I feel the need for speed....I think. - RC Groups
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Dec 22, 2017, 10:01 AM
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I feel the need for speed....I think.


Guys,

I've got a couple of vintage model kits that were designed for nitro power and I would like to build them using electric power. I use the e-Calc program to find good combinations of motor and prop for models, but this is only part of the question. For instance, the e-Calc program will tell you what the projected thrust and speed are for a particular combination, and what the projected stall speed will be. The question is, how do you know how much speed that you will need for a particular application it order to fly the model in the manner that you want. For example, a Cub doesn't need much speed to fly well, whereas a model with a thin airfoil and small wing will need more speed.

So what criteria do you use to be fairly sure that not only do you have the correct power combination selected, but also that the model will perform well within the flight envelope that you desire?

Thanks in advance.
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Dec 22, 2017, 04:49 PM
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scirocco's Avatar
If you work on a pitch speed at least 2.5 times the estimated stall speed, you should get a nice flying sport model. Particularly with well powered (ok, overpowered) models 100W/lb and greater where most flying is at part throttle, I often target pitch speed over 3 times stall speed.

If pitch speed is less than double stall speed ECalc will warn you that is a bad idea.

If you are looking for outright speed then with over 200 W/lb, you should get within about 15% of predicted pitch speed. Static thrust will not be a good measure when looking for speed. It's a matter of trading voltage, Kv, pitch and diameter to get what you need. It's worth noting that it's hard to know how much thrust is required for a given speed, but if your predicted pitch speed is not high enough you can't get there, period.
Dec 22, 2017, 05:40 PM
Registered User
Scirocco,

Thanks for the information. Merry Christmas.
Dec 23, 2017, 06:45 PM
I just want to go fly!
walter3rd's Avatar
He is the go to guy for questions like this. Hss helped me in the past. Thanks again! walt
Dec 25, 2017, 12:34 PM
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vollrathd's Avatar
Quote:
Originally Posted by Warthog_Fan
Guys,

I've got a couple of vintage model kits that were designed for nitro power and I would like to build them using electric power. I use the e-Calc program to find good combinations of motor and prop for models, but this is only part of the question. For instance, the e-Calc program will tell you what the projected thrust and speed are for a particular combination, and what the projected stall speed will be. The question is, how do you know how much speed that you will need for a particular application it order to fly the model in the manner that you want. For example, a Cub doesn't need much speed to fly well, whereas a model with a thin airfoil and small wing will need more speed.

So what criteria do you use to be fairly sure that not only do you have the correct power combination selected, but also that the model will perform well within the flight envelope that you desire?

Thanks in advance.
Check out www.motocalc.com for another PC electric model calculator. This program is free for 30 days, then $39. Skip all it's graphs and so on, and just go straight to its "Opinions" tab for predictions on how a given combination of motor, battery, model airplane will fly.

Ref
http://www.motocalc.com/about.htm
Dec 31, 2017, 01:59 PM
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E-Challenged's Avatar
Lucien Miller has an advice thread on the Power Systems forum, also is proprietor of Innov8tive Designs, sells Scorpion and Cobra motors and speed controls. If you provide him specific information about your vintage kits, he can provide you good motor/prop/battery pack, speed control advice to give you desired performance. His website also has comparison charts showing other equivalent brands and models of motors.

I prefer his Cobra brushless motors because they have sturdy bolt-on prop adapters, his more expensive Scorpion motors have collet adapters. Use a speed control with switch-mode BEC (not linear type BEC)
Last edited by E-Challenged; Dec 31, 2017 at 02:05 PM.


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