Choosing Brushless Motors to fit Vintage Models


My way of choosing modern brushless components for vintage model aircraft designs.

This explanation is meant for vintage style FF and RC models of around 30 to 50in span. I have little experience with very small or very large models, so we're talking Tomboys, Matadors, Deacons, Cardinals, Schoolmasters etc and getting them to fly the way they were meant to fly when they were first designed.

Pulling the information together

I always write my build notes down in a notebook. When I build new models, I refer to my notes from previous ones. It helps. Particularly useful are component weight data, engine test runs, how I set up and programmed the RC components etc. And, of course, what went wrong.

There's helpful information to get us started on Outerzone plans!

Let's take the Outerzone Frog 45 Mk II plan [see oz2249] as an example (see fig.2).

Engine used : Frog 150
Weight of the model : 14 - 14.5 ozs
Wing area : 260 sq ins (but always check this)

fig 1. Frog 45 mkII with diesel engine and 8x6 prop

Now start hunting for test data on the engine. A really good place to look online are sites like and in old Aeromodeller Annuals. If the plan doesn't specify a particular engine, at least it will mention an engine size e.g. 1 - 1.5cc. Look for engines from the time period when the plan was published.

Remember that the test data will be directly applicable to electric power as the air only 'sees' the propeller and doesn't care what's turning it. I got used to this in Germany as car engines are no longer specified in horsepower but in Kilowatts. Some EU ruling I believe. Watts don't just apply to electric motors.

fig 2. Specification for the model

The Frog 150 test data from the February 1956 Aeromodeller (see fig.3) shows the Frog 150 peaked at 0.108 BHP @ 12,400 rpm with a 6x5 Stant propeller (see fig.4). The plan (see fig.1) shows an 8x6 (presumably the nylon one from Frog) which gave 0.076 BHP @ 7 400 rpm. Now the magic : convert BHP to Watts (1 BHP = 745.7 Watts) and we get around 57 Watts.

The 57 Watts from the engine test were measured at the prop, but it's easier for us to work with the Amps and Volts at the battery as we can easily measure there with a Wattmeter (fig.5). They're not expensive and provide valuable data (like avoiding overloading the ESC). So we add a fudge factor of 20% for efficiency loss through the ESC and come up with roughly 70 Watts power from the battery to the ESC.

Now we have some really useful information! If we start by using round numbers of 7 Volts from a 2S Lipoly and 11 Volts from a 3S Lipoly we are looking at a setup of something like:

2S Lipoly @ 10 Amps or 3S Lipoly @ 6.4 Amps. That would mean choosing an ESC of 20 Amps for the first case or 15 Amps for the second case. Always leave the possibility to increase power later when choosing the ESC.

For battery capacity I usually use a battery weight of roughly 20% of the total model weight and look for a battery to match that.

We can repeat the sums later with a calculator.

fig 3. Frog 150 engine test pages

So now we look for a brushless motor. As we want to be able to adjust the power downwards later, we are looking for a motor that will turn an 8 ins prop at 100 - 120 Watts to comfortably cover the 70 Watts. My buddy Leo has an early 90's ZR1 Corvette which has two engine power settings depending on which key you use. It's sort of the same idea.

We will probably find a suitable outrunner brushless motor in the lower kV values i.e. 1000 - 2000 rather than 3000 - 4000. I usually start looking for motor data on the Hobby King site. The reviews often show test data from customers which is a great help. I trust these the most. One possible motor would be the FC 28-12 1534kV. An HK customer, Steeve, gives test results of : 10.2A / 114.7W on 3S and a 8x4 GWS prop. Sounds good! As the motor will go to 15.5 Amps according to the specs, there is some upwards stretch if needed. I used this motor in my Ken Willard Schoolmaster and can recommend it.

What's also very helpful is the motor weight which is given as 39g. The Frog 150 (3 ozs = 86g) plus tank, heavy 1950's prop, engine bearers etc would probably be well on the way to weighing three times that of the brushless setup, so the warning bells start ringing with regards to getting the model to balance in the right place. As is usually the case with vintage models, we need to do the conversion so that the equipment is well forward and the tail kept light. On the Frog 45, the rear fuselage side sheet will have to be very light stock. Maybe even punch in some holes. If it looks almost hopeless, like on a model like the 'Karoro' [see oz4334] it will mean stretching the nose. I have done that myself on one model, but purists may prefer keeping the dimensions and adding lead. I have a semi scale Piper Cub I converted from brushed to brushless and it required 80g in the nose. That's easily the amount that a Frog 45 could need without careful planning. However, whatever it takes, make sure that model balances at the right place.

There is nothing to be gained by picking an extremely light motor or prop for such models. It's the wrong place to try and save weight.

fig 4. Engine test data

Model weight and wing area

A good starting point is 12 ozs/sq ft. Although I have lived in Germany for 40 years, I have never been able to get a feeling for grams/sq decimeter. Check the wing area excluding the fuselage section. I get approximately 256 sq ins, pretty close to the given 260 sq ins (not always the case). 256 sq ins = 1.78 sq ft. At 12 ozs/sq ft that means a flying weight of 21.33 ozs/sq ft = 606g. That sounds a bit too high for the added RC equipment compared to the FF weight of 14-14.5 ozs (397 - 411g), so a better target may be 540g = 10.7 ozs/sq ft.

A rough guide on where to start with component weights is :

Airframe : 50%
Motor & Prop : 10%
Lipoly : 20%
RC Components : 10%
Miscellaneous : 10%

Keep a running check as you build to avoid surprises. If you catch 'indiscretions' early on, you can work hard finding lighter solutions later on. So for 540g flying weight this would mean :

Airframe : 270g : Easiest place to lose (and gain!) weight.
Motor & Prop : 54g : Fits in well with the 39g motor
Lipoly : 108g : This would mean a 3S1000
RC Components : 54g : 2 Servos, Rx and ESC at an average of 14g each
Miscellaneous : 54g : Usually the small bits added after the airframe is built

A check back to the plan data gives some confidence regarding the airframe weight. With the original upper limit of 14.5 ozs (411g) and taking away an estimated weight for the engine, tank and bearers of 5 ozs (140g) we get 271g. With careful construction there's a good chance of getting below this!

fig 5. Wattmeter

Getting ready for the first flight

I started with electric flight 20 years ago. In those days it was with Speed 400's and 600's and there was never a surplus of power. These days it is far too easy to grossly overpower a model and make the first flight a nightmare. Who would put a 2.5cc Oliver Tiger in a Frog 45, so why do it with electric?

This is what I have checked out on at least 10 models and is now my standard procedure :

Buy or borrow a selection of props. For the Frog 45 I would get some 8x4's, 8x5's and 8x6's. You will be surprised at the big differences in performance, even between different makes with the same diameter and pitch. I am now using props intended for glow motors as they are much more robust and put weight at the right end of the model. A prop saver is a must to ensure that the motor shaft doesn't get bent in a nose over landing.

When the model is finished, take the final model weight ready to fly in grams and divide by 6, 7 and 8. If the Frog 45 did actually turn out at 540g, that would be 90, 77 and 68. I use the 3 position switch on my Spektrum via Flight Mode to restrict the 3 full power throttle settings on a Wattmeter to 90W, 77W and 68W. It's what they do on RC helicopters - it's called 'throttle curve'. Try the different props to find one that comes closest to the maximum 90W at unrestricted throttle.

If you only have a 2 position switch, take 77W and 68W. If you don't have a throttle curve function on your transmitter, find a prop that gives as close as you can get to 68W. Normally you can't just simply reduce the throttle throw on the transmitter as most ESCs will reset to full travel when you switch on and defeat what you just did.

My buddy Harald & I each built a Schoolmaster at the same time. Harald fiddled with different props and got nowhere near to 68W (always too high), so he dropped down to a 2S from my 3S and it was immediate success.

The first flight is launched at the lowest (i.e. 68W) setting. It WILL fly on that amount of power. Remember launching diesel FF models on trimming flights with the compression backed off and the motor gently misfiring? It's the same. If you want more power, switch to 77W or 90W. At a setting of weight/8 you will not be all over the sky for those first, anxious seconds.

As an example : I built a Tuff Trainer 2 foamy from Hobby King for my son. I tested the manufacturer's setup on the model with my Wattmeter and it gave 236W. The model weighed 542g. I set the throttle curve to 70W for the first flight and it flew very nicely. After trimming I upped the power to 90W. I cannot imagine having done the first flight with 236W. In a raw beginner's hands it would have lasted 2 seconds. In mine, probably not much longer.

Hope this helps!

Chris Pinn



10th January 2016


User comments

Chris, I really enjoyed your article, it had never occurred to me how you converted horsepower to watts. I've done similar conversions like you have of vintage designs, not all work out OK without changes. My most recent Ken Willard Roaring 20 turned out nose heavy and I had to cut some off the nose and still move the two tiny servos to the tail, finally balanced without any added weight. I was able to make these changes during construction, checking the balance at each stage, that way I didn't have to un-do anything after it was finished. It seems most all the vintage designs can benefit from weight reduction, no electric vibration means you can build lighter without problems. As an example, my Sig Kadet Senior (68") weighs exactly 4 lbs, with battery, about half the usual weight. A small-looking AXI 2820 pulls it just fine at about 400 watts, takes off in about 10 feet. It's good to see someone else contribute to Viewpoint, I thought I might be the only one. Gimme some more.
DougSmith - 23/01/2017
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