About fixed flaps

A serious question about fixed flaps. Which is better, a wing that is full airfoiled, or one which is barndoor with fixed flap of the same overall area? Example a 10 inch root, with 8 inch tip that has full ribs all the way, or an 8 inch barndoor with two inch root fixed flap? My experience with my best models of all time says, the fixed flap flys better. Aerodynamically speaking, it would seem that a full airfoil would have better lift and flying ability. What are the differences and advantages of each. Which is the best design idea?

Hi Shel

I don't think you can compare these 2 ideas fairly as there are so many variables.

If you are going to make some kind of comparison, then at least the
average thickness, nose radius and thick point would have to be the
same on both wings inclusive of the flaps. This would ensure at least
a similar lift and trim range. If that were the case, the sections
themselves would be differently shaped with the flapped version having a tighter curve aft of the thick point. That might
promote separation in that area with possable reattachment at the
flap. If reattachment does occur, performance is not too greatly effected however this sort of thing promotes a phenomenon known a hysteresis loop. That is, starting at a low incidence, gradual increases in incidence a separation bubble begins to form.
Increasing incidence causes the bubble to grow toward the the TE until
the boundary layer detaches altogether (STALL). Decreasing incidence will not imediatly reattach the boundary layer. Incidence must be decreased to a point near that at which the separation bubble first formed. The result is a range of angles of attack in which 2
distinctly different performance values can be produced both in lift and drag. That can make control less predictable, considered undesirable for stunt. However, depending upon the curve of the airfoil, with a forward thick point, this may not present a significant problem.

Histeresies tend to be more an issue with very high lift airfoils,
usually cambered where the surface curvature is tight behind the point of maximum flow velocity. The usual practice of adding fixed
flap to an existing airfoil tends to preclude this but it is not
fair to compare such a wing to another of the same section with no
flap because the two wings are designed for different opperational lift ranges.

There is also the question of what is meant by better. Performance parameters may be different, some greater, some less. You improve some at the expense of others simply ending up with a different compromise. If you prefere that compromise to another, you call it
better. So we need to understand which performance parameters you want to improves and which you are prepared to suffer loss.

LAST EDITED ON Jul-20-04 AT 09:09 AM (CDT)
 
There are several threads that include discussion of fixed flaps. Try searching with key words "flapless" or "fixed flap".

I cannot look up my info right now, but I know John Miller has found to his satisfaction that fixed flaps are an advantage in certain applications. I don't know about making a generalization concerning "barn door" applications, but if he chimes in here, I think he'll advocate the "Flight Streak" set-up. I can't speak for him though. However this I do remember:

1) Al Rabe found that thin deflected flaps out performed deflected flaps that were formed from the trailing edge of a full airfoil section.

2) NACA data in limited mentions (ca. 1930-37?) indicate that the thin flaps undeflected do outperform standard configurations. I don't remember what Reynolds numbers were mentioned.

3) X-Foil applications in Profili at our reynolds numbers indicated that for a section I designed, attachment of a fixed flap of about 18% (total) chord outperformed the same section without the fixed flap, but with chord equal to the total (airfoil + flap) chord of the first section.

I don't know that X-Foil is reliable at our low RN's of <500,000, but comparitively, it supports the fixed flap.

If you take time to do the search, you may find that I posted some information and sources on this.

SK

Edit: I did a search on all my posts, and found that most of them were missing. I could only access about a year's worth. However, useful info can be found here:

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=7765&forum=DCForumID1

I did a search of this forum from current to back one year as well as archives using flaps as the subject. I pulled up an amazing amount of info. I have long wondered the same question and was able to get it answered completely with this search. Check it out, you will be amazed. Aloha!