Do you know how powerful your RC Jet is?
Chance are you don’t, so this quick guide will show you how to measure the Watts per Pound of your model aircraft.
It’s actually notoriously difficult to measure the performance of remote control aircraft relative to each other. You can measure top speed or climb rate, but how do you take into account the weight differences? Or the power plant?
The most common figures given to compare one model to another are:
Wing loading – g/dm2
Wing area – dm2
EDF Size – mm
Engine Thrust – g
There are of course many other measurements such as wingspan, ESC amperage, weight etc but these are the ones that you could use to reasonably compare rc jets like-for-like.
For performance sakes it would be tempting to only look at the engine thrust and wing area data. After all, if you’ve got 2000g of thrust on a jet with only a 3.5dm2 wing area (approx 450mm wingspan) then you’ve got a beast on your hands!
But again this is comparative and subjective as two people could look at the same numbers and get different ideas of which jet is better. There’s also the unfortunate fact that many suppliers don’t offer Engine Thrust data (likely because the manufacturers have not performed the tests). On top of that other measurements are sometimes not supplied and are difficult to test yourself – so clearly another solution is needed.
The answer is Watts per Pound, or w/lb.
Providing an easy measurement for cross-jet comparison the watt per pound measurement is also easy to work out for yourself. You only need a Multimeter and some scales and you’re good to go.
To determine the wattage of your aircraft take your Multimeter (watt/amps/volts meter) and measure in between the battery and your ESC – this will tell you your wattage, or number of watts.
Next simply weigh your remote control jet whilst fully loaded and ready to fly – convert the weight into pounds (lb) if it’s not already, and then divide the watts by the weight.
This should result in a figure that looks like this : 147 Watts / 1 Pound.
You’re running an 11.1v Battery with a 25A ESC, in an A4 Skyhawk for example, and you measure the wattage between them at 133.2 watts.
(if you can’t measure the wattage, simply measure the amps and multiply that by the battery voltage)
The aircrafts weight is 420g = 0.92 pounds.
So divide 133.2 watts by 0.92 pounds to get your answer of.... 144 Watts per Pound. Not a sloppy machine by any measure!
For comparison you can think of these figures as a guide:
0 – 50 Watts per Pound = A slow, gentle flyer.
50 – 100 Watts per Pound = A decent performer – comparable to powered gliders, rather than jets
100 – 150 Watts per Pound = Good performance – capable of basic and moderate aerobatics and a high top speed
150-200+ Watts per Pound = Scale fighter jets, advanced aerobatics and phenomenal performance all-round.
Now, are there any pitfalls you need to know about?
Well there aren’t any pitfalls as such but there are limitations to the Watts per Pound measurement when applied to jets. Many enthusiasts point out that you could have a fantastic W/LB figure, the battery could be pumping amps through your electronics and the motor could be screaming away – and your jet could still be on the runway.
If you taped Scotch tape over your exhaust duct that’s exactly what would happen, and the point is that thrust is the ultimate measurement. It’s devilishly tricky to get right though, and bench rig tests seem to show different results each time or for the same configuration etc.
In conclusion the watts per pound stat is very useful and is widely used as a benchmark, but be sure that your other factors (such as plane design, air intake etc) are all OK to avoid being misled by the numbers.