Setup feasibility question - RC Groups
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Mar 07, 2012, 05:51 PM
Registered User

Setup feasibility question

Hi everyone,

I was wondering if you could tell me if this setup is realistic. We are trying to design a plane that is as light as possible, so we are trying to cut the propulsion weight down (the structure is as light as we can afford to go already). If we have a 3 lb. plane including propulsion, would the following setup have enough power for us to fly? We don't need to be very maneuverable or fast - we just need to be able to fly.

Two prop system:
2 packs of 9 cells in parallel - Elite 1500 NiMH (we're not allowed to use Lipos)
Hacker X-20 SB Pros (we're not allowed to go over 20A)
Neumotors 707 Neutrons 3125 Kv, 150W
4.75 gearboxes
Props TBD - the best we can get without breaking 20A

Thanks for your help!
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Mar 07, 2012, 08:30 PM
jackerbes's Avatar
The wing loading is an important consideration, do you have any idea what the wing size is going to be?

In general it is considered that it takes about 50-70 watts per pound to fly a plane, that would be like the minimum level of power for decent performance in park flyer/slow flyer models.

Most planes will sustain flight on less power then they needed to take off nicely, once in the air it is not unusual for planes with wing loading around the 15 oz./sq. ft. or so area to be able to maintain flight on 25-30 Watts per pound.

We really need more details or the specs for the plane go give you a good answer.

Mar 07, 2012, 11:31 PM
Registered User
Originally Posted by jackerbes
The wing loading is an important consideration, do you have any idea what the wing size is going to be?

Right now our wingspan is 52". The wing area is 558 sq. in. and we have an aspect ratio of 5.8.

We are also using a V-tail (deflected at 30 degrees) with a span of 12" (per side) and a chord length of 2.1"
Mar 07, 2012, 11:55 PM
Registered User
You didn't say what the flight task is, but note that at 11 volts (~9*1.2) if you pull 20 amps you will need to dissipate close to 220 watts. That's input watts, but if your motors are only rated for 150 you will be limited to about 13 amps for long term running. You might get away with 20 amps for a short burst, but over 30 seconds of that may cook the motors.

Mar 08, 2012, 07:44 AM
jackerbes's Avatar
if the 18 NiMH cells are 25 grams each they will weigh 450 grams.

If the wing is 558 sq. in. (3.9 sq. ft.), and for the plane to have a favorable 15 oz./sq. ft. loading, it would need to weight about 58.5 oz. or 1660 grams (3.65 lbs).

The two motors will have a combined continuous power of 300 Watts and, against the weight I mention above, that would be about 82 Watts per pound. And that is more than enough for basic flight. Here are some rules of thumb for electric flight power requirements:

Approximate power requirements:

50-70 watts per pound; Minimum level of power for decent performance, park flyer/slow flyer models
70-90 watts per pound; Trainers and slow flying scale models
90-110 watts per pound; Sport aerobatic and fast flying scale models
110-130 watts per pound; Advanced aerobatic and high-speed models
130-150 watts per pound; Lightly loaded 3D models and ducted fans
150-200+ watts per pound; Unlimited performance 3D and aerobatic models

To get the 150 Watts from each motor, the 9 cell packs would have to supply 15A at 10V and I think that might be were some problems start to show up.

I've not used NiMH packs much at all and I'm not sure what discharge rate those cells will give you or for how long. Once in flight, you can reduce power and you may be able to maintain flight on as little as 1/3rd or 1/2 of the 150W so maybe you'll have a chance.

But it looks like you have a chance if the batteries are up to it.

Mar 08, 2012, 01:12 PM
Registered User
I'll keep those guidelines in mind. The cells have a discharge rate of 20A, so I'm pretty confident they can get the job done.

Thanks so much for your help!

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