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Author Topic: Prop thrust angles?  (Read 677 times)
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BR549
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« on: April 26, 2017, 09:01:14 PM »

I just built my first Poonker and I'm having trouble getting it to climb. It circles exactly how I want, so my left thrust is OK. However it circles at the height I release it and will not climb. I've played with incidence, and the balance is good. I'm starting to think my up/down thrust angle is the problem.

What effect does up/down thrust have on a model? Any tips would be welcome.

Bob
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cglynn
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« Reply #1 on: April 27, 2017, 09:01:36 AM »

Bob,

Downthrust is usually used to prevent a stall at high launch torque.  Between 1 and 3 degrees is usually sufficient. 

If the model is not climbing, it might just want more power.  What are you using for a motor?  How long is it?  How much does the motor weigh?  How many turns are you putting in?  What is your launch torque?

Without the information above, it will be hard to give you a definitive solution.  If you don't yet have a torque meter, the length, weight, and turns will be enough to at least give some advice.

CG
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BR549
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« Reply #2 on: April 27, 2017, 05:17:29 PM »

I've tried motors from .200 grams up to .380 grams, various number of turns on each. At the upper end it hardly flies from torque roll. Once it unwinds a bit it flies fine at a constant altitude.

I'm pretty new at this, it must be something fundamental I'm missing. I built a Fly as my first mini, and with the Ikara prop and hanger. I get real nice flights from it, around a minute and a half at a gym.

Lots to learn, and next purchase (or build) will be a torque meter.

Thanks


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cglynn
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« Reply #3 on: April 28, 2017, 12:14:06 PM »

Alright, so we've got a start here.  More data on your motor would be helpful.  Barring that, lets start from the beginning:

Lets make sure your model is trimmed.  Start with a motor loop that is 7" long and 250mg.  This motor is too short for serious duration, but that's not what we are looking for here.  We want to confirm trim, and that's it.  Put 750 to 1000 turns in the motor.  Write down the number of turns put in.  Fly the model, observe its behavior, and record the flight time.  What you want is for the model to climb about 10-15 feet, cruise, and descend.  You can observe all phases of flight this way, while not using a ton of time for full flights.  For a ministick with this motor, you should be getting around 2 to 3 minutes.  When the model lands, record the number of turns left on the motor.  Based on the models behavior, change ONE thing, and fly again.  If the model stalls and torque rolls at launch, add downthrust.  If it doesn't torque roll, but stalls, try a small (1/32" or less) change in wing incidence (raise the TE of stalling, lower it if diving).  For maximum performance, you want the model to be flying just under a stall.  I like to add incidence until the model just starts to stall on climb, then take out a tiny bit of incidence until the climb is smooth.  While I am doing these tests, I use my turns remaining to calculate turns used during the flight.  I divide turns used by flight time to get my average prop RPM.  What I have found is that as incidence is added to the model, prop RPM decreases, and initially flight time increases.  Eventually, as incidence is added, prop RPM will start to actually increase as the model starts to stall.  Once I reach that point, I revert to the previous incidence.  This should give a minimum RPM and maximum flight time.  Once I find that point, I measure the trim settings and write them all down.  The next time I fly, I have a baseline.  Adjustments will always be needed, but they will be small, and if and adjustment doesn't work out, its good to have something solid to go back to.

Once you get the model trimmed, you can start playing with different sized motors to maximize duration. 

Note, this is what I do.  I like to fly my models starting with as few variables as possible.  By confirming trim in the way I do, I am reasonably certain that the only variable to flight time will be the motor size and length, which we can talk about about once you confirm your trim, and get a torque meter.

Hope that helps, or at least gets you in the right direction. 


CG
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mkirda
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« Reply #4 on: April 28, 2017, 12:44:26 PM »

I've tried motors from .200 grams up to .380 grams, various number of turns on each. At the upper end it hardly flies from torque roll. Once it unwinds a bit it flies fine at a constant altitude.

I'm pretty new at this, it must be something fundamental I'm missing. I built a Fly as my first mini, and with the Ikara prop and hanger. I get real nice flights from it, around a minute and a half at a gym.

Lots to learn, and next purchase (or build) will be a torque meter.

Thanks

Another possibility here is just a malformed prop.
It might just not be efficient at the speeds you are flying at. Pitch distribution might be off, or potentially excessive flare.

Or it could have lost some twist - I saw this before with LPP props.
I put one back on the form they were made with and found that at about 5" radius the LE was approximately 1/4" off the form.
No matter how much power you put on that prop, the plane would barely climb.

I recall Jeff Annis saying recently that for ministick, his best props were made with 0.016" wood. Any thinner and they lost their twist.

Good luck!

Regards.
Mike Kirda
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BR549
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« Reply #5 on: April 28, 2017, 06:01:03 PM »

Thank you both very much. This give's me lots of info to work with and some things to try.

I'm bad about jumping in the deep end with a new hobby.

Bob
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cglynn
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« Reply #6 on: May 01, 2017, 10:00:20 AM »

Nothing wrong with that.  The learning curve in indoor flying is pretty steep.  The good news is that almost every indoor flyer is willing to offer assistance in some form or another to help lessen that curve.

Let us know how you get on.

CG
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