On a 7 kW motor being used to accelerate the
glider about 6 kW is going into the kinetic
energy of the glider. Only 1 kW or so appears as
heat in the motor. Maybe a kW or two in the tyre during takeoff roll.
Now take a 500Kg glider touching down at 20M/sec (about 40 knots).
Kinetic energy is 1/2 mv^2
so 250 * 20 * 20 joules = 100 kilojoules
If the glider stops in 10 seconds the average
rate of energy dissipation is 10 kilojoules/sec or 10 kW.
Note that it doesn't matter whether you use the
wheel brake or not. ALL the energy is dissipated
as heat anyway. If you don't use the brake where
do you think the heat ends up- yes that's right the tyre.
I'm aware of some fires caused by glider wheel
brakes where the glider was being towed to the
takeoff point with the wheelbrake at least
partially applied but that is a different matter.
When taxiing at low speed the power dissipation
is likely to be only a few tens of watts in the
motor while the motor delivers a few hundred
watts drive to the glider which gets dissipated
as heat in the tyre while taxiiing at constant speed.
As we don't hear about fields being set on fire
by powered aircraft tyres much ( hot exhausts are
a different matter) I don't think we need worry.
I'm also sure the Glow people looked at the
alternatives to the dual wheel before choosing it for good reasons.
-------------------------
The current crop of two stroke two cylinder
motorglider engines aren't too bad. It is just
that the vibration and other characteristics
cause problems with other parts of the
installation such as reduction drives and belts,
ignition systems failing, engine mount cracking etc etc.
I've owned two and had a fair bit to do with a
few others and heard from owners and maintainers.
I put them at the level of 1950s Pommy
motorbikes. Can be made to work reasonably if
thoroughly inspected and frequently maintained
(particularly if you get rid of the Lucas electrics:-) )
Mike
.
At 07:15 PM 21/04/2015, you wrote:
Good one David,
Actually the electric motor only runs for
seconds, not minutes to get the glider up to
speed. It cannot contribute anything at speeds
above about 30 kts. It all kool!
On Tue, Apr 21, 2015 at 5:28 PM, DMcD
<<mailto:[email protected]>[email protected]> wrote:
>>Like the wheel brake when stopping?
Yes, that's it. Same heat, same fires except with the wheel brake, you
are normally there to enjoy them. 7 kW is a whack of current at most
safe voltages.
>>I call all piston motor glider engines boat anchors.
My great grandfather called them that too
at least the water cooleed
two cylinder two stroke in his Scott. However, they worked OK back in
1928 and they're working that way now.
The rotary, why, they sell a 10 hour version don' they! And who of us
is going to do the claimed 400 hours of most piston or rotary engines.
One will vibrate to pieces and the other burst into flames :-)
D
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