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Thrust required for level flight (cruising)

#1
I've got an eRc Cub I picked up a few years back loaded with the servos but no motor/ESC. I'm finally getting around to getting it in the air..... I have a few unmarked motors I can put on the plane. The flying weight should be 650-700 grams, the motor I'm looking at using measures out to be about a 3310, 1200 RPM, 4.2 amps no load on a 3s give or take. It will turn a 9.6x7.5 prop and produces a max thrust of 750 grams @11 amps. I'm thinking that should be enough thrust for take off, but 1/2 -2/3 throttle, thrust produced is 400 grams. Is that enough to maintain flight/cruise? It's a cub, not looking for sport performance, just cruiser. The motor doesn't seem to be overly taxed spinning the 9.5, I'm guessing it could spin a 10 or 11 in prop, but don't have one to test with right now.
 
#4
thanks for confirming, getting back into the swing of things after a few years off :(. Here's the details: 43-3/4 in. wingspan, 29-1/2 in. long, 291 sq. in. wing area. https://www.modelairplanenews.com/erc-piper-j-5-cub/

I have no specs on the motor. I'm guessing I'm not overpropping it, but only guessing at best. It not getting hot running at full load (short bursts) on the bench with that prop. I'll pick up a 10" prop and test as well. I thought I had a few laying around but looks like I don't anymore. Any other ways to tell other then heat?
 
#6
Didn't really think about it at the time, but yes I would agree with that. I must have had the hold setting still on and didn't realize it. I'll have to go back and remeasure.
 

Bricks

Well-known member
#7
9.6 for that motor is a little on the small side not that it hurts but that size motor should swing 10-12 inch prop.
 

quorneng

Well-known member
#8
You calculate the minimum cruise power of a plane by measuring its glide angle. If your Cub with the prop off achieved say a 5:1 glide angle then it would requires a thrust of 1/5 of its weight to maintain level flight but it would need to be flown pretty close to the stall speed.
 
#10
I've got an eRc Cub I picked up a few years back loaded with the servos but no motor/ESC. I'm finally getting around to getting it in the air..... I have a few unmarked motors I can put on the plane. The flying weight should be 650-700 grams, the motor I'm looking at using measures out to be about a 3310, 1200 RPM, 4.2 amps no load on a 3s give or take. It will turn a 9.6x7.5 prop and produces a max thrust of 750 grams @11 amps. I'm thinking that should be enough thrust for take off, but 1/2 -2/3 throttle, thrust produced is 400 grams. Is that enough to maintain flight/cruise? It's a cub, not looking for sport performance, just cruiser. The motor doesn't seem to be overly taxed spinning the 9.5, I'm guessing it could spin a 10 or 11 in prop, but don't have one to test with right now.
That seems like a good value for the throttle. A general rule of thumb you can use is that sailplanes and gliders require a minimum full throttle thrust value of about half the plane's weight, park flyers require about three-quarters the weight, and, 3D and Aerobatic fly well with about 1.5 times the weight or more.
 

Merv

Well-known member
#11
the motor I'm looking at using measures out to be about a 3310, 1200 RPM, 4.2 amps no load on a 3s give
So, the motor your looking at, how many amps is it rated for by the manufacture? They should be able to tell you the recommended amps. Don't use a prop that would not exceed the amp rating.
 
#12
So, the motor your looking at, how many amps is it rated for by the manufacture? They should be able to tell you the recommended amps. Don't use a prop that would not exceed the amp rating.
that's just it, I have no clue. It was in a pile with some other things I picked up a while ago and has 0 markings on it. So everything on this one's a guessing game :)
 

Merv

Well-known member
#13
that's just it, I have no clue. It was in a pile with some other things I picked up a while ago and has 0 markings on it. So everything on this one's a guessing game :)
Sounds like you will need to sneak up on a good prop size. Put the motor on a stand, run a prop for 20 seconds or so, then test the temperature. If it’s cool or just warm give a longer run, 60-90 seconds. If it’s still cool or just warm, repeat with a larger prop. What you don’t want is hot, if you grab a hold and have to let go right now, it’s too hot. You need to down prop. If its uncomfortable but you can hold on to it, you need to stop.
 
#14
Thanks Merv, I've got a handful of props from 9"-12" of varying pitches on the way. I assume I'm running the motor at 100% throttle for the testing?
 

Chuppster

Well-known member
#15
A good rule of thumb I use is around 4 grams of motor/watt of power. So, if you know the weight of your motor, divide it by 4 and don't exceed that wattage. As for cell rating, I typically look at similar sized motors to get an idea. Higher KV motors should be run on lower cell counts. If its 1200 kv it's probably best to run 3s. Have you weighed the motor?
 

quorneng

Well-known member
#16
Glide angle cannot be simply 'calculated' unless you have access to a lot of data and some serious computing. For a model it comes down to actual testing, how far does it glide from how high, coupled with much experience of what worked well in the past.
Models are aerodynamically less efficient compared to full size so they have to have relatively rather more power installed simply to take off and climb so 'cruising' tends not to be an issue.
Electric motors are pretty efficient at much reduced throttle so having considerable excess power available but not used for cruising flight is not a great penalty.
 
#17
A good rule of thumb I use is around 4 grams of motor/watt of power. So, if you know the weight of your motor, divide it by 4 and don't exceed that wattage. As for cell rating, I typically look at similar sized motors to get an idea. Higher KV motors should be run on lower cell counts. If its 1200 kv it's probably best to run 3s. Have you weighed the motor?

Interesting, did you mean multiple by 4? The Motor weighs in at about 80 grams, if I divide that by 4 and take that as the wattage its' only 20 watts.
 
#19
I'm waiting for the 10x6 props to show up still. I tested last night on a 3S 1300 with a 11x5.5 prop. I didn't take notes for some unknown reason but if I recall correctly it put out 3.1 lbs of thrust, 18.x amps, about 240 watts. beastly little motor, I was quite impressed. I didn't think it would do anything close to that. I tried testing with a 12x6 but it was too much for the little motor mount to handle, not to mention it's way to big of a prop for that plane. It would look silly.
 

Chuppster

Well-known member
#20
I'm waiting for the 10x6 props to show up still. I tested last night on a 3S 1300 with a 11x5.5 prop. I didn't take notes for some unknown reason but if I recall correctly it put out 3.1 lbs of thrust, 18.x amps, about 240 watts. beastly little motor, I was quite impressed. I didn't think it would do anything close to that. I tried testing with a 12x6 but it was too much for the little motor mount to handle, not to mention it's way to big of a prop for that plane. It would look silly.
A 10x6 will probably make less thrust but you'll get more topline speed. With a 1200kv motor you shouldn't have trouble getting up to speed with the 11x5.5 but with the numbers you're getting I think it would fly better with a 10x6 or, even better, a 10x7.