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Bob Axsom

Well Known Member
I've bee noodling and actually working on a mod to plug the recesses in the outboard end of the elevator and stabilizer for a while. I finished and test flew it today and the results indicate a 1.8 kt gain in TAS. The top speed is now up to 184.4 kts. Same test method as all previous tests. The pockets are filled with balsa wood plugs held inplace with one 1" #8 flathead sheetmetal screw and a dimple washer.

Bob Axsom

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Bob... did you...

...plug the horizontal stab. side to make it flush, or just block off the lightening holes?

A simple mod for a reasonable result....:)
 
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To make it flush

My control surfaces have no lightening holes at this location. I bought three 2"x4"s of balsa wood to use for this work (this is just the beginning but maybe the most effective part of a planned pocket filling effort). I cut off a 3/4" slice to make the four plugs used to fill the pockets at the outboard ends of the horizontal stabilizer and elevator. Once I got the shape right and notches and holes to accommodate the rivets and flutes, I used #60 sand paper to thin each plug to exactly fill the depth of it's pocket. I drilled a centrally located 1/8" hole in the plugs then used the plugs as a drill guide for the 1/8" mounting hole in the mating rib. 1/8" is the correct pilot hole size for the #8 sheet metal screws. The 1" flathead sheet metal screws and dimple washers were bought from Aircraft Spruce. The washers are required because the screws head will crush right through the light balsa wood. I painted all surfaces of the plugs with two coats of polyurethane. I lightly sanded the exposed surface and installed each plug then painted them with two coats of color (poorly and with a brush as you can see) before installing the screws and washers for the final assembly.

Bob Axsom
 
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A great return on investment!

Really a lot for pennies...and verified too. FWIW, Bob, proseal will hold them in place forever...just hold them in with rubber bands overnight.

Regards,
 
PUR-foam instead of balsa wood?

If there is no lighthening holes, why not just fill the area with PUR-foam (from a can) , cut it flusch after it cured and put two or three layers of glass over the foam before painting? I think I will do that before the plane goes to the painter.

Now that you have seen such an improvement, are your also going to do the ailerons, flaps, etc?

Regards, Tonny.
 
Be carefull that that sheet metal screw does not get loose and cause that thing to jam your elevator...
 
Exactly right

Pilottonny said:
If there is no lighthening holes, why not just fill the area with PUR-foam (from a can) , cut it flusch after it cured and put two or three layers of glass over the foam before painting? I think I will do that before the plane goes to the painter.

Now that you have seen such an improvement, are your also going to do the ailerons, flaps, etc?

Regards, Tonny.
Click to expand...

Unfortunately I wasn't into this kind of mod back in the building stage. That is the better approach for sure.

Yes I am going to do the same thing with the ailerons and flaps but I do not expect the same magnitude of improvement. If you look at the aero/static balance horn on the elevator and consider the flow path when it is not in perfect alignment with the stabilizer I think you will see why I think that. I did something similar on the 3" racing wing tips I developed for my RV-6A. In that case I made a closure web/stiffener of 0.016 2024T3 aluminum sheet with the mounting flanges turned into the wing tip and installed it with pop rivets. This gave me a flat wall opposite the end of the aileron rather than the pocket that would result from a more traditional solid rivet installation with the mounting flanges turned toward the wing/aileron. I have a speed gain from the wing tips but I can't allocate increments to each feature - in other words I don't know how much this closure/stiffener design helped the speed.

Bob Axsom
 
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You are right

Brantel said:
Be carefull that that sheet metal screw does not get loose and cause that thing to jam your elevator...
Click to expand...

In my case it is the correct size pilot hole for the aircraft sheetmetal screw and the balsa wood has an elastic function to resist loosening as well but it could be a problem if not done properly.

Bob Axsom
 
Bob Axsom said:
In my case it is the correct size pilot hole for the aircraft sheetmetal screw and the balsa wood has an elastic function to resist loosening as well but it could be a problem if not done properly.
Click to expand...

(another gentle nudge).....perhaps "no positive retention" = "not done properly" in the case of a potential control jam?

Sheet metal screws have a long history of littering the landscape. No point in adding 1100 lbs of aluminum.
 
Aileron and Flap Pocket Plugs Installed

I followed the method used in the tail and have now plugged both ends of the ailerons and the outboard end of the flaps. After one more coat of blue on the plugs I will use some non-permanent loctite on my sheet metal screws and the mod will be done. I will not be able to test for speed because I am converting from the race configuration to the travel configuration for a California trip which will not be completed until early June. The pressure to convert back to race configuration for the Courtland Alabama race and speed trial on June 13 will be extreme. Hopefully I can work a test in before another mod is incorporated.

Bob Axsom
 
Yes I did

The test results for the aileron and flap plugs showed a decrease in speed. I have rationalized why this is the case but the bottom line is when I removed them the speed came back. My rationalization is that the installation is similar to holding a bottomless cup out the window of a moving car then closing the bottom and seeing an increase in pressure. I am convinced that there is drag at these locations but to reduce it you have to close off the entry port with a streamlined fairing and follow that with an after fairing behind each installation. I'm going over that in my mind. I think angle brackets with platenuts on the aileron supports will enable a good strong installation as a starting point.

Bob Axsom
 
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