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-   -   Electric motor selection (https://www.rcuniverse.com/forum/questions-answers-154/7963695-electric-motor-selection.html)

SteelheadAero 09-17-2008 01:21 PM

Electric motor selection
 
My team and I are designing a 36" wingspan top-wing mounted RC plane that will probably weigh about 8 lbs (very simple design). We want it to carry an additional amount of weight (say 8-10 lbs). Does anyone have any recommendations for a light-weight electric motor that can haul at most 18 lbs? We have been researching a lot, but there are quite a few to consider. We are open to different prop styles as well, though we have been hitting around a 6" 4 thin electric propeller. Thanks!

blvdbuzzard 09-17-2008 01:40 PM

RE: Electric motor selection
 
36 inch span and 18 pounds. maybe a nice JATO from a C130 will fly it. Average 36inch span plane is around 1 pound or less. From the weight you listed the span should be around 65 to 70 inches for the listed weight. A plane of 18 pounds is going to have a longer span.


I think you need to change your design some. Check the specs on a 60 sized trainer and compare weights with it and what you want to do.

Dru.

JohnW 09-17-2008 02:19 PM

RE: Electric motor selection
 
Really? Is there a typo here or are we really talking 18lbs on a 36" wing? What is the AR? Even if the AR is 3:1, that is a wing loading of nearly 50oz/sqft. Not going to be easy to make that fly with any motor, except maybe a gas turbine, but even then it would be "interesting." It would help to confirm that weight/span and maybe a bit more info about the wing/plane design.

Flying freak 09-17-2008 05:19 PM

RE: Electric motor selection
 
well count on that 36 inch wing giving your basically no lift (it might make 18 pounds of lift just as it cross over into the transonic range)

So maybe try and get something that will give 20+ pounds of thrust and some killer lipos!

SteelheadAero 09-17-2008 09:39 PM

RE: Electric motor selection
 
my bad! I meant 63" wing -span... hahaha..

blvdbuzzard 09-17-2008 10:16 PM

RE: Electric motor selection
 
That's a little bit better. Still 63 inch at 18 pounds is way heavy. I was worried that my Cub was 8.5 pounds at 71 inches. The Cub could handle a lot more weight but the speeds go up and stall speeds go way up.I figure it could do 12 pounds with a good pilot. Much more and it would not be fun to fly.

With a wing of 63 you are looking at a 12-16 inch electric prop with some hefty lipos. lots of current and amps to get 18 pounds air born. Not saying it is not do able, just a lot more then the 6 inch prop you mentioned. You could take a look through the glow to electric conversion forum for some ideas. They do a lot of planes in there.



Dru.

Campy 09-18-2008 09:11 AM

RE: Electric motor selection
 
If you have not done so already, take a look at Bellanca's planes from the 30's. EVERY surface except the vertical stab was a "lifting surface" and this included the fuselage.

You are putting A LOT of weight on a 63" wingspan. As mentioned above, figure somewhere around a 14" - 16" prop and some heavy duty lipos.

As a "GENERAL RULE OF THUMB" (this will get you in the ballpark) figure 80 - 100 watts of power per pound of weight READY TO FLY. For instance, if your plane, ready to fly, weighs 18 lbs, you will need a motor capable of generating 1450 watts of power at a MINIMUM. I would personally feel better having 1,800 watts (or more) of power.

Do a Google for HobbyCity. They list the wattage on a lot of their motors.


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