BillS

Barn door vs. strip ailerons

I always thought barn door ailerons were more efficient than strip ailerons. Not sure now.

A Sig Kadet Senior kit was flown in original configuration with strip ailerons. Aileron authority was good. An ARF wing with barn door ailerons was installed for other purposes. A minute into the first flight it was realized that aileron authority was not very good. The clevis was moved in on the horn and retested until the last hole was reached.

Aileron differential (more up than down) was added to the barn door wing by changing the horn position in the servo post. No additional aileron authority was observed.

Current setup
Strip aileron wing – hinge to clevis 1”- area = 60 1/2 sq in. - No differential
Barn door wing – hinge to clevis 15/16”- area = 87 sq in. - Slight differential
Servo horns are in the 1/2”hole on both wings.

Maybe more important it appears that the barn door wing almost requires rudder to turn but the strip aileron wing didn’t even hint at needing rudder in a turn.

It seems that what I have thought for years about barn door aileron efficiency may not be correct. Can anybody shed light on the observations?

Tall Paul

How much dihedral?
Usually we cut the dihedral in half for ailerons, both strip and barn door.
Strips do seem more responsive.
On my Dynaflite P-51, the kit strip ailerons were too responsive, making the plane very twitchy.
When I changed to barn-doors (scale sized) the response was much more acceptable, but still active.

ksechler

What's the area of the respective ailerons. Maybe the strip ailerons were significantly larger. The other question would be what kind of ARF wing were you using.

I can see why there would be more adverse yaw with barndoor ailerons since the area you're deflecting has a longer moment-arm. The fuselage on the Kadet is kind of large and slab-sided. Perhaps as the barn door aileron deflects and induces yaw the side-slip of the fuselage starts to work against the aileron.

BillS

I’ll measure the dihedral on both wings when I get home.

Thanks

BillS

Current setup
Strip aileron wing – area = 60 1/2 sq in. - No differential
Barn door wing - area = 87 sq in. - Slight differential

The wing is from the Sig Kadet Senior 80” sold by Tower. Don’t know how else to describe the ARF.

Thanks.

Bill

BillS

The wing dihedral is:
Strip aileron wing – 2 1/4” dihedral – 78 1/4” long
Barn door wing – 2 5/8” dihedral – 80” long

The 3/8” dihedral difference seems small compared to the wing length.

Bill

Rotaryphile

The reason that barndoor ailerons produce more adverse yaw is simply that in producing the differential lift required to roll the airplane, less of the wing is involved. Getting the increased lift from a smaller portion of the wing requires a proportionally higher lift coefficient from that part of the wing than would be required if more of the wing was working in concert with the aileron. Since induced drag is proportional to the square of the lift coefficient, the yawing force produced by the lowered aileron is roughly inversely proportional to the amount of wing involved. If you reduce an aileron's span by, say, 50%, you will need roughly double the increment of lift coefficient to get the same rolling moment, and doubling the lift coefficient will produce four times the induced drag increment per unit of area - the net result is considerably increased yaw.

Full scale wind tunnel tests conducted in the early 30s indicated that ailerons spanning roughly 60% of the wing produced the necessary lateral control with minimal adverse yaw, but this doesn't leave very much room for wing flaps, so a compromise is made, with the ailerons shortened. Airplanes without flaps perform best with ailerons covering most the the wing span - a good examples are the unlimited aerobatic full scale birds such as the Extras and Edges. Full span, relatively wide chord ailerons are also very effective at counteracting engine torque, enabling flying rather deeply into the stall with good aileron control, simply by deflecting propwash against the roll, as evidenced vertically nose-up hovering with no rolling.
Barn door ailerons produce much more adverse yaw, and are almosts useless for hovering without torque rolling.

BillS

Thank you very much for the explanation. It really helps to understand that the underlying premise was flawed.

I intend to continue increasing the aileron differential hoping for less adverse yaw.

Will β€˜booster trim tabs’ increase or decrease the aileron authority?

Thanks again.

Bill

Rotaryphile

Hi Bill: Because boost tabs move in the opposite direction from the rest of the aileron, they will slightly reduce aileron authority for a given amount of aileron deflection. Boost tabs, however, greatly assist the servo in moving the control surface to the desired deflection, and the additional travel that they will produce beyond where the servo would otherwise be stalled can give several times the control surface authority. If the servo is able to move the control surface to full travel, unassisted by boost tabs, there is little reason to use boost tabs. I like to use boost tabs on models with largish control surfaces simply to permit the use of lighter weight and less costly servos and smaller battery packs.

Ben Lanterman

Bill also keep in mind that there isn't a big cliff where you fall off the strip ailerons down to the barndoor ailerons effectiveness and side effects. Visualize a image of going from one to the other (kinda of motphing a good guy into the werewolf in the movies). As each of the parameters is incremented - span, chord, position, etc. - the resulting roll and yawing moment also changes.

We tend to build the strip aileron because it is easy, or, if we don't mind doing the building thing, we make a barndoor installation - so we think of them as either/or. You can indeed make a aileron that is 3/4 span and 15 percent chord that is inbetween that has the good or bad characteristics of each.

At one time, 20 years ago, I had summarized the appropriate graphs to let a person make an estimate of aileron effectiveness but can't find them anywhere now. But the curves with respect to span and chord of aileron sizes were smooth variations.

BillS

Rotaryphile and Ben,

Thanks. This has been very helpful in understanding the flight characteristics. I made two more test flights today with a slightly more throw (different horn with clevis closer to the hinge line) and no aileron differential. Without differential and with current throw the adverse yaw is … well very uncomfortable. Earlier tests with no differential must have been with less aileron throw. Incidentally the CG is 1/4 to 3/8” forward for a higher sink rate that is preferred.

Bill