane to be 1/2 to 1 degree higher. Some prefer the bottom and some the top. What this does is allow one of the wings to stall first and the other to recover. This adds to stability. As far as RC I'm not sure if the benefit is necessary as I do not fly RC.

Steve

[davidchoate]

Thanx

[David Lewis]

The difference in incidence between biplane wings is called decalage. (There is longitudinal (wing and stab) and interplane decalage. We're discussing interplane decalage here.) Decalage could improve stall behavior when the wings are staggered.

In the case of positive stagger, the incidence of the top wing is greater than incidence of the bottom wing. The intention is for the top wing to stall first, and since the top wing is further forward, a stall creates a nose down pitching moment. With small model airplanes, however, stall behavior tends to be gentle and mild so interplane decalage is, in my opinion, not worth the decrease in maximum lift coefficient it imposes.

[davidchoate]

Some very basic cheks

[davidchoate]

If You decide to add ailerons to a Plan that was designed for rudder and elev. only. Decrease the dihedral to only 1 or 2 deg. I crashed a 30" Aeronca last summer that I modified. Now I know why. The dihedral lessens the effectiveness of the ailerons, and makes worse the drag effect on the wing with the down aileron. The more You try and compensate the worse the problem becomes. Until You spin into the ground. I wish I had found out about that before. I have been studying a kot on this stability subject, and it has had a good effect on My piloting skills.

[davidchoate]

One more question about bipes. Is the wing stagger (top ahead of bottom) for stability, or strength reasons?

[dirk gently]

Stagger is not for strength, it actually affects structural rigidity in a negative way.
It's also doesn't affect stability in a direct way (however, there is an effect that the when the top wing in a plane with positive stagger is at higher angle of incidence than the lower one, it stalls first, which moves the center of lift aft, causing the nose of the plane to be pushed down and thus helps to recover from the stall. With negative stagger the bottom wing should have higher incidence to achieve the same effect, of course).

The main reason for stagger is to increase lift, see the last five post in this thread for details: viewtopic.php?f=2&t=3457

[davidchoate]

David, I have purchased a 48" high wing short kit. It has a asymmetrical airfoil, but the tail also is of a similar airfoil. Does this affect the wing area and wing loading? Also the plans call for a 2.5 deg incidence on wing and 1.5deg on stab. What for? Also (sorry for the 3 part question) What are the benefits or disadvantages of a flat bottom airfoil? Thanx

[dirk gently]

For standard layout the lift from the stab (whether it's positive or negative) is negligible as far as wing loading is concerned *)
The difference in incidence of the stab and wings is called decalage. For longtitudal stability you want to wing to stall before the stab.
What are the benefits or disadvantages of a flat bottom airfoil? Very difficult to answer in general terms, without discussing some specific airfoil. For me the biggest advantage of a flat-bottom airfoil is that the wing tends to be easier to build. Symmetrical airfoils are good for inverted flying



*) not entirely true, in some rubber powered classes with wingspan limits the contestants tended to use larger than scale lifting stabs to increase total lift

[David Lewis]

dirk gently wrote: "For standard layout the lift from the stab (whether it's positive or negative) is negligible as far as wing loading is concerned *)"

David Lewis wrote: That may be, but according to my experiments lift from the stab is significant as far as performance is concerned. And, as a side benefit, with a lifting stab, pitch trim is relatively invariant with respect to air speed.

One advantage of symmetrical or semi-symmetrical airfoils is that they make vertical manuevers, such as loops, easier and more graceful. You rarely see them on free flight models unless it's for scale fidelity purposes.

[davidchoate]

OK. Next Question. And thank You Guys For answering My Rudamentary needs. But It's not enough for Me to just follow a plan and assemble a Plane that Flys,but I want to know right down to the molecular level WHY? It is flying. OK so I got the whole wing loading, stall speed, and prop basics, but what exactly is " CUBE LOADING"), and what does it have to do with My building.

[David Lewis]

Cube loading is a performance parameter that lets you compare wing loading of different size airplanes on an equal basis.

As an airplane gets bigger, it's able to carry with equal ease a higher wing loading than a smaller plane can. That means you will get more bang for your buck by increasing wing area than reducing weight. Of course you want to do both, but you will find returns diminishing more quickly with weight reduction than with area increase.

The value of cube loading is found by dividing weight by "wing area raised to the 1.5 power" (= w/S^1.5).

[MMitch2782]

In the past I built Guillows scale Skyraider and Spitfire and did not consider weight because I wanted scale but now I am interested in Free Flight rubber. I'm amazed by some YouTube videos of FF rubber a/c that have been trimmed out perfectly and stay airborne for minutes! Beautiful. It looks to me that builders build them much lighter than a stock Guillows. Where do I go to learn this?

[tom arnold]

Two web sites are devoted to these types of free flight scale models:
http://www.hippocketaeronautics.com/hpa_forum/index.php
http://www.stickandtissue.com/cgi-bin/yabb2/YaBB.pl

I'd also recommend getting the Flying Aces Club Newsletter and that website is:
http://flyingacesclub.com/