[ao.frog]

Goal:
To fly with 0 aileron trim

Scenario:
You're going for a soloflight in your side-by-side RV and you're ready for fueling.

To be able to fly the RV with neutral aileron-trim, how much more fuel would you put into the tank with the empty seat?
(assuming you don't need full tanks for the planned trip ofcourse...)


Or said in another way: assuming you take off solo with no imballance in the tanks, how much lighter would you make the side you're sitting on before starting to use the other tank?
(and from then on you'll feed equal amounts from both tanks ofcourse)

 

[DakotaHawk]

Using some assumptions...

This is a simple weight and balance question.

Assume that the "Center of Mass" of the pilot is 12" to the left of the aircraft centerline. The pilot weighs 175 lbs.

Assume that the cockpit is 20" from centerline to fuselage side wall, and then the fuel tank is 36" long (from fuselage side wall to outside edge of tank). Further, assume that the portion of the fuel tank that is left empty is at about 24" from the fuselage side wall (because of the dihedral of the wing - the fuel at the outboard end of the tank goes away first). The total distance for fuel calculation is:

20" (inside cockpit distance) + 24" (fuel tank distance) = 44".

Fuel weighs 6.6 lbs.
The pilot's arm is:
12" x 175 lbs = 2100

So the fuel that has to be left out is

2100 / 44" = 48 lbs

48 lbs of fuel / 6.6 lbs per gal = 7 gal

So - leave out 7 gallons of fuel on the pilot side and you should be pretty close.

 

[Bob Axsom]

None!

I put in the aileron trim spring system and it does work BUT I NEVER USE IT! It doesn't matter if I am alone or flying with a passenger. The airplane flies beautifully with no roll tendency unless I really get away from my ".5hr,1hr,1hr,.8hr,.3hr, empty" tank switch process. Since you are already flying you have some special need that is not satisfied by my limited sensibility but I come from the "you can only have too much fuel if you are on fire" school of thought.

Bob Axsom

 

[pierre smith]

Errrr......

So - leave out 7 gallons of fuel on the passenger side and you should be pretty close.

Should it not be left out of the PILOT's side?

Best,

 

[David-aviator]

Should it not be left out of the PILOT's side?

Best,

It works better to leave it out on the pilot's side.

I burn down 10 gallons on the left side and it is close. On a cross country the auto pilot keeps the thing fairly level.

Truth is, the airplane needs aileron trim and rudder trim.

 

[DakotaHawk]

It works better to leave it out on the pilot's side.

I was just testing to see if anyone was paying attention! It's good to see that at least two people noticed the intentional gaff.

 

[AlexPeterson]

With both tanks full, and just me in the left, I need about half aileron trim to keep it level. Once I've burned about an hours's fuel from my side, the trim is neutral.

 

[Walt]

No aileron trim here... about 4-5 gal difference puts me pretty close.

 

[Ron Lee]

Personally I minimize the use of the fuel selector valve so I let it go at least 10 gallons on the left side first before switching tanks. I use aileron trim.

 

[L.Adamson]

I installed my auto-pilot & aileron "trim" servo in the right wing to offset solo weight a bit. I'll also draw off the left tank first. However, I really do like my 1*6 aileron trim tab that sit's out on the wing. This was the type sold before the bias spring setups. Even though aileron forces are light, I use it all the time. It moves very little to have the required effect.

L.Adamson --- RV6A

 

[airtractor8]

With both tanks full, and just me in the left, I need about half aileron trim to keep it level. Once I've burned about an hours's fuel from my side, the trim is neutral.

Same for me as well.

Graham
RV6 220hrs

 

[JonJay]

Yep, really close to that...

This is a simple weight and balance question.

Assume that the "Center of Mass" of the pilot is 12" to the left of the aircraft centerline. The pilot weighs 175 lbs.

Assume that the cockpit is 20" from centerline to fuselage side wall, and then the fuel tank is 36" long (from fuselage side wall to outside edge of tank). Further, assume that the portion of the fuel tank that is left empty is at about 24" from the fuselage side wall (because of the dihedral of the wing - the fuel at the outboard end of the tank goes away first). The total distance for fuel calculation is:

20" (inside cockpit distance) + 24" (fuel tank distance) = 44".

Fuel weighs 6.6 lbs.
The pilot's arm is:
12" x 175 lbs = 2100

So the fuel that has to be left out is

2100 / 44" = 48 lbs

48 lbs of fuel / 6.6 lbs per gal = 7 gal

So - leave out 7 gallons of fuel on the passenger side and you should be pretty close.

Great exercise and nice to see someone did the math.

And time to remind the side by side building community that a "balanced" side by side with equal fuel will have a "heavy" left wing.
I leave my fuel selector on the left as many here have posted until about ten or so gallons are burned off before switching.
I did not install aileron trim as most people I spoke with who had it did not use it or felt it was unecessary.
Good question and good info in this post.

 

[L.Adamson]

I did not install aileron trim as most people I spoke with who had it did not use it or felt it was unecessary.

Some people just let the auto-pilot take care of it, or just fly the plane with a slight pressure to keep the wings level. In my case, I'm always checking trim just out of habit. If it's off just a bit, I'll trim it perfectly neutral. I do prefer my trim setup (servo in aileron) over the bias spring method. IMO, it is nessesary if you're not depending on a slight constant loading on the auto-pilot motor. Unless fuel and passengers create the perfect balance, the perfect trim will be off in a side by side.

L.Adamson --- RV6A

 

[rvmills]

I also thought Scott's math was good work, as in the end, the number made good sense, passed "TSAR".

Like Bob Ax, for X-C I normally want all the gas I can carry, so I start off burning from the left side, with the aileron trim about halfway from neutral to full right, give-or-take.

I have my Garmin 396 set for a 30 minute fuel tank warning, and if I'm by myself, I'll let it go off twice before switching. That burns down about 12-14 gallons with my motor, and the trim has migrated over to about half left (thus the TSAR comment on Steve's math). Then I swap tanks and switch every 30 minutes.

If I'm with an adult, I just switch at the first 30 minute warning, and each one thereafter (though not a slave to the gizmos...similar to what others have said, I'll also burn down to 10 on one side, then 10 on the other, then 5, 5, and would probably be getting somewhat close to landing at that point).

Referencing the AP and the trim, I've noted with my Dynon that the wings start to rock or bump a little as my ailerons get out of trim. I use it as an additional indicator to retrim, and like Larry, I check it from time to time by disconnecting the AP (all FWIW). Not sure if TT or other APs exhibit this behavior, or they balance it out more (or just fly wing low/ball out when out of trim).

And good catch Pierre! Went right on by me like a fast ball (no wait, I was just concentrating on the numbers...yeah, that's it!)

Cheers,
Bob

 

[ao.frog]

An update.

Hi again.

I've used the "7-gallon trick" for a while now and it works really well.
(Actually I use 25 litres since litres is what we use in Norway)
With that imballance, soloflight needs 0 aileron-trim.

I either fuel for that imballance when refueling, og feed from the wing with the pilotseat until I have 25 litres and keep that imballance.

It's very rare that I fly with full tanks for short flights so this procedure works for me.

Lastly a comment to those who always takeoff with full tanks: if I do that, then I can't fly acro with two persons onboard my-7 since I then is above max acroweight... so the statement regarding "fuel in the truck" and "when on fire" isn't always quite correct...

Thanks again to Dakota Hawk for doing the math for us!

 

[RVbySDI]

. . .48 lbs of fuel / 6.6 lbs per gal = 7 gal

So - leave out 7 gallons of fuel on the passenger side and you should be pretty close.

I have always been taught the weight of 100LL was around 6 lbs (the sources I just looked up shows 6.02 @ 59 deg F).

At this weight your calculations should be closer to 8 gal ( 48/6.02 = 7.9734).

 

[rvmills]

At this weight your calculations should be closer to 8 gal ( 48/6.02 = 7.9734).

"220, 221...whatever it takes"

(Name that movie!)

Another way of looking at it would be how far below the top of the tank would you leave it to equal the amount needed to have neutral trim ( that 7-8 gals, +/-). My guess is an inch and a half. I have a marked dowel to check, but if it was missin', that might be a good ball park. Call it a field estimate. What sez you guys?

Alf: acro, racing (short) and speed mod testing are when I leave it low as well. I usually go most places with full tanks, but my engine is a thirsty beast! However, for the short, fun stuff, its nice to go light, as you said!

I like the aileron trim though...just in case I get some lighter or heavier gas, and miss the guestimate! (I'm kiddin' around now!! )

Neutral trim is kind of a transient thing...kinda like being on course. You're there once in a while, and the rest of the time you're trying to get back there!

Cheers,
Bob

 

[Hawkeye7A]

7 to 8 gallons sounds about right.....

...in mine. -7A with no aileron trim. I usually burn off the left first starting with both full and run it down to 12 or 13 gal to get a slight right tendency then back to the left after 45 minutes or so. I've never landed with less than 10 gallons remaining (total). Works well for me. People occasionally ask what my cruise range is and I tell them about 4.5 hours but I've got a 3 hour bladder. So that combo is a good measure on when to stretch my legs and top off the tanks.

 

[gereed75]

I typically do the 10 gallons per side thing, burning out of the left first.

That is about a 30 degree adjustment on my Van's spring aileron trim gizmo. Your mileage will vary. It is generally all a non-factor as the ailerons are so effective and so are most autopilots.

I basically don't worry about it. I think the actual aileron displacment to compensate is small small small. Small enough to get lost in the constant shuffle of holding the wings level or driving back to course in anything other than perfectly smooth air.