Electric Glider Winch

[PhillG]

(It's probably too difficult/expensive, but what the heck. I'll ask the questions anyway)

I'm doing some research into the possibility of launching a glider using an electric winch. Currently winches consist of a 400ci V8 coupled to an auto, coupled to a heavy truck diff driving a 1metre diameter winch drum. The V8 runs at full throttle for most of the launch. I've worked a few numbers -
drum diameter 1m
tow-in speed 30m/s (yes, 55kts!)
Max pull 900kg
Duration 30s (reducing from max pull to 200kg over 30s)

These numbers suggest (if my math is right)
250kW
30021kWh (reducing to 2426kWh)
Will need to control both the rotational speed and the output torque.

And for the cherry on top, I'm considering driving the entire thing from batteries, but that might be very impractical. Slightly impractical may be OK.
The attachment is an Excel s/sheet I've used to make the calcs.

Is the idea completely outrageous?

TIA, PhillG

 

[GregLocock]

Your maths is wrong, as posted.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

Also, there are many electrically launched sailplanes, and of course thousands of cessnas and the like, which weigh more than gliders yet I doubt they have 100 hp motors.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PhillG]

Thanks for the responses, Greg.
Cessnas have 150 and 180 hp engines, and take 2 or 3 minutes to get to 2000' from rest. A glider makes 2000' in 30 seconds. Raising the dead weight of a 600kg glider by 2000' requires quite a bit of effort. And dead weight does not take into account induced drag)

I'd appreciate a hint as to where the maths is incorrect.

TIA
PhillG

 

[itsmoked]

A V-8 will be farrrrrrr cheaper to run and build than a brute force motor system. A motor that size would probably cost more than $50K.

A better design would be a flywheel storage system and a clutch. Then a small motor and electrical system would be sufficient to provide a launch.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

250kW is a big LV motor - in Europe I would probably put it on a 3.3kV or 6.6kV system given the chance and keep it off the 415V system altogether. You will be looking at a generator in the 400kW - 500kW range to start this motor under load, a fairly expensive piece of hardware and at least twice the size of your engine rig, before you even consider the motor itself.

As for batteries... see if the Navy is scrapping a submarine. You will be looking at a massive battery just to handle the peak current, likely weighing several tonnes, and some pretty big switchgear to control the power to the motor. You'll be lucky to get a 250kW DC machine for less than a 6-digit price, and figure on about the same for the battery and switchgear.


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

PhillG; If you need 250kW for 30sec how do you arrive at 2426kWh???

2.083kWhr (with no load reduction)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[PhillG]

Dear itsmoked,
The s/sheet attached in my first post show my workings-out.
There are several parts to a launch -
a) the initial acceleration and rotation into full climb. This is taken at max power.
b)The first half to two thirds is at near max power.
c)The last third is under reduced power.
d)Release is at close to zero power.

I did two calcs -
One at max pwr (450kg/m torque = 250kW for 30s = 30021kWh)
One at min pwr (100kg/m torque = 20kW for 30s = 2426kWh)
The reason was to figure out what the utility cost might be for a launch by multiplying the kWh by utility cost then relating the $$$ back to the 30s.

And it needs *full* speed control to make sure the launch is gentle acceleration. There wouldn't be big inrush or startup current drain (says me, The computer expert).

In any event, it looks like the project is shelved based soley on the cost of a 250kW motor. <sigh>

Thanks for the replies. You are a most informative group.

Best,
Phill.

 

[davidbeach]

Gee, without even breaking out a calculator it appears to me that 250kW for 30s would be 250/120 or just over 2kWh and 20kW for 30s would be 20/120 or 1/6 kWh. Probably time to check your math.

 

[itsmoked]

PhillG; How many 250kWhours do you use in 30 seconds??

You are using how many hours? None!! You're using a 120th of one hour!

You divide your 250kW by 120 to get the kW-hours you're using.


Now did you not understand my flywheel comment? Flywheels are used everywhere in industry for exactly this purpose. To supply a large amount of power briefly.

Be smart! Instead of using a $100k 250kW (335hp) motor hooked to an expensive utility connection thru $30k of controls for a measly 30 seconds, use a much smaller motor (~$300 ebay)hooked up to an inexpensive power connection thru a small speed controller for 10 minutes. That's 20 times the time of a launch. So you need one twentieth the power supply and motor. Is not a 16hp motor easier to find and provide power to??

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Then there's the small matter of the 10 tonne flywheel to consider...


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

Make it like a Trebuchet?

Hey that's a good idea!! Just use water.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Why do I have an image of Wile E Coyote sitting in an ACME glider waiting for launch...


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If we learn from our mistakes I'm getting a great education!

 

[roydm]

How about a bungee cord?

 

[MikeHalloran]

A trebuchet might yank the wings off a sailplane.

It would be fun to launch the 400ci engine with...



Mike Halloran
Pembroke Pines, FL, USA

 

[jraef]

250kW, so let's assume 415V just to make it easier to acquire, that makes the motor current roughly 483A. Assuming (again) that we use an inverter so inrush is not a problem, 483A /120 = roughly 4A/Hrs so that's not bad. But assuming 12V lead-acid batteries, to get 415V AC from the inverter you will need 590VDC, so you would need about 50 automotive batteries in series. Charging them would be problematic, let alone hauling them around; 7lbs each maybe? That's 350 lbs. Typical 350HP AC motor, 1500lbs. VFD; 500 lbs? Frame to hold it all? 1500 lbs easy. So you are a little short of 2 tons to execute this. The engine would be lighter...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[ScottyUK]

Hi jraef,

A 4AH cell will have too much internal resistance to deliver 483A into anything other than a short circuit, and probably not even then. The 12V battery would need to be more like a truck battery to deliver the current while maintaining a useful terminal voltage, and with 60 blocs to maintain the 590V under load. Swap 7lbs for maybe 140lbs per bloc and the problem is even worse. Over three tons of battery alone.

A generator would be a better bet if the motor was a necessity, but you might as well use the engine from the genny to drive the winch through a hydraulic coupling to avoid snatch on the tow and wear on a clucth.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

I would look for two old welding generators. Use a Ward Leonard control system. For 30 seconds you can survive a significant overload. An alternative would be four welders in series. Two generating and two as motors. Use the V8 to drive the generators. (I have worked on systems with two 1300 Hp. motors in series with two similar capacity generators.) The control was a modern version of the old Ward Leonard system.

Bill
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Jimmy Carter