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Calculating rotor sizes

PostPosted: Wed Apr 03, 2013 11:03 pm
by jomac
Is there a set formula for calculating max rotor sizes from a given engine power?

In my case i want to use a 2 Hp 20cc engine to power a tandem heli, similar to a Chinook and i would like to find out if my (hoped for) rotor sizes are optimistic or realistic!

I would be half way there if i could find a large tandem heli and know what the engine sizes are, this would give me a rough idea.

All comments welcome.


Re: Calculating rotor sizes

PostPosted: Thu Apr 04, 2013 12:01 am
by cynr100
Not quite the same scale (engine wise) but may help

Re: Calculating rotor sizes

PostPosted: Thu Apr 04, 2013 6:17 am
by jomac
Hi Cynr,
I actually did find that link useful, and im greatful. Im not sure if im approaching the situation from the right way, but looking at what was required to drive a 1Mtr rotor(approx) which was a 50 nitro engine and what was used to drive a 1.5Mtr rotor which was a 20cc petrol engine, and looking at the swept area of the rotors, im hoping im not to far out using a 20cc 2 Hp engine and 2 90cm/1Mtr rotors.

But there again ive seen "quoted" horse power ratings vary wildly for the same size cc engine. Ive a 3.5cc engine with a higher rated Hp figure compared to a 10cc engine i have.

But surely there must be a more scientific approach to this problem?

Re: Calculating rotor sizes

PostPosted: Thu Apr 04, 2013 8:27 am
by cynr100
Depends on the design application of the motor.
I had a 2m stiletto f/glass RC speed boat, originally it was equipped with a purpose built long stroke 23cc Zenoah w/cooled 2 stroke. I wrecked that during an "Oh my God" incident with a very large wave ( I was wave hopping 0.5 m waves at 50kph, got the bounce wrong) anyway I digress. The replacement was a short stroke Homelite 30cc 2 stroke, RC adapted for marine application and went like a pig, wrong motor for the wrong application, it would have been better on an air boat.
I have never liked HP ratings I prefer torque values that is where you get your real mojo.
Your 3.5cc may be rotating at 4x the speed of the 10cc to get the higher Hp rating but which one is going to turn the largest prop.
A 1m rotor is a fair size, I would opt for an engine that has a high torque rating (if you can get that info). You might also consider a diesel engine, known for there higher torque values compared to the same size 2 or 4 stroke petrol.

Re: Calculating rotor sizes

PostPosted: Wed Apr 10, 2013 1:23 pm
by jomac
More useful info to digest, i fully understand what you are saying, unfortunately this version of the engine was destined just as a grass and weed cutter, so the carb and silencer arent exactly tuned for high performance. The manufacturers website only lists data on the revised engines which were modified for RC use.

I do have a couple of nitro engines and the specs exceed this engine, but nitro is not a fuel i want to use. After many years in engineering ive developed a 'gut ' feeling when it comes to using an electric motor for a particular job, its this gut feepling im trying to use here.

Many sites seem to suggest that this engine 'may' do the job, and ive not seen any information to say it wont