Tried a Bob Axom Speed Mod

Thanks, Bob.

I think Bob came up with the best cost-benefit ratio in speed mods ever. I taped the gaps in the elevator leading edges which provide access to the attach bolts, four in all, top and bottom. I used HVAC sliver tape, a very thin and very sticky tape. I also covered the holes in the underside of the wing where the tie downs go. Cost what - ten cents at most?

The conditions for testing were not ideal, but at a minimum I gained 2-3 knots of TAS tested at a density altitude of 3000'. When we get a break in the Michigan WX I'll try to do a better test, but this is 2-way GPS verified. 177 KTAS versus previous best of 174 or 175 using a 3-way GPS test at DA of 8000 or more. It might have gone a little further, but I ran into my upper limit that I programmed into the autopilot! It kept cutting out with a warning and the air was too unstable for me to hold it well in level flight for this purpose.

The RPM was pretty much the 2650 I always get a full throttle.

Now I'm trying to think of a way to get this improvement with something more permanent and more attractive than silver tape.

What about...

Thanks for posting. I'll try it too.

Has anyone covered the openings on the rudder the same way?
Guess that schould give a few KIAS too?

Yes cover the rudder openings too

I actually copied the idea from John Huft. When we were in Dayton in 2005 on the day before that year's AirVenture Cup race to Oshkosh. I photographed his RV-8 taped control surface attach point openings but I didn't actually try it until last year. Like you, I was surprised at the extent of improvement. I expected a fraction of a knot but as I recall it was more than a knot. In this forum it was suggested that I should use Gaffer's tape and that is what I used. It worked well and did not leave a mess when I had to remove it for the annual condition inspection last month. I have watched Tom Moore tape his RV-7A for racing and he uses some thin and narrow light blue colored tape that he puts over every exposed seam - including the cowl joints and the ends of all of the fairings (not control surface gaps) - if he could manage it he would probably tape his canopy joints but I haven't seen him do that yet. Tom's plane is pretty fast but I never asked if he made a comparative test to evaluate the difference between the basic airplane and the taped airplane and I have not tried it. Back to the attachment opening closure methods - some people have attached extensions to the skin forward of the openings to shield the holes. I have experimented with this a little bit without success. I have read in this forum that some have tried gap sealing the ailerons with very bad control response as a result. You have to be really careful to not change the aerodynamic function - just close holes. Another experience I had trying to cover the hole in the fuselage around the rod connecting the left to right elevator was unsuccessful - in the race where I tried this the wind pulled the rear portion of the tape loose and turned it into a tangled flailing mass. Tape doesn't work everywhere, it takes careful thought and application to get it right and it can cause trouble. In other words there are no sure things and if you try this, it is at your own risk.

Bob Axsom

I can imagine quite well what you been doing but couple of pictures wouldn't be bad (again)... definitely something to try when getting that far.

W/O seeing a picture, I assume that you are still leaving a gap wide enough for the end rod to pass through during up/down elevator movement.

Instead of taping just the holes, couldn't you put some length of gap material that bridges the gap between the HS and Elevator along the length?? Same could go for the ailerons.

(Gap just below the letter A)

hevansrv7a said:

Thanks, Bob.

I think Bob came up with the best cost-benefit ratio in speed mods ever. I taped the gaps in the elevator leading edges which provide access to the attach bolts, four in all, top and bottom. I used HVAC sliver tape, a very thin and very sticky tape. I also covered the holes in the underside of the wing where the tie downs go. Cost what - ten cents at most?

The conditions for testing were not ideal, but at a minimum I gained 2-3 knots of TAS tested at a density altitude of 3000'. When we get a break in the Michigan WX I'll try to do a better test, but this is 2-way GPS verified. 177 KTAS versus previous best of 174 or 175 using a 3-way GPS test at DA of 8000 or more. It might have gone a little further, but I ran into my upper limit that I programmed into the autopilot! It kept cutting out with a warning and the air was too unstable for me to hold it well in level flight for this purpose.

The RPM was pretty much the 2650 I always get a full throttle.

Now I'm trying to think of a way to get this improvement with something more permanent and more attractive than silver tape.

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