electric flight uk - British Electric Flight Association - Jan Bassett's

Spitfire IX Boucher (1/3) Bunting (1/9)
Stall Speed 70 23.3 7.8
Manoeuvre Speed 215 71.7 23.9
Maximum Speed 408 136.0 45.3
The Balsacraft Spitfire Mk IX is 1/9th scale and will be used to test the figures
above. A typically flying weight for the Balsacraft Spitfire with a Graupner Speed
600 Race 8.4v motor and 8 RC-2400 cells is 1.6 kg (3lb. 8oz.). This is a 14 oz.
below the Bob Boucher weight of 4 lb. 6 oz.
For this weight ElectriCalc estimates a stall speed of 17 mph. The Bob Boucher
figure is reasonably close and will be achieved. The figure using the scheme that
John proposes (7.8 mph) is not achievable unless the model weighs only about
300g (~11 oz.). Even if this low speed could be achieved, I don’t think that a
Spitfire would look credible at only twice walking pace. Certainly when it comes
to stall speed, I have to side with Bob Boucher.
It must be remembered that Bob Boucher says that the weight and power should
be reduced to one half of the calculated values for high power aircraft. This reduction
in power will inevitably mean that scale speeds will not be achieved. The 50% loss
of power will mean a speed reduction of around 25% at maximum power.
Another important factor at the upper speeds is that it is extremely rare for a
model to have a variable pitch propeller. This means we have to compromise on
propeller selection to get acceptable performance for take-off and climb out.
Applying the 25% reduction to the maximum speed from Bob Boucher’s system
gives a speed of 102 mph. This speed is still excessive and couldn’t be achieved
with the 266W (at the propeller) available at half scale power. Using this power,
it is probably realistic to expect 65 mph allowing for the scale lines.
I am fortunate to see the Royal Navy Historic Flight Hawker Sea Fury FB11 fly
regularly at RNAS Yeovilton. When you see this superb war bird hammer across
the sky at around 450 mph it is an amazing sight (and sound). It is surprising
how much sky an aircraft covers at these speeds, at a scale distance you could get
whiplash. Because of that I think that John’s maximum speed of 45 mph is just
too low, the model would appear very slow.
I have to agree with John about the times to complete aerobatic manoeuvres, but
it is a tricky problem. I think it important that the Manoeuvre Speed is considered
here as this is typically the limiting speed for such antics. The Manoeuvre Speed
using John’s recommendation is 23.9 mph, which would make it almost impossible
to complete a loop. Again Bob Boucher’s speed is unreasonably high and would not
be achievable.
The solution is probably to fly at an intermediate speed and to enlarge the diameter
of the loop slightly. You may not get to 10 seconds, but it will look better.
10 E.F.-U.K.