Watts per pound revisited...

[stu78]

I know there is a general rule that states that you need a given number of watts for a given flight performance. i.e.

60 watts per pound for general sports flying
75 watts per pound for aerobatic sports flying
100 watts per pound for vertical performance.

(all numbers are from my memory and may not exactly match the numbers in your memory!)

I believe these numbers are watts IN, and must, therefore, assume a certain system efficiency to predict performance.

My guess is that these estimates were derived some time ago and were based on inefficient can motors (60-70% maybe).

Modern brushless (and some brushed) motors can now exceed 80% efficiency. so do the estimates above still hold true, or can you now expect to get near veritcal performance from an efficient 75watts per pound?

Stuart

[Bill Glover]

Many brushless motors aren't as efficient as you might think (certainly the 'budget' brands), often no better than a well-matched can motor. Of course AUWs have fallen significantly due to nimhs and now LiPos, few people seem to bother with watts/lb nowadays whereas it used to be a major fixation!

PS we'd probably need a new "3D" category now anyway! Normally 2:1 thrust to weight (or thereabouts) required.

[vintage1]

The watts per pound get split up as follows

- lost due to motor inefficiencies: Going from a 50% can to a 75% brushless nets you 50% more, but after that its diminishing returns. Yes, you can fly on 40W /lb where you used to need 60W/lb...

- gearbox and prop inefficiencies. These haven't changed, and typically sit at something like 70%.

- overcoming drag. Typically between 1/5 and 1/20th of the model weight, at cruise. Multiply by cruise speed and you get watts...

- climb rate. Or speed. Anything left over either makes the model fly faster or climb.

You can directly calculate climb rate from 'left over watts per lb' its about 44 fpm per watt of excess power.

If you work on overall drive train of 50% - 70% prop/box and 70% motor, and take a slow flyer with a glide angle of one in ten, that flies at 30mph, you get directly that power for level flight is 2 x 4.4 feet pounds force per second, per pound.

Or 12 Watts per pound input. Thats what it takes to keep a 30mph reasonably clean plane in the air with those inefficiencies. Until you try and turn it :D

It takes - at those inefficiencies - about another 4.5 watts per pound for every 100fpm climbrate you want to achieve.

Which is why an IPS parkflyer that might have 200 fpm climb, can zoom around on about 20 watts per pound.

I like betwen 400 and 600 fpm, as a general guide, so that comes out to about 37.5 watts per pound all in at 500fpm

To go straight up at 30mph, (2640 fpm) takes 130 watts per pound, in this theoretical plane.

The issues of gearing, thrust and pitch speed are simply trying to improve prop efficiency to achieve what theory says you should..

Bu the fact remains, you can easily fly a plane in a sort of lightplane/scale fashion on as little as 30W/lb these days, or less.

LIPOS bring the weight down, and hence cruising speed, and that helps as well.

[peterangus]

The rules are well overdue for an update. They still hold true, but they need to be extended

Electric flight has developed greatly since they were established, more than ten years ago.

Much lower wing loadings are now possible, higher power loadings are attainable, and new styles of flying have evolved.

Look at www.rcgroups.com/forums/showthread.php?t=378838

[slipstick]

The rule as originally stated assumed good quality brushed motors like Astro cobalts. These have very similar efficiency to many current brushless motors e.g. around 70+%. Cheap can motors are often much worse which is why people kept complaining that the numbers didn't work for them :(.

Of course loads of people have played with the numbers since so who knows they were thinking ? E.g. in the early 1990s when the rule was first coined "vertical performance" didn't really exist and I'm pretty sure no-one had even thought of hovering a plane however common it may be now.

More to the point we now have excellent simulation programs like Motocalc. It's hardly worth bothering with over-simplified rules like watts/pound ?

Steve