B

Bob Axsom

Well Known Member
I have done just about all the drag reduction I can on our RV-6A. I am planning to fill all the control surface end air pockets to see if there is any speed to be picked up there. There are many standard ways to go this but I have a strong aversion to messing up a pretty paint job if it is unnecessary so the suitable method decision has been rolling around in my head for months. I've decided to make plugs of balsa wood and mount them with flat head stainless steel sheet metal screws through the plugs and into the end ribs on the ailerons, flaps and elevator (no rudder pocket). I ordered 9 feet of 2"x4" balsa from Specialized Balsa Wood, LLC (www.specializedbalsa.com) for $47.88 including shipping. I'll let you know the results when the mods are done and tested - good or bad.

Bob Axsom
 
Sheet metal screws???

What keeps them "locked" in place? If one screw, or the entire plug, should come loose, will it jam and prevent movement of the control surface???

I guess I am not visualizing properly what you are describing...
 
I agree, Bob

....Also glue/bond them to the ribs for extra safety. I've also seen gaps as small as 1/16" since none of the control surfaces can move sideways and interfere.

Good luck,
 
Once my plane is flying, I was thinking of doing the same thing. My approach was with foam inside those areas held down by velcro on the inside and tape on the outside.
 
Good Feedback

If I were smart enough to do this before I painted I would have used blue foam and fiberglass but after unbelievable amount of thought this is the best way to go in my opinion. I can't imagine a sheetmetal screw backing out but the glue couldn't hurt as long as I'm careful with it. The friction in the balsa wood will be similar in anti-spinout function to the elastic AN365 nuts. The friction in the interface between the screws and the aluminum of the rib and the screw (given proper hole size) should be enough to prevent backing out as well but belt and suspenders will work too. If the mod is ineffective in gaining speed I can cut out the balsa. By the way, the baseline maximum speed in the current configuration at 6,000 ft density altitude flown per the www.us-airrace.org handicap procedure and the three leg products entered into the NTPS spreadsheet is 182.6 kts. 183 kts would be a fine improvement. My major gains so far were achieved by cooling drag mods, wingspan reduction and wheel subfairings.

Bob Axsom
 
Couldn't you skip the balsa and use some tape over the pockets just for testing purposes? If it gives a real speed increase, work up a more permanent solution.
 
szicree said:
Couldn't you skip the balsa and use some tape over the pockets just for testing purposes? If it gives a real speed increase, work up a more permanent solution.
Click to expand...

I was thinking the same thing on a cross-country this morning. My 200 mph duct tape holding a wedge on my rudder, has held up well.

L.Adamson --- RV6A
 
I've had some recent bad experience with tape

Tape may be a workable test method for the fill the pocket concept but I don't think there is any risk in what I'm doing and It's just my way. If it achieves an increase in speed I want the work to be as close to being complete as I can make it. As I said earlier I have been thinking out the details and options for months and I can't motivate myself anymore with baby steps in the development/implementation phase. My plan is to round out the ends of the control surfaces as well as fill them in. I plan to reduces the gaps as well as fill the pockets. The untreated sharp edges I suspect generate transition disturbance as well the cavities create air entrapment drag. Depending on what I run into on the inboard edge of the elevator, I may actually bring that out to a point

I have had bad experience with tape recently at high speed though I recognize this particular application should be safe if it is applied carefully leaving no edges to catch air.

(Quoted from AirCap 200 Race Memories at www.sportairrace.org) After the race was over I looked at the tape on the aft fuselage. A corner was lifted on the left side so I removed all of the tape there and checked the right side. It was far worse with all three pieces clinging at the front end but resembling a tangled and knotted mop at the rear end. Now I know there is a lot of air action in this location and I will work on a permanent fix for closing the holes.

Bob Axsom
 
Good hard data - Thanks a lot

pierre smith said:
....Also glue/bond them to the ribs for extra safety. I've also seen gaps as small as 1/16" since none of the control surfaces can move sideways and interfere.

Good luck,
Click to expand...

I really appreciate that actual observation. Opinions are fine but actual data is bith rare and valuable. I think I will go for 1/16" gaps as well since you have seen that dimension used safely. Maybe later I can do a centerline groove with my router and put a wiper in there to completely close the gap with controls centered. Maybe an actual segment of a windshield wiper would work. I'm getting ahead of myself.

First I'm going to make the pocket plugs and take them out to 1/16" gap. Going to Kansas City this weekend for a birthday celebration (mine) then I have to get back to cutting wood for the fireplace this winter and the Rocket 100 air race at Taylor Texas is on the 22nd of November. S-o-o-o-o it will probably be December before I start cutting Balsa. It probably would be a good idea to do the ailerons first and test the result before doing the elevators.

Bob Axsom
 
My take from my builders research point of veiw has been that it doesn't do much for speed but does a lot for looks. For my money those pockets are getting glassed in from the get go.
 
It will have to be sealed

The plugs I make will be sealed to keep moisture out and match the appearance of the airplane. I will decide how to do that during the implementation. The first tasks will be to fit the plugs and attach them in a removable manner then take them off and finish them.

Note: This is not the material and process I would use if I didn't have a several thousand dollar paint job on the plane that I want to preserve.

Bob Axsom
 
You must log in or register to reply here.