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Watts/lb misleading?

Vs Static Thrust etc

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Matt Carlton13/01/2021 21:19:58
240 forum posts
42 photos

Is the well used watts/lb formula a little misleading?

In trying out various things, I imagined a model. 50" span, 40oz AUW, intended for sport flying. So, 250W or thereabouts?

Plugging figures in.

Turnigy 2830/1100, 3S Lipo, APC 9x5 prop.

Nearly 1:1 thrust/weight ratio (0.8), 54mph pitch speed, 17A current.

182W. Or put another way. 73W/lb, which should be barely sufficient according to the watts/lb rule, but which seems, on the face of it, more than ample!

Why would I need to spec a 250W power system if 180W will be more than good enough?

Or am I missing something?

dan h13/01/2021 21:50:17
123 forum posts
36 photos

Hi Matt.

What plane is it.

A 50" wingspan sports plane would need a motor bigger than the 28mm one you have it seems a bit small

Bob Cotsford13/01/2021 21:54:24
8942 forum posts
498 photos

You are only missing that these guides are just guides and if you select 100w/lb then you would expect a sprightly performance. 60-80w/lb will give a vintage style performance to match a lightly loaded 50" model weighing 2 1/2lb, 120 w/lb will give an aerobatic performance for a 50" model weighing 5lb. 150 w/lb will give a modest 3D performance with a 50" model weighing 3 lb.  All these are IN MY OWN OPINION!!  I've flown WW1 50" bipes on 60w/lb and aerobatic models on 150+w/lb.

Nowt in life is simple, we can only get a ballpark figure as there are so many variables in terms of drag, speed, manouverability and pilots preferences.

Edited By Bob Cotsford on 13/01/2021 21:58:04

J D 813/01/2021 21:58:38
1750 forum posts
88 photos

I have found the watts per lb to be a guide that errs on the side of having more than enough power .

Denis Watkins13/01/2021 22:13:34
4684 forum posts
135 photos

Just because I could Matt, I built 18" - 4 8" span, Depron, EPP and Balsa models for indoor and outdoor flying

weighing from 100 grms to 1300 from

Generally flying 100 - 500 grm at 50 - 60 W/lb

And the 1000 grm flown at 75-80W/lb

Just because I could Matt

Matt Carlton13/01/2021 22:20:16
240 forum posts
42 photos

It wasn't a particular model as such, although it's similar to the Derek Woodward "Amelia" that I'm converting (.20 2 Stroke).

I'd originally decided on a 3530 with 331W on the basis that it'd fly on 2/3 throttle with a bit of spare if I need it as I don't want to have to fly at WOT all the time.

Really, I was just wondering how 'light' it would be feasible to get as the electric conversion is likely to be a bit heavier than the original glow engine.

I did just check the spec of the .40 powered SLEC "FunFly" that I fly a lot and that's about 150W/lb so it probably is right.

Edited By Matt Carlton on 13/01/2021 22:21:11

Peter Jenkins13/01/2021 23:47:57
1724 forum posts
314 photos

In general, I wouldn't expect an electric model to be heavier than a similar IC model. Remember you need to weigh them when the IC model is full of fuel. An electric motor is much lighter than an equivalent power IC motor. If you include fuel load in the equation with battery/ESC weight then I wouldn't think there would be much difference between them.

It all depends on what performance you want from your model. As I fly 2 mtr F3A models (competition aerobatics) I am limited to a max AUW of 5 Kg with the flight pack fitted. My motors produce around 2,800 watts (254 watts/lb). With that power they have unlimited vertical performance. That's 3.75 HP.

My Wot 4 with an Irvine 53 weighs in at 4.5 lb and the Irvine produces about 1 HP or about 750 w. That gives it 166 watts/lb but that is insufficient for unlimited vertical. With a throttle pipe on it I that requires a much coarser prop to keep the WOT revs the same so say 1.2 HP or 894 watts making 198 watts/lb. That is unlimited vertical.

I have no light weight floater type aircraft so I don't know what a viable minimum watts/lb figure would be.

Phil Green14/01/2021 01:26:14
1682 forum posts
349 photos

I use 100w per cc of glow.

Simon Chaddock14/01/2021 02:11:02
5844 forum posts
3100 photos

The Watts per pound is only a guide to ensure you have sufficient power, An electric motor can always be throttled quite happily and is likely to be more efficient at converting Watts to thrust if you do.

It is possible to fly on a lot less than the guide. I got a 48" span plane to maintain height using just under 14W/lb.

SIMON CRAGG14/01/2021 05:11:19
681 forum posts
5 photos

I have converted most of my model fleet (18), to electric.

From powerful 3D TO 1/4 scale.

I have used e.calc as my guide, and all the models have turned out to be more or less exactly the same weight as they were with an ic engine.

The beauty of e.calc is that you can try unlimited variations of lipo / motor / prop etc.

Its certainly not as easy as "Its a .46, a 10x6 APC will do it" !.

e.calc do point out a 10% error margin as well.

Matt Carlton14/01/2021 05:41:00
240 forum posts
42 photos

Thanks everyone. It just seemed that the W/lb measure was not necessarily telling the whole story. But I'll lean on it as a guide.

100W/cc looks interesting actually.

Converts the .20 size model I'm doing to around 300W which would fit in with an AUW of 2.75lb.

Sorry about my general ignorance on this!

Chris Walby14/01/2021 07:33:45
1455 forum posts
386 photos

Slightly off topic, but IMO its a guide and thus only a guide for general consideration. A good starting point however the more performance (extreme in any direction) then the less useful it is.

I had a funfighter and fancied the need for the end it was pushing 400 w/lb but really went little faster than the original set up. Due to quite a thick wing, frontal area and high drag it was never going to go fast no matter how much power was dissipated by the electric set up.

If the model has a high wing loading or low lift wing as part of the design then the power train will be designed to match the performance required, my ham fisted attempt achieved little compared to designs that are intended to fly fast in comparison.

I think the really clever/good designs are those that achieve very wide ranges in flight performance without any nasty traits. They are easy to they are very popular with model flyers.

Peter Miller14/01/2021 10:00:49
11758 forum posts
1416 photos
10 articles
Posted by Chris Walby on 14/01/2021 07:33:45:

I had a funfighter and fancied the need for the end it was pushing 400 w/lb but really went little faster than the original set up. Due to quite a thick wing, frontal area and high drag it was never going to go fast no matter how much power was dissipated by the electric set up.

The reason is simple. If you want to double the speed you need four times the power because doubling the speed increases drag 4 times. This a known constant

Nigel R14/01/2021 10:45:13
4396 forum posts
717 photos

Airplanes go fast when they are low drag.

"100W/cc looks interesting actually."

100W/cc is a pretty good rule of thumb for a glow 2 stroke equivalence.

80W/cc works well for glow 4 stroke equivalence.

Maurice Dyer14/01/2021 11:39:52
173 forum posts

Just finishing a Flair SE5a, modded for sparks. My first one weighed in at a tad over 5lbs and had an OS52 and is lovely. This one has a motor I've propped for 700 watts on 4S. About one horse, which is what the OS was putting out.

Makes sense or ??.


Geoff S14/01/2021 11:46:16
4025 forum posts
68 photos

As several have already said- it's just a guide. Moreover the 100 watts/lb rule of thumb was used when electric drive trains were rather less efficient with cheap brushed can motors being the most commonly used. My Ballerina, for example, flies as well as I need on about 80 watts/lb.

I've converted a few models designed originally for glow to electric power and the electric drive trains tend to be lighter. Unfortunately, the model as a whole has to be the same or a similar weight to when it was glow powered because it still has to balance in the same place. The only way to reduce the weight and take advantage of the lower weight of the motor/battery combination is to lighten the tail or lengthen the nose (difficult in a scale model).

I like to have the potential for greater power by choosing heavier motors and higher current escs but prop for more modest power. The flexibilty of electric drive trains is both an advantage and a problem because it confuses until properly understood. But isn't understanding part of the attraction?


Peter Christy14/01/2021 14:19:53
1947 forum posts
Posted by Phil Green on 14/01/2021 01:26:14:

I use 100w per cc of glow.

I'm with Phil on this one, as it actually works quite well for helicopters, too! Watts per pound doesn't work at all well for helis, which tend to be much heavier for a given engine size than their fixed wing counterparts.

(Big "prop" turning slowly is more efficient!)



Geoff S14/01/2021 17:06:40
4025 forum posts
68 photos
Posted by Maurice Dyer on 14/01/2021 11:39:52:

Just finishing a Flair SE5a, modded for sparks. My first one weighed in at a tad over 5lbs and had an OS52 and is lovely. This one has a motor I've propped for 700 watts on 4S. About one horse, which is what the OS was putting out.

Makes sense or ??.


Not sure if your description of your Flair SE5a as 'lovely' was intended or if it was a typo for 'lively' but my similarly powered SE5a's performance would fit both. Though mine is quite a bit heavier at almost 7lbs in both glow and electric versions.

Unfortunately I haven't been able to fly it since the conversion but it's turning a 12x6 prop at around 8500 rpm, about the same as the OS52 did. It's also on 4S but didn't need any of the lead ot needed in its glow form, mostly because the battery (4S 4AH) is right at the very front over the motor. IIRC the power is also about 700 watts which is the magic 100 watts/lb and probably far more than is needed. I'm looking forward to its test flight whenever. I had my Corvid (Pfizer) vaccination today, so here's hoping yes


Matt Carlton14/01/2021 17:46:29
240 forum posts
42 photos

As a side note, I assume that if one were to find that a particular model was overpowered, such that full throttle were rarely, if ever, used, does it make sense to 'under prop' to reduce the wattage and current draw to keep things cooler and happier?

It looks like there's a fairly wide range of suitable props within any set up, certainly compared to IC and definitely less fiddling with tuning, exhausts etc.

So, for a slow flying model like a Junior 60 say, a big, low pitch prop, on a hefty motor should be fairly efficient? So less current = less capacity required for same flight duration = lighter weight = less power required!

Is it possible to gear an outrunner down to drive some really slow high pitch props like those used for rubber power??

Dad_flyer14/01/2021 20:52:01
355 forum posts
320 photos

Certainly it can make sense to prop down a bigger motor. It is probably only really necessary if you are using so little power that the stick travel is too small to be comfortable.

For example, I have a 50-odd inch trainer, 3 1/2+ lbs. Initially I ran it on a small motor, 3s and 300W. Flew fine, but not much in reserve, everything was at 3/4 throttle or more and felt on the edge. Now I have a bigger motor, 4s and max 550W. I expect I still actually _use_ well under 300W most of the time, but it is much nicer to fly around with the stick near the middle. There is also plenty of go for many practice landings in a flight.

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