Rules of thumb for electric flight

[Geoff S]

Whilst clearing out a lever arch file I came across an article I must have down loaded and printed back in 1999/2000. It's a presentation by Jim Bourke to the 1999 Electric Performance Rally in Dallas USA. I thought it may be of interest.

Watts/lb

30 to 50 for ROG

40 to 60 for sport aerobatics

70 to 100 for 'good' aerobatics

Not much different from now and we're getting more useful output power for input watts than they got with NiCads and brushed motors.

Orme's Law

1 cell (NiCad) per 35 square inches of wing area for sports planes

1 cell per 50 square inches for trainers

So with LiPos it's 1 cell/ 100 square inches for sports planes and 1/150 square inches for trainers.

Wing Loading

1 lb/square foot for very light planes

1 lb/square foot for sport planes

The writer gives a lot of importance to wing loading and aims for the lower rating.

Power Rules (for propellers)

Power is proportional to diameter ^4

Power is proportional to rpm^3

Power is proportional to pitch

These are rough approximations but he claims them to be near enough. eg doubling rpm means the power (and presumably the battery/motor/esc current) is increased by a factor of 8

Theoretical Pitch Speed

mph = pitch (inches) x rpm/1000

This is again a rough approximation (level flight, I assume) and in practice the actual speed is a bit less than this.

Duration

= 120/current in amps

Amps = 120/duratiom

This assumes a 2 aH battery so much the same as our 2200mAh LiPos

Obviously adjust for capacity. He does mention a 'miracle' battery with a capacity of as much as 4 AH! We're lucky.

 

Propeller diameter and pitch

The diameter to pitch ratio should be between 1.5:1 and 1:1

He suggests 1:1 is most efficient and it seems popular sizes like 12x6 are not very. I can't think what he would say about the 13x4 on my DB Tiger Moth. Trouble is with higher pitch the pitch speed goes right up as well which isn't what I want with a scale biplane. I think this issue touches on the static thrust measurement arguments, too ... oops!

Jim's Quick and Dirty Propeller Guesser

D^5 = I/(V^2 x Kv^3)

Where D is prop diameter in feet, I is the target current, V is the battery voltage and KV is the motor's rpm/volt.

He suggests that pitch is less of an issue because it affects the power much less than the diameter. In any case it will be greater than half the diameter according to one of the earlier 'rules'.

 

The whole paper is much longer than my post and offers several examples to support the rules. I think a lot of it is relevant to modern electric flight. They were struggling in 1999 (I know I was) and it was relatively expensive. Just thought it might be of interest to some of you.

Geoff

 

Edited By Geoff Sleath on 15/02/2016 15:51:44

[MaxG]

Looks a good set of rules and the modern ones do not seem to be far from them.

May well help the beginners in electric flight.

Maxg

[Dave Hopkin]

Interesting watts/Lb ratios - the "modern" rule of thumb seems to be 100 W/lb much lower...

[Delta Foxtrot]

One correction:

Power is proportional to N^3 and D^5 not D^4

Thrust is proportional to N^2 and D^4

where N is rotation speed and D is the diameter.

Edited By Delta Foxtrot on 15/02/2016 16:51:40

[Denis Watkins]

Interesting ,in that 5 of my indoor planes are 50 watts/Lb and less, 1 is 40w, and all 5 ROG and are fully controllable. And I do fly a foam/tape composite at 75w/Lb outdoors more than adequately. As we are Earthbound, we add weight and power cheeply, but a recent satellite left the Earths atmosphere at a cost of £50,000 per pound to reach orbit, and this was a cheap one. We are lucky.