Yardstick mods for AP

[Bill Harris]

I have a Great Planes Yard Stick ARF slowflyer that I am prepping for use as an aerial photography platform.

Since I will be carrying more weight with the camera, larger battery and motor, and wing/airframe strengthening I plan to install a supplemental CF spar/wing joiner at the 33% chord point. This mod is still "on the board" and I've not finalized it, yet.

The weight will increase from 12 oz to 20 ounces, and the wing loading from 3.5 ounces/ft^2 to 5.8 ounces/ft^2.

One area of concern is the wing incidence.
The tailplane is at -0- degrees, the wing incidence at the root is 5.3 degrees, with the wing tip incidence at 3.5 degrees (washout twisted into the wing). The stock engine down-thrust is 6.6 degrees.

The airfoil is flatbottomed with a thickness of 5%. I think that the large wing incidence is present because it is a slow flyer and it might cause problems when the airspeed in increased with load. The wing incidence will be fixed once things are glued together and will not be easy to change. My intuition hints that the wing incidence ought to be reduced by a couple of degrees to 3.3 at the root to 1.5 at the wingtip.

Reports from others suggest that the engine downthrust should be reduced to -3 degrees.

What advice do we have on this situation?

--Bill

[Ollie]

If you like the handling characteristics that the unmodified plane had, don't change the CG, thrust line or wing and tail incidence. With sufficient thrust, the plane will fly faster to produce the lift necessary for the added weight. If the thrust is insufficient, the plane will fly at a more nose up attitude and the stall margin will be reduced. Changing incicence, elevator trim or thrust angles won't solve this degradation.

It is the power loading (ounces per watt) that has to be maintained or reduced by adding power in proportion to the weight and drag increase. If your unmodified model will take off and climb on less than full throttle then all you may have to do to fly with the added weight and drag of the modification is to advance the throttle.

If you are using nicads or nickle metal hydride batterys, switching to LiPo batteries will save weight to at least partially offset the weight increase of the AP modification and reduce the need for added thrust.

[Bill Harris]

I have no idea how the unmodified YardStick flies. I'm doing some pre-emptive mods as I assemble it, based on AP modifications required for the (somewhat similar) GWS Slow Stick. All in all, the YardStick is an interesting design, and ought to prove to be a good AP platform...

Thanks for the feedback!

--Bill

[Ollie]

The prudent thing to do is to build it stock first and evaluate its performance and handling. If it is not at least a little over powered in the stock condition, it will perform poorly with just the AP modification. Then you will be faced with the need for a stronger motor and more battery capacity to restore performance.

I would want to know what I was letting myself in for before starting modifications.

[Sparky Paul]

I just attached* the Aiptek SD to the battery pack.
Filled in the slot between the wings at the root.
Went flying.
8x800 NiMh.. Runs about 15 minutes.
Gets 3 climbs to "gotta bring it down" altitude.

.
*(Used special aerospace universal pliable polymer fasteners.
$1.00 a bag at Staples.)

[Bill Harris]

Agreed, Ollie: building it stock at first would be prudent and reasonable and makes perfect sense. I _was_ going to do it that way at first. But the Yard Stick, in it's ARFedness, is difficult to unbuild and then modify and rebuild.

By looking at the way Paul Burke's (et al) Yard Sticks fly I can guesstimate the changes that may be needed.

--Bill