So we have designed a pusher-prop UAV with a balsa frame. We are using a foam wing that was scavenged from a puller-prop airplane kit (the design files of the frame are posted at https://github.com/ssayer/Open-UAV in SketchUp format). The components we are using are...-Motor: Park 450 Brushless Outrunner Motor, 890Kv-Propeller: 10 x 4.7 Slow Flyer-Battery: 800mAh 3S 11.1V 15C LiPo, 20AWG JST/Balance (although we are intending to use a 11.1V/1200MAh battery for future flights)-Weight: ~2 pounds-Wing: foam, 39.5" wingspan-The center of gravity was under the wing, at the same place it had been with the kitWhen we tried to fly, the plane moved slowly forward, then lost its altitude and fell to the ground slightly nose-heavy. We are wondering what would be the best step to take to increase thrust and get our plane flying. We have thought about...-using a longer/higher pitch propeller (10x7, 11x5, 12x7?)-using a more powerful motor-upgrading the batteryWould would appreciate any input on which would be the most effective solution to get more thrust.Thanks.
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I've been scratch building RC planes for a little while, so here are my recommendations.
Don't use a 8" prop with that motor. General rule of thumb; the lower the Kv, the bigger the prop. The higher the Kv, the smaller the prop. With a 890 Kv motor, I would use a 10x6 prop. That motor is rated for planes up to 30 oz for sport flying, so it should lift a 2lb plane off the ground. Double check the orientation of your prop. Make sure the writings on the prop face forward with the direction of flight. If they are facing away from the motor (aft), you are losing thrust.
Like everyone else mentioned, make sure the CG is correct. Glide test it with everything installed. It should fly for several feet without wanting to do a nose dive. Throw it back and forward with a buddy and make adjustments until it looks right. Normally you can just slide the battery a little bit to adjust CG.
That motor is a little small for your plane, but it should work. The battery wont last long, I would use ~ a 2100 mAh battery. I hope this helps, let me know if I can answer any other questions.
then play with different prop/motor/battery combinations in motocalc, and when you have a few ideas about what might work better in real life, try them.
MotoCalc Electric Flight Performance Prediction Software
MotoCalc electric flight performance prediction software
I uploaded a couple pictures. The plane we flew is the same as the one in the pictures, except with polyfiber wrapped around the fuselage.
So we should use a smaller prop? We were under the impression using a larger prop would be best, partly because of E-flite's recommendation that the prop for the motor should be "9x6 to 11x3.8 Slow Flyer or 10x5 to 12x6 Electric".
A bigger motor will always give you more thrust but might not be an elegant solution. Eriks idea with the prop is a good one. Heres mine why not try some gliding flights from a slope to make sure your design is trying to fly at the correct CoG then once you have it gliding well, is the thrustline right? Is the motor driving it into the ground or bringing the nose too high and into the stall??
If you put a big enough motor on a brick it will fly.
I would say from reading the above details you need to up the battery power in the first instance. I would aim for a 3s with about 2100 mah battery capacity. The kv of the motor is the rpm per volt, so this should be more than enough to power a larger prop and give you lots of thrust, which would be my second upgrade to your system please check with the motors manufacture for prop sizes that work with that motor. As a rule I always try and aim for a power to weight ratio of 1:1, so if you point the plane towards the sky under full power the plane will be (or feel like) it weighs nothing or slightly trying to pull upwards out of your grip.
The correct centre of gravity is a critical aspect of a good flight, get this in the wrong place and you will have to trim the plane and this causes drag, which uses more power to drive the plane through the air. If the centre of gravity is correct the plane should almost sink in a level manner until the air speed is to slow, at this point you will have a stall and the nose or a wing will drop, wing dropping is the least desirable, with the nose dropping while high enough will allow you to gain air speed and recover for a controlled crash (aka a landing)
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Don't use a 8" prop with that motor. General rule of thumb; the lower the Kv, the bigger the prop. The higher the Kv, the smaller the prop. With a 890 Kv motor, I would use a 10x6 prop. That motor is rated for planes up to 30 oz for sport flying, so it should lift a 2lb plane off the ground. Double check the orientation of your prop. Make sure the writings on the prop face forward with the direction of flight. If they are facing away from the motor (aft), you are losing thrust.
Like everyone else mentioned, make sure the CG is correct. Glide test it with everything installed. It should fly for several feet without wanting to do a nose dive. Throw it back and forward with a buddy and make adjustments until it looks right. Normally you can just slide the battery a little bit to adjust CG.
That motor is a little small for your plane, but it should work. The battery wont last long, I would use ~ a 2100 mAh battery. I hope this helps, let me know if I can answer any other questions.
Regards,
Andrés
measure the workrate on the bench (e.g. with a Watmeter).
then have a play with motocalc, e.g. to get an idea of the thrust reduction as airspeed increases with that setup.
http://www.motocalc.com/
then play with different prop/motor/battery combinations in motocalc, and when you have a few ideas about what might work better in real life, try them.
So we should use a smaller prop? We were under the impression using a larger prop would be best, partly because of E-flite's recommendation that the prop for the motor should be "9x6 to 11x3.8 Slow Flyer or 10x5 to 12x6 Electric".
IMG_0344.JPG
IMG_0348.JPG
A bigger motor will always give you more thrust but might not be an elegant solution. Eriks idea with the prop is a good one. Heres mine why not try some gliding flights from a slope to make sure your design is trying to fly at the correct CoG then once you have it gliding well, is the thrustline right? Is the motor driving it into the ground or bringing the nose too high and into the stall??
If you put a big enough motor on a brick it will fly.
The correct centre of gravity is a critical aspect of a good flight, get this in the wrong place and you will have to trim the plane and this causes drag, which uses more power to drive the plane through the air. If the centre of gravity is correct the plane should almost sink in a level manner until the air speed is to slow, at this point you will have a stall and the nose or a wing will drop, wing dropping is the least desirable, with the nose dropping while high enough will allow you to gain air speed and recover for a controlled crash (aka a landing)
Regards
Martin