X-Mozilla-Keys:
Russell Kent wrote:
Paul wrote:
Ken Powell wrote:
And the answer is: use 1 alternator and 2 RG batteries, replacing the
oldest battery during the Annual. Just like Tracy showed us in his
newsletter. AH, but now I am educated and understand WHY, so I can
sleep at night. Thanks to all of you for your input. (I still like
the idea of having a backup alternator but I am not sure it is worth
the weight.)
A RAT is no big deal. Take any 12V brush type DC motor and put
a model airplane propellor on it.
This question gets asked so many times over in rec.aviation.homebuilt I
think it should be in the FAQ (if we had one). Airplane propellors (model or
otherwise) work by turning rotational energy into linear displacement (thrust).
A RAT (or any power generating turbine) works by turning linear energy (the wind)
into rotational energy. If you do what Paul says, you're using the wrong prop.
Go take a gander at a sailing mag. Cruising sailors have these nifty
stay-mounted wind-powered generators. You need to make a prop shaped like those.
Hey this is just proof of concept here. Use two or three model air plane
props
if you like. They will turn the shaft.
Stick it out the window of your car and wire it to a six ohm, 25 watt power
resistor (2 amps).
Eh? Since the voltage from the DC motor is unregulated, how can you say it's 2
amps? It will only be 2 amps when the motor puts out 12 volts.
Yes of course. Why you use the regulator or zener is so you don't blow
your $900 EFI/IGN computer with over voltage. It does nothing
for under voltage. Tracy tells me it works down to six or seven
volts as I recall.
Better yet get a rad fan and stick it out the window.
Rad fans have the same problem as the R/C propellor.
Yes but they do spin in the wind. We have a guy out here with a
Cirrus VK-30 that just burned out a cooling blower because
it was spining so fast. We don't really care about efficiency here.
Any impellor that will turn the generator with enough juice
to operate the EFI/IGN is fine. Most of the time it will be cooling
the radiator. The advantage is the RAT weighs nothing
because it is already there to cool the engine on the ground.
Here's my quick&dirty suggestion:
1. Get a big *WOODEN* R/C prop (you can get 10" - 14" no problem).
2. Carefully cut the blades off near the hub at the point where the blade looks
most cylindrical.
3. Carefully bore a dowel hole into the cut ends of the blades, and matching
holes in the hub where the blades formerly attached.
4. Re-assemble the propellor using epoxy and dowels, but angle the blades. The
angle needs to be such that at 80-100 kts (WAG) they are at "optimum" AOA
for turning wind parallel to the axis of rotation into lift within the plane
of rotation (arc of the propellor). The propellor is now a wind turbine.
5. Check the balance of the wind turbine. It doesn't need to be perfect (like
when on an R/C engine turning at 14,000 RPM), but we don't want it to shake
the generator to pieces.
6. Mount the wind turbine on the DC motor, and perform the tests that Paul
mentions.
You don't even need a second battery. You can use a large capacitor across the
RAT and a solid state $5 linear regulator or zener diode to keep the voltage
down to less than 15 volts.
What about undervoltage? I guess we'd better desing it so that doesn't happen.
There will be a range of aircraft speeds to fly at in an emergency. Too
slow
and your engine will quit. Too fast and your generator will blow up.
That is why putting it in front of or in back of the rad will help.
The airspeed through the rad is going to be limited.
Disconnect the offending shorted battery entirely from the circuit. You just
need enough juice
to operate the EFI and ignition until you can land.
Hmmm. Might want to do this through a relay wired to the RAT deployment
mechanism.
What RAT deployment mechanism? More weight saved by using the cooling
blower.
Disconnect all other loads.
Probably only a couple of amps. Right Tracy?
If you want to get really trick use the electric
fan on the radiator and make it do double duty.
RAT and cooling fan.
Ugh. Now we're using a lousy generator system that when it works makes the
engine run hotter. Bleah.
Russell
Hey you are not climbing out hopefully. You are in cruise looking for a
place
to land as soon as possible. Why the hotter engine???
Paul Lamar
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