9 Nov 11
Finally finished the flat plate Speedy Tips to the point I could test fly with them. I've been working on them off and on for several months and I've been anxious to test fly them to see if they actually result in increased speed as reported by others.
I'm sad to report they do not improve speed. I noticed (maybe) 1/2 knot speed increase. So, the flat plate wingtips are not recommended to increase speed.
One interesting thing I noticed (and I'm going to do more test flights to verify) was that the stall characteristics are significantly changed. The airplane does not stall - instead it acts like a canard-equipped airplane and as it reaches the stall it buffets moderately (normally it is a sharp break with little or no warning) and bobbles the nose up and down as it approaches a stall and then recovers. I'm assuming the fences at the wingtips is contributing to the new stall characteristics. The clean stall speed is 61 vs 59 for the normal straight Horner tips.
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Left - It's easy to see how much the end
plate (I call it a tip fence) extends beyond the wing shape. There
is nothing firm about the dimensions and I may change them as I go
along. The top edge of the upper fence is aligned with the relative
wind in level flight - thus no added frontal area. The higher fence
toward the rear was also discovered in earlier tufted flights with the
Hoerner tips to help control the vortex generation.
Right - The attachment hinges for the aileron and flap are exposed below the wing and are quite draggy, so I incorporated a fairing for the outboard hinges. |
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Left - The aero info I've gathered indicate a
slanted trailing edge creates less drag so I applied that idea to the
fence trailing edge I was worried that the fence was too thin - and
thus flimsy - but it turned out to be fine since it is perfectly aligned
with the relative wind. I tried some rudder deflections to see if it
was affected but it appeared to not be. Later I'll spin test it as
well.
Right - The frontal area was significantly reduced. The Speedy Tips extend out only 1.5 inches from the edge of the wing skin. |
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Left - An inboard view of the tip
fence. The entire wingtip will get another layer of glass as well as
filling and sanding and primer.
Right - Cockpit view of the tip. |
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14 Nov 11
More test flying and I'm still stumped about the new stall characteristics. It absolutely stalls like a canard-type aircraft. With idle power and about 10 degrees nose high heavy buffet begins about 65 KIAS. With the stick held full aft the nose bobs up and down at the horizon and continues buffeting as a 1200-1400 fpm sink rate develops. There is no tendency to drop a wing and ailerons are responsive throughout.
Click here for a video showing the stall characteristics.
I added tufts to the left wingtip at 0.8 inches inboard from the fence and at 3.5 inches inboard. The tufts that were 3.5 inches inboard laid flat until approaching the aileron, whereas the tufts that were 0.8 inch from the tip fence are drawn upward beginning at about 50% chord. Initially the tufts are drawn upward slightly, but by the trailing edge they are standing up at a 45 degree angle relative to the wing surface. It's interesting to note that even though the tufts were long enough to extend above the fence, they did not go higher than the edge of the fence for its entire length.
My assumption is that the upward air movement is due to skin friction along the tip fence. One reason for that assumption is that the tufts are drawn upward to a greater degree as the height of the fence above the wing surface increases.
I'm still stumped as to why I'm not getting the 4 knot top speed gain reported by other experimenters.
According to Fluid-Dynamic Drag by Dr. S. F. Hoerner the Cd on the original Van's Hoerner-type tips is .002 while the Cd on flat (no fence) tips is .015. While there is a theoretical drag increase, it should not be sufficient to offset the reduced parasite drag from the reduced frontal area. It seems to me that the tip fence, both top and bottom, should help control the formation of the wingtip vortex thereby reducing the drag coefficient of my tips.
According to Dr. Hoerner, "In low-aspect ratio wings, the lateral (wingtip) edges have an important influence upon lift and drag." He goes on to say that on wings where the vortex begins to form closer to the leading edge rather than closer to the trailing edge indicate a larger effective span than otherwise shown. Further, he says that wings with sharp lateral edges produce the widest effective span while those with rounded edges result in a loss of effective span or aspect ratio. And in his book, Fluid-Dynamic Lift, he discusses the effect of end plates on a low aspect ratio wing. He states that adding end plates to a low aspect ratio wing results in lift graphs that are equivalent to the graph of a high aspect ratio wing. Thus, the induced drag one would expect on a low aspect ratio wing does not occur when end plates are added.
For my RV-8A, the aspect ratio of the wing is about 1.88 so adding end plates to the wings should make it perform more like a high aspect wing without the normal parasite drag that accompanies a high aspect wing.