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Help getting plane to turn LEFT

Posted: January 24th, 2016, 12:16 pm
by jrwood
We're looking for some help - we are using a plane from Freedom Flight models (designed to the 2016 rules), built it and it worked great. First flights turned consistently left, good lift, achieved 1:10 - 1:20 flight times relatively consistently; using a rubber motor 0.089", 305mm long, 900 winds. As we sought to adjust lift angle to slightly decrease lift so we could fly longer in the gym we tested with: it crashed, front wing separated. Reglued, reflew, slightly variable but ok, then a few more crashes and wing post mount separated, rudder separated. All of these were carefully repaired.

The plane now still gets good lift and speed, but won't turn LEFT any longer, using the same motor and 600 winds (we tested previously with 600 winds and it had turned consistently). Looking for assistance regarding what adjustments to make. The plane design is generally asymmetric to overcome the left turn induced by the propeller, but it appears that there is too much compensation at this point. Obviously something didn't get repaired in quite the same position as it was when flying originally, but it "looks" the same as when first constructed.

Any advice? Thanks

Re: Help getting plane to turn LEFT

Posted: January 25th, 2016, 9:56 am
by bjt4888

Congratulations on getting a good start. You will want to inspect all the settings that are designed to create turn. Since it sounds like most of the repair work related to the wing, I would start there. Per the Freedom Flight kit design, turn is generated by left thrust and left stabilizer tilt (the kit does not indicate any tailboom offset as the subrudder already causes a certain amount of turn effect). Also, double-check the amount of left wing washin and any other wing or stabilizer warps that might have changed during repair. The kit recommends about 1/16" to 1/8" of left wing washin (see the excellent description and diagram of this setting in the instructions). If you now have more washin than recommended, this will cause the left wing to generate excessive lift and will hamper the left turn. Washin can be adjusted by carefully bending and heating with your breath or by softening wingpost to wing joints with acetone (if they are joints glued with Duco or Ambroid) or by cutting the wing LE or TE near the wing post w/ a diagonal cut partially through the spar and cracking the spar and supergluing while bending in less washing (usually bend about 2X farther than the resultant desired change, it will spring back a little after drying). The kit also recommends a small amount of wing skew and you will want to verify that this wasn't changed by your repairs.

By "changing the lift angle", I believe that I understand that you are pushing in the front wing post (or pulling out the rear post) a little. This adjustment change will not change the rate of climb. There is an ideal wing incidence setting for each center of gravity location and once you find this incidence setting, you will only make the airplane less efficient by changing. The ideal setting is such that the airplane is just shy of stalling. Read last years wiki and you will get a very good understanding of what this means and how to do it. Climb rate can be reduced by using thinner rubber or by increasing the propeller pitch (maybe increase it about 3 to 5 degrees).

Also, you mention winding to 900 turns and don't mention winding to a set torque or backoff turns. If you read last years wiki section about winding you will learn a much more effective methodology. If you use a progressive method to your test flying (also described in great detail in last years wiki) you should not ever damage your airplane and will progress very rapidly to much longer flights.

Have fun and keep at it.

Brian T.
AMA since 1972 (off and on)

Re: Help getting plane to turn LEFT

Posted: January 26th, 2016, 5:54 am
by jander14indoor
See also a reply I just wrote to a team having trouble getting the plant to climb. Extensive discussion on how to control turn.
<SNIP>The plane design is generally asymmetric to overcome the left turn induced by the propeller, <SNIP>
I need to correct that thinking.
There are two asymmetric features to overcome the left bank induced by the propeller rotation. The longer left wing and the left wing wash in. The goal is to fly a left turn with relatively level wings.
All other asymmetric features are designed to cause a consistent left turn over the range of speeds the plane flies.
- Left prop thrust induces left turn at high thrust, influence dropping as the rubber winds down.
- Tail boom bent to left acts as a rudder to control turn radius in the middle of the flight.
- Horizontal stab tilted to right (left side high) causes left turn and controls the radius as the speed slows.

If you don't have enough turn at the beginning of the flight (flies straight initially and then starts turning), you need more left thrust or more bent tail boom.
If your turn opens out wider through the flight, you need more stab tilt (raise left side higher).
If your turn is steady but too large, you'll need BOTH more bent tail boom (easier than thrust angle adjustment) AND more tail tilt.
If your turn is steady but too small, you'll need to reduce both tail boom offset AND tail tilt.

If you broke enough things, look at the plane when it is fixed, and make sure all the tweaks are still there as intended. Then treat the plane as if you are trimming a new one from the start.

Jeff Anderson
Livonia, MI

Re: Help getting plane to turn LEFT

Posted: January 26th, 2016, 8:59 pm
by jrwood
Thanks for the replies, VERY helpful.

We haven't gotten into torque measurements yet, or into the wind-down procedures. Those will be an excellent next step for us. I think the primary area we'll start looking at is the rear stabilizer (it is slightly down on the left side, as opposed to being elevated), and confirming the washin on the left wing.

As I'd mentioned earlier, the plane does fly level. By adjusting the rear stabilizer will that affect how level the plane stays while flying?

Thanks again

Re: Help getting plane to turn LEFT

Posted: January 27th, 2016, 3:38 am
by jander14indoor
If your left stab tip is below the right, the stab is trying to turn the plan right, cancelling some of the other left turn adjustments. Definitely something to correct.
It SHOULDN'T affect flying level, but watch to be sure. Part of the reason we use airplanes for this event is the non-linear, interactions of the relevant parameters.

Jeff Anderson
Livonia, MI