Test of end plugs - Ailerons & Flaps

[Bob Axsom]

I spent 14.5 hours developing and installing plugs in the outboard end of the flaps and both ends of the ailerons in an attempt to get more speed out of the RV-6A. Once again the anticipated gain in speed didn't happen! The speed turned out to be 181.4 kts TAS using the same test method as I have used for several years (triangular course, 6000 ft density altitude per www.us-airrace.org handicap procedure and results plugged into NTPS spreadsheet).

Not only did it not produce a speed gain it affected the roll control significantly. It flew like a 2.5 ton 6x6 without power steering. Roll control stability seemed almost neutral.

Next I am going to remove the aileron plugs and leave the flap plugs in to see how that works.

Bob Axsom

[kentb]

Originally Posted by Bob Axsom

Not only did it not produce a speed gain it affected the roll control significantly. It flew like a 2.5 ton 6x6 without power steering. Roll control stability seemed almost neutral.

Bob Axsom

Just doesn't make sense the blocking off the ends would have this effect.

When you have it all figures out, I plan to copy your best modes.

Thanks

Kent

[Mel]

to plugging the ends of control surfaces might be that water can be trapped inside. Typically the end ribs leave a "drain" hole a the aft end.

[roee]

Originally Posted by Bob Axsom

Not only did it not produce a speed gain it affected the roll control significantly. It flew like a 2.5 ton 6x6 without power steering. Roll control stability seemed almost neutral.

Wow, interesting! I wonder why it had that effect. Glad the airplane was at least still controllable and got you home safe.

Was there any appreciable effect on control characteristics from your elevator & HS end plugs?

Even if it seems well behaved in "normal" flight, now I wonder if there could be surprises lurking at some corner of the envelope... Sounds like this warrants some thorough (and careful!) testing. Your experience with the aileron plugs goes to show how seemingly innocuous changes can have unintended effects.

Bob, I admire your pioneering spirit, but be careful out there!

[Bob Axsom]

For my flight domain there was no observed effect except the increase in speed from plugging the stab & elevator but based on the aileron experience I'm rather sure there is some effect somewhere in the flight envelope. When you think about it as the "pockets" get exposed the object passing through the air is different and if you take that away the reaction will be different. I am removing the aileron end plugs tomorrow.

Bob Axsom

[qiutong]

Bob,

This may be a stupid question, but did you re-balance the ailerons after you installed the end plugs? I'm just trying to think what could have made it feel so heavy with what would appear to be such an minor aerodynamic mod.

Brad

[Bob Axsom]

I did not rebalance the ailerons. The weight and the arm were small but it can't be ignored.

Bob Axsom

[Bob Axsom]

If I took the ailerons off and carefully balanced them about the mount point by screwing different length plugs into the rod at the fwd end, that may very well solve the control degradation problem. I'm getting into a time crunch now though so I'll pull the aileron plugs and keep them on the shelf for another try someday. I did roughly rebalance the elevators by drilling out a 1/2" hole 1/4" deep at the forward end of both of them to offset a tendency for the trailing edge of the plugged elevators to rise when left in neutral under static conditions.

Bob Axsom

[Kevin Horton]

I can't see how the extra weight aft of the hinge line is the cause of the change in aileron feel. The two ailerons are connected together, so the weight of one is balanced by the weight on the other, thus there would be no change in static forces. The extra weight has to be a very small percentage of the total weight, so the force required to accelerate the ailerons would be essentially unchanged, so no change in the dynamic forces. There would be a small effect on aileron flutter speed.

I'm guessing the change was due to an effective reduction in the gaps at the ends of the ailerons. Look at the profile of the ends of the ailerons, flaps and the inboard ends of the wing tips. The gap between the aileron skins to flap skins, and aileron skins to inboard end of the wing tips is small. Consider the inboard end of an aileron that is deflected down. As you deflect the aileron a bit the effective gap is between the end of the aileron skin and the web of the end rib on the flap. There is a lot of room between the aileron skin and the web of the flap rib, so the effective gap has increased a lot, which lets a lot of air through, perhaps reducing the pressure differential between the upper and lower surfaces of the aileron, and thus reducing the stick force required for a given amount of aileron deflection.

If you insert those plugs, now the gaps at the ends of the ailerons stay small, even when the ailerons are deflected. Less air escapes from the bottom to the top, so the pressure differential between lower and upper surfaces is higher, and that increases the needed stick force for a given amount of deflection.

This is just a theory. It could be wrong.

[scsmith]

Hi Bob,

This surprises me too. Can you elaborate a little on what you mean by 'roll control stability' seems neutral?

The two possible interpretations that come to my mind are that:
1) The airplane lateral stability seemed neutral, meaning that it doesn't seem to care about flying wings-level or not. If you yaw with the rudder, the wing tip does not come up. Loss of effective dihedral.
or
2) The control surface stability seemed neutral, meaning that the control surfaces don't care where they are, not much return force through the stick to make the ailerons seek neutral position. This would imply very low control forces, which seems to conflict with your other statement that the control forces got heavy.

Kevin's idea makes sense, but the smaller effective gap would also make the control surface more powerful, so less deflection would be necessary for the same control moment so one might guess that those two effects would cancel.

With the stock arrangement, there may also be a detailed flow effect where the gap flow produces some suction on the back side of the first skin lip, and some pressure rise on the internal face of the other lip This would increase the control force, suggesting that your end caps should have made the forces ligher.

I assume that in filling in the recess, you maintained the sharp edges where the filler plug would be flush with edges of the skin, and not build up any significant edge radius.

Steve.

Originally Posted by Bob Axsom

Not only did it not produce a speed gain it affected the roll control significantly. It flew like a 2.5 ton 6x6 without power steering. Roll control stability seemed almost neutral.

Next I am going to remove the aileron plugs and leave the flap plugs in to see how that works.

Bob Axsom