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Aug 23, 2019, 09:16 PM
Foamy and Glider fanatic
Blacky's Boy's Avatar
Thread OP
Question

Motor power question


OK, here's the deal, I'd ask this over in the warbirds forum in the Wildcat thread but after being dog piled the last time I was there I get the feeling that ANY questions I ask there is labeled "stupid" and worth only derogatory comments. Sort of like dealing with an electronics professor who starts busting on you for spelling mistakes rather than answering the question you ask. So I'm coming here because I know I'll get the straight poop without a lot of drama.

So a buddy gave me a Parkzone F4f Wildcat that he augered in. I repaired it and dropped a new Leopard LC-3536-5T 1520kv motor in. Turns out it's SCARY fast. I mean it's exhilarating as hell but WAY too fast for me. One of my FBs who loves jets took it for a spin and LOVED it. But he's a speed freak. When I take it up I'm flying fast (for me) at a quarter throttle. So I want to drop a lower power motor of the same form factor in it's place.

This plane came with a 980Kv motor installed and I don't have a lot of info about that one. BUT I do have all the info on those Leopard motors.

My choice for a replacement is a LC-3536-6T 1270kv motor or a LC-3536-7T 1100kv motor. I was wondering if someone could give me a hand choosing between the two. Or perhaps tell me that the differences are negligible.....

I fly with a 9045 prop and a 3s 1300mah Lipo. So I'm using the data relevant to this configuration. For your reference, the F4f Wildcat weighs 774g with the lipo installed.

According to the chart I linked to, the 1520 kv motor develops 333W, the 1270kv motor develops 260.85W, and the 1100kv motor develops 183.15W.

If we look at this on a percentage basis using the performance of the 1520kv motor as a metric, there is 22% difference in power output between the 1520kv motor and the 1270 kv motor. And there is a 45% difference in power output between the 1520kv motor and the 1100kv motor.

The way I see it, if I go with the 1100kv motor I'll be getting just a bit less than 50% of the performance of the 1520kv motor. Which might not be a bad thing. But I would still like a bit of "get up and go". So I'm more inclined to go with the 1270kv motor.

Is my logic sound here or am I barking up the wrong tree?
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Aug 23, 2019, 10:53 PM
Registered User
rajazo's Avatar

Motor power question


Well, your are right with the math but is not too easy the motor propeller “science “. Lot of things are involved in the performance of the plane and specifically about speed you need to check some things:
- check if the plane geometry can fly at the speed you want... wardbirds never ever could fly as slow as a trainer.
- about watts, it’s a good approach to define the class of the motor that you need, but is only the beginning . Check this rule of thumb about watts vs weight

25 W/lb = minimum for level flight, with a reasonably clean plane.
50 W/lb = Trainer/Casual/scale flying
75 W/lb = Sport flying and sport aerobatics
100 W/lb = aggressive aerobatics and mild 3D, effortless loops from level flight.
150 W/lb = all out performance.
200 W/lb = Unlimited high-speed vertical flight.

(Info posted by RobinBennett in this thread https://www.rcgroups.com/forums/show...o-Weight-Ratio, and I followed this rule with success

-lot of variables produce a desired (or not) result , a certain kv involve speed turns , pitch of the prop, airspeed produce by turns, more pitch more resistance per turn, then need more power (or can not reach to much turns), bigger propellers move more air by turn, then more thrust or same thrust but with less turns, more power more weight and so on... a complex mess... but be patient and check the last point


- I check the specs of the plane and the motor that you installed is too big (heavy) over 120grs vs the original motor that should be weight 50grs. A heavy motor increase the weight of the plane, then the wing load, then the stall speed. About the power, If you check the previous point, you put near 240w/lb.

- the slower motor that you propose should be reduce the top speed, but I would not buy it because their weight.
Check this https://alofthobbies.com/dualsky-eco...0kv-motor.html motor.

-if you want to check and play about motor propeller “science “ check the ecalc calculator. With this you can get a close idea about the results of your combinations, including level protected flight, top speed and lot lot more. Worth every dollar you pay here
https://www.ecalc.ch/index.htm

Have a nice day
Last edited by rajazo; Aug 23, 2019 at 11:12 PM.
Aug 23, 2019, 10:55 PM
IMO ( In My Opinion ) →
balsa or carbon's Avatar
The main determining factor for the slowest speed ( minimum stall speed ) a plane can go is wing loading . If the weight-to-wing-area ratio is high ( high wing loading ) , there's no way to slow down the minimum stall speed ..... unless you can reduce the weight and/or increase the wing area . If you want to slow it down , your best bet is to use the lightest motor/ESC/battery that will still give the plane a flyable CG .

Also reduce the pitch ( second number ) of the propeller .
Aug 24, 2019, 08:10 AM
treefinder
springer's Avatar
BB, spinning off what you are calculating and the other guys said, the first thing I would try is propping down (understanding that from what Rajazo said, that it is over motored). If you are running a 9x4.5 prop now, try an 8x3.8 prop - I would suggest a lower pitch 9" prop, but don't think you will easily find one. Your pitch speed, and ultimate top speed is proportional to the motor Kv and prop pitch. Remember the prop is a screw more pitch goes faster if there is sufficient power and sounds like you have plenty. Reducing pitch reduces top speed while reducing diameter reduces thrust (swept area).

Considering that the original motor was 980kv and 50grams, and probably flew acceptably; at higher motor weight and 1.5 times kg, you just made a rocket! I'll bet it's fun to fly!

If it balances ok with current motor and lipo setup, I would switch props first, but if you decide to redo motor, I have had great luck with the Racestar 2212 from banggood https://m.banggood.com/Racerstar-BR2...l?rmmds=search it is currently on sale for $6(!) 50 gram, a Kv just a bit higher than original should be a good fit.

The 2212 is an interesting motor, comes in 1000, 1400, 1800, 2200, 2450Kv ratings. So same weight & form factor, many Kv. I have used 1k, 1400 and 2200 so far.all worked well.
Aug 24, 2019, 11:19 AM
Foamy and Glider fanatic
Blacky's Boy's Avatar
Thread OP

great info!!


Thanks you all for the great responses. Lots of good stuff to go through here. I have a couple of 8" props laying around and will give them a shot before I go and buy that new motor.

One thing I want to add is that the weight of the new motor was never an issue. With a 1300mah Lipo installed, it balances out perfectly and flies like a dream. Just a tad on the FAST side. I know this video doesn't do it justice but look at the 2:04 mark and it may give you an idea what I'm talking about.

Wildcat F4F video 1 (2 min 41 sec)


Additionally, this plane LOVES to glide. I killed the power with it at least 50 feet above the ground and was able to make a full circuit of the airstrip with no power and almost hands off. One of the guys there even commented that even my warbirds behave like gliders (cause glides make up about 50% of my hanger!)

One of the reasons I chose this motor was the form factor. It's almost identical in size to the stock motor and mounts right to the stock motor mount without any adaptation. The shaft is the perfect size to take the stock spinner and it has enough length to clear the cowl.
Aug 24, 2019, 11:48 AM
treefinder
springer's Avatar
Yup,I you got a rocket. You may find after flying it like that, when you prop it down it will seem slow! I think I would just keep flying and learn how it needs the left stick modulated. Then you can fly "scale" but still show off when ya want!
Aug 24, 2019, 12:56 PM
Registered User
Perhaps try current setup on 2s instead of 3s?
Aug 24, 2019, 02:28 PM
Registered User
rajazo's Avatar

Motor power question


the video clarifies everything, it’s rocket capable but fly ok at low speed.
Both solutions posted above will work (lower kv or 2s same weight).
Another solution is make a program mix to limit the working range of the throttle, activate for normal fly and switch off for WEP mode (or rocket mode).
You could use same switch to change dual rates and Expo of the controlls to get different sensibilities at high speed
Aug 24, 2019, 05:56 PM
Foamy and Glider fanatic
Blacky's Boy's Avatar
Thread OP
Quote:
Originally Posted by rajazo
Another solution is make a program mix to limit the working range of the throttle, activate for normal fly and switch off for WEP mode (or rocket mode). You could use same switch to change dual rates and Expo of the controls to get different sensibilities at high speed
Oh, that's a GOOD idea!! I need to investigate this further.

Oh BTW, I flew this plane with an 8045 prop today. It was a lot tamer. It's still got a lot of pep, but it wasn't as freakishly fast.
Aug 25, 2019, 03:03 AM
flyin' fool
goldguy's Avatar
Quote:
Originally Posted by Blacky's Boy
……………. the 1520 kv motor develops 333W, the 1270kv motor develops 260.85W, and the 1100kv motor develops 183.15W.
Those are watts 'IN' readings, 'volts X amps', and nothing to do with power at the prop or watts out. Watts 'in' is the amount of power the motor CONSUMES to turn the prop at the RPM your telling it to do by the mount of volts (RPM) your feeding it. Generally it's interpreted as how efficient a motor is.

It's the thrust that flies the model. So, using different motor KVs, an assortment of props, a watt meter to make sure you don't fry anything and comparing the thrust is the most accurate way. Then again, if done on the bench it's static and will register more than when unloaded when actually flying.
Last edited by goldguy; Aug 25, 2019 at 03:22 AM.
Aug 25, 2019, 03:24 AM
flyin' fool
goldguy's Avatar
There's lots of ideas here on RCG on how to make a thrust stand or thrust measuring devise.
Aug 27, 2019, 11:57 AM
flyin' fool
goldguy's Avatar
I always work from the prop back. First I decide which prop I want to turn, then which bits will allow me to get the best performance at the lowest amp draw.


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