B

Bob Axsom

Well Known Member
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
 
Bob, I sure do enjoy your testing efforts.

Bob Axsom said:
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
Click to expand...

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
 
A possible side effect...

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.
 
Wow!

Bob Axsom said:
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.
Click to expand...

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!
 
I wondered the same thing

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
 
Re-balancing

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
 
I did not rebalance the ailerons

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

Bob Axsom
 
You got me to thinking

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
 
Last edited:
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.
 
A bit more detail, please

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.

Bob Axsom said:
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
Click to expand...
 
Last edited:
What about the inboard ends of the elevators?

Bob,

Not directly related to the puzzle, but:

It seems like some fairings on the inboard ends of the elevators would be helpful in drag reduction. If you stand directly aft of the tail, you will see a fair amount of aft elevator (inboard ends) surface not faired. It may sound like a fairing would interfere with the rudder, but I don't think so. The aft, inboard ends would not require the fairing to extend inward towards the aircraft center line much at all.
 
Some elaboration

The control forces were higher than normal (it felt like a pig) but the airplane did what I asked it to do. When it was banked over to one side there was no tendency to level itself. Dihedral would normally be expected to help here but I had to force the plane back to level with firm roll control input. The plugs are straight up and down flush with the end of the aileron skins. When I disconected the aileron from the control it hung flat (upside down horizontal) from the mounting brackets under the wing indicating a balance around the pivot point. I was not able to get to the airport today and the weather is getting bad again so plug removal and retest will take a while. I like Kevin's theory.

Bob Axsom
 
It is in the queue

AlexPeterson said:
Bob,

Not directly related to the puzzle, but:

It seems like some fairings on the inboard ends of the elevators would be helpful in drag reduction. If you stand directly aft of the tail, you will see a fair amount of aft elevator (inboard ends) surface not faired. It may sound like a fairing would interfere with the rudder, but I don't think so. The aft, inboard ends would not require the fairing to extend inward towards the aircraft center line much at all.
Click to expand...

Good observation. Both John Huft and Tom Martin have added the fairings you describe and I plan to do it too. There is plenty of room and the elevator interconnect hole in the tail cone needs to be addressed as well.

Bob Axsom
 
Actual clearance distance between plugs?

Hi Bob,
Were the plugs very flexible? Is it possible that they could have been deflecting outwards due to a differential pressure effect if you sealed the tips? Perhaps both the aileron plug and flap plug were touching each other when the aircraft was in flight giving the feel of increased stick force...

Just a thought.
Chris
 
I think You are on the right track

I just woke up and with all of the thinking over the past many hours I think the problem is friction. I believe the problem may be corrected when I loosen the aileron pushrod attachment bolts!

I will retest this configuration. On the day I flew the test it was gusty and the speed varied more than 1 kt after settling down in each leg for 5 twenty second interval GPS ground speed readings so I need to look at the speed again as well as the roll control.

Bob Axsom
 
Last edited:
Common misunderstanding??

PMFJI, but aileron pushrod attach bolts should not be loose, unless your system is different from mine.

There is a widespread misunderstanding of rod ends, and how they should be attached. Rod ends are designed with the bearing inside, and are meant to be attached with the attach bolts torqued to standard torque values. (ie: TIGHT)

The rotation of the rod-end is accomplished INSIDE the bearing. You shouldn't be able to alter the friction of the ailerons by tightening the rod-end attach bolts too tight.

I don't understand your most recent post. Maybe I am missing something?? FRICTION in control systems is best detected while the aircraft is on the ground.

AIRLOADS can only be felt when airborne.......... Duhh:eek:
 
I know the rationale but ...

If you really torque them down you can swell the insert (uni-ball) OD and increase the friction. I am guilty. I did not have a good second wrench for the job, I was rushed and I Cranked it down using the logic you present and I over did it. I'm sure this is going to prove out when I re-fly the test without the over torqued assembly. Two things I have to remember I may have distorted the rod end bearing to where if will have to be replaced and the nuts probably be replaced with new ones as I have removed and replaced them 3 times. In this particular application it is a concern.

Bob Axsom
 
Mystery Solved

The plane appears to be slower with the plugs so they are coming out. The outboard plugs are on the shelf and the inboard plugs will come out in the next few days. The roll control anomally was due to me installing the the aileron attachment hardware essentially backwards so that the rod end bearing shaft was forced against the edge of the large washer. The plane flys great again now that the large washer is outboard of the rod end bearing. Sorry for the confusion.

Bob Axsom
 
Last edited:
another theory

First off, I'm hanging on every word and experiment you're doing...Keep up the good work Bob, I like to think you are also doing a good service to all us RVers.
My theory....the open ends of the ailerons when deflected catch the airflow in the direction moved thereby getting an assist in their movement which should help lighten the felt control forces. By filling in the "holes" no airstream catches and the boost is eliminated making the ailerons feel heavier.

BUT I could be completely WRONG but it sounds good to me!
 
God Bless the Thinkers Glenn

Here is an image of one of six plugs that were installed. I try to be pragmatic about these experiments and if they don't work out they come back out. This one bothers me more than usual because the mod is so instinctively good for drag reduction. Like the cooling air inlet plugs they go on the shelf to be pondered and perhaps someday tried again. For now the racing season is starting to intensify (www.sportairrace.org and www.airventurecup.com) so it is time to secure everything in the best configuration I have developed for now and go run the good race.

Bob Axsom

IMG_3580.jpg
 
You must log in or register to reply here.