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Thrustline and incidence for Extra 300s?

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Old 01-29-2014, 10:02 AM
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jester_s1
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Default Thrustline and incidence for Extra 300s?

You guys have been so helpful so far on this project. I had to move my engine box back to accommodate the muffler on my engine, so now I am unsure how the thrust angles should be set up. The plane is a Midwest Extra 300s 27% scale.
Also, any advice on incidences would be appreciated. Is this a 0-0-0 kind of plane or do they usually fly better with some up wing incidence and down thrust on the engine? I am using an SD-10 radio so I can set up whatever mixes I need to if that's a factor.
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Old 01-29-2014, 10:29 AM
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Quote:
Originally Posted by jester_s1 View Post
You guys have been so helpful so far on this project. I had to move my engine box back to accommodate the muffler on my engine, so now I am unsure how the thrust angles should be set up. The plane is a Midwest Extra 300s 27% scale.
Also, any advice on incidences would be appreciated. Is this a 0-0-0 kind of plane or do they usually fly better with some up wing incidence and down thrust on the engine? I am using an SD-10 radio so I can set up whatever mixes I need to if that's a factor.
Make your own downthrust angle measurement device. Get a 3/4" square stick, about 10" long and drill a hole on one end to fit the engine shaft. On the outside of the hole, parallel to it, pin a weighted string (sewing thread works fine) and let it hang down towards the opposite end of the stick. On the opposite end, scribe several angled lines in 1/2 degee increments which start at the pin hanger point.

With stab at zero, hang the instrument on the engine shaft and secure it. Let the string hang and let it find the downthrust angle....

Remember the MattK rule for angles, 1/4" deflection of a line in 14" of span, equals 1 degree. For right thrust determination, measure the distance from tail post to each prop tip. Assuming a 14" prop diameter, you have 1 degree of right thrust for every 1/4" length difference...QED!
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Old 01-29-2014, 11:41 AM
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Matts, technique to determine thrust angles is a great one and works well. For this type of airplane I pretty much have a bench setup that gets me 90% of where I want to be. The other 10% comes slowly during the course of about 50 flights. The Midwest is a fairly easy airplane to work on but it does have a fixed stab so the first thing to do is block up the tail until the stab is at zero degrees. Next set the engine to zero degrees and 2.5 degrees right using Matt's math. Now comes the hard part, you will want to set the wing to .5 degrees positive. A symmetrical wing needs a slight amount of positive to generate enough lift to support the weight of the airplane. Without the positive angle you would be flying the entire airplane at a positive angle. Now adjust CG during flight tests so that no elevator trim is required. If you needed up trim then move the CG back 1/8" at a time. Once CG is set do some uplines. If it pulls to the canopy on an up line the add some down thrust, if it tucks the add some up. Next will be mixing. Do a flat turn using rudder only, it should want to tuck. Add a slight amount of up elevator mix and opposite aileron mix with rudder so that when you apply rudder you get yaw only. You will never get it 100% perfect but you will notice a huge difference while sequence flying.
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Old 01-29-2014, 12:34 PM
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That's what I'll do then. 0-0 on engine and stab with 1/2 positive on the wing with 2.5 right thrust. Thanks guys.
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