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Old Jul 09, 2011, 11:27 AM
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Its not so much a lack of power thats a concern. Its the asymetrical thrust. Vmc refers to "Velocity, minimum controllable". Thats how I remembered it.

It was the minimum airspeed at which there was enough air going over the vertical stab and rudder to offset the asymmetrical thrust. If you get below Vmc on one engine the aircraft yaws and rolls over on the dead engine and kills everyone on board.

So if you lose an engine below Vmc, you end up having to reduce engine power on the good engine. Reducing drag on the dead engine helps to decrease Vmc, which buys you margin. Thats why in WWII movies the bomber pilot would always close the flaps and feather the propeller of the dead engine.

Don
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Old Jul 09, 2011, 11:40 AM
JustFlying
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Quote:
Hey if you wire it up right, it'll be a generator and so give more power to the live engine.
As Don said,

and -

A dead motor (not an engine ) will be a dead generator as well. Besides, the asymmetrical thrust ignored like with an inline push-pull setup, if you want to generate power it will increase the drag so the other motor will need to work harder to overcome the drag. A total efficiency lost will make it use more power than it can generate.
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Old Jul 09, 2011, 08:14 PM
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It was a joke guys and yes I'm well aware of Vmca and Vmcg - I used to be a flying instructor and would demonstrate those things for real.
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Old Jul 10, 2011, 01:10 AM
JustFlying
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And it was just an explanation for those not knowing the realities.
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Old Jul 11, 2011, 02:48 PM
Dale
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JM1 full battery cyle test

If I am happy with the plane by Oshkosh I will go to Oshkosh.

Vmc on a Lazair varied with what engines and control system it had. The first Series I had no independent rudders. Only on the large JPX and KFM engines or on the two place would I say there even was a Vmc. I have flown single engine with the Lazair quite a bit but I usually fly with no airspeed so I can't really answer the question. There was never any time I lost control, if the rudder wasn't enough and the plane started to turn, I just pushed the nose down until it stopped (no snap spins). At some point I plan to make a sort of rubber finger to stop my props for soaring purposes.

Anyway, I spent the weekend doing more testing and for the purpose of the next flight, this is what I know.

As far as power and thrust goes. On a fresh charge my peak power is 11 kw (4000 rpm and 85 lbs thrust) which drops to 10 kw in 3 minutes, by this time the stator coil temp has climbed to about 250 degf from 85 deg ambient, at this point I reduced power to 8.4 kw (3600 rpm and 70 lbs thrust) for a further 3 minutes where the temp seemed stable at about 250 degf. I then ran the batteries out at 2.0 kw (2300 rpm and 29 lbs thrust) for a further 74 minutes (80 minutes total). Attached is a graph of this battery cycle run.

I am not sure of the voltage irregularity between 7 and 17 minutes. I am fed up with the Eagletree loggers and display panels. I have had no tech support on my issues and I have requested help many times in the past 6 weeks. I am still using them for lack of any alternatives.

I am pretty sure that I could run a fully charged pack out on the ground at WOT and stay below the motor wire max varnish temp of 390 degf but I am not willing to try this right now in case I am wrong.

I spent quite a bit of time trying to optimize motor timing and what I ended up with was 12 degrees. This seems low to me but this was where I felt the motor ran the 'smoothest' and the coolest, I may revisit the timing later.

If all goes well I hope to fly again this Thursday.

Dale


P.S. I just opened the box of the freshly delivered 'second' JM1 motor, no stopping me now.
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Old Jul 11, 2011, 03:13 PM
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What was the hp of the 2stroke engines typically installed on a Lazair?

Do you think you'll be able to make enough thrust to fly off of a float? Those photos at the beginning of the thread looked so cool.


Don

To those who are wondering 745w = 1 hp.
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Old Jul 11, 2011, 03:20 PM
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Engines are rated brake HP and the electric motors are being measured watts input not watts output. Electrics have a lot more torque. Comparing thrust and prop speed would be a fairier comparison, IMO.
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Old Jul 11, 2011, 03:31 PM
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Originally Posted by everydayflyer View Post
Engines are rated brake HP and the electric motors are being measured watts input not watts output. Electrics have a lot more torque. Comparing thrust and prop speed would be a fairier comparison, IMO.
Your point about input watts vs output watts is certainly valid, thanks for that clarification. What is the typical efficiency of a large brushless motor? 80+%?

Re your comments about thrust and prop speed, you're making a common error.
Power is power. Power measures the amount of work that can be done over a given amount of time. Torque measure's turning force. A gas engine has a torque curve that varies with RPM. An electric motor has pretty much consistent torque at all speeds. So its power is a smooth, straight line.

All things being equal, and they never really are, an electric motor operating at a certain power level will push an airplane at the same speed as a gas engine making the same amount of power.

Everything else is just tweaking the drive system for its optimized use. For example, thrust is not an accurate measure of power. You can put a large, multi-bladed prop on an aircraft and increase its static thrust. It will accelerate harder. But thrust at speed will be decreased and the plane will actually be slower.

Being able to swing a large propeller is a function of Torque (another definition of power) at the prop shaft. But if you aren't also talking about RPM, you've only got half the picture. You can't just talk about torque. I could rig up a 1/10th hp motor that can twist with 1000 ft-lbs of torque. It just takes the right gearing. That motor combo would be very useful for pulling your truck out of a lake, but wouldn't be much good getting an airplane into the air.

Let me give an example. I used to own a 125cc Honda MT125. It was a 2stroke GP roadracer. It had a 14,000 rpm redline and made 46 hp. It only made 6 ft-lb of torque. The engine could produce very little turning force, but it could produce it at a very high speed. Driven directly off the crank this would be useless. It would need to turn a tiny prop and would generate very little static thrust.

However, put a gearbox on this engine and you can do a lot of things with it. If you halve the speed, you double the torque at the output shaft. You vary that until the engine is optimized for its best RPM and turning a useful prop. What is a useful prop depends on what you want to do with it. if you are building an ultralight helicopter, it may be a prop 20 ft in diameter turning at 300 rpm, ie. optimized for static thrust. If you want to put it into a Sonerai kit airplane, then it may be a 50 inch prop turning at 3000 rpm.


So in the end, power is power, and torque is just one factor in the definition of power.

Don
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Old Jul 11, 2011, 04:00 PM
Expo/DualRates = Lack of Skill
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Quote:
Originally Posted by donmei View Post
To those who are wondering 745w = 1 hp.
Wrong.

745W ≠ 1 horsepower

746W ≈ 1 horsepower
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Old Jul 11, 2011, 04:04 PM
Southern Pride
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This is not my thread niether is it a general power discussion thread. I was just pointing out that rated gas engine HP versus watts motor input were not a valiade compasison IMO.

Gearing and such do not apply here as neither the gas engines nor the electric motors are geared. Thrust generated by the prop and pitch speeds are a fairer comparison IMO.

Charles
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Old Jul 11, 2011, 04:13 PM
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been following this thread from the start, nice project, not something you see everyday, well, at least for me anyway, keep up the good work!
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Old Jul 11, 2011, 04:18 PM
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Quote:
Originally Posted by everydayflyer View Post
This is not my thread niether is it a general power discussion thread. I was just pointing out that rated gas engine HP versus watts motor input were not a valiade compasison IMO.

Gearing and such do not apply here as neither the gas engines nor the electric motors are geared. Thrust generated by the prop and pitch speeds are a fairer comparison IMO.

Charles
But power is still the ability to do work in a given amount of time. Or to put it in airplane terms, the ability to provide a certain thrust at a certain speed. What I mean there is that if you assume no gearbox, a reasonable assumption, then you need to measure power at whatever RPM you intend to run at. Varying the size and pitch of the prop is the equivalent of the gearing in a transmission. (to some degree) You then seek to optimize the size and pitch of the prop based on speed and efficiency desires, while also constrained by other things like the need to be able to accelerate to take off speed in a reasonable amount of time and the space requirements of a given airframe. Its all a big tradeoff.


I'm done with the power discussion. I just was curious as to what the original lazair flew on. These electrics seem to be in the 8-10 hp range.
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Old Jul 11, 2011, 07:12 PM
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joby resistance

very interesting project to follow KiloOne, and a well balanced approach to all the tech stuff, whilst getting into the air without long delays.

a question though, ive been testing the turnigy 120 motors and have a good measure now of their capabilities and in wye they also have a kv of around 84rpm/volt, so for comparison im very curious what the unloaded rpm/amps were from your kv test of the joby, and what resistance of the windings are? (prolly needs a good 5a+ dc to test volt drop)
-we can then work out their efficiency also.
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Old Jul 11, 2011, 08:18 PM
Dale
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Quote:
Originally Posted by donmei View Post
What was the hp of the 2stroke engines typically installed on a Lazair?

Do you think you'll be able to make enough thrust to fly off of a float? Those photos at the beginning of the thread looked so cool.


Don

To those who are wondering 745w = 1 hp.
Don,

The first few hundred Lazair flew with two 5.5 hp Pioneer Chainsaw motors (100cc).

Most Lazairs flew with two 9.5 hp Rotax 185 cc motors.

Fifty to 80 Lazairs had larger JPX or KFM two cylinder motors of about 18 and 25 hp each.

In my lighter days I flew a Pioneer powered Lazair on floats.

Yes, I expect to fly this on the monohull float later this summer now that my cross country trip is postponed indefinitely.

And as a pilot I remember the watts/hp conversion as 747 (like the airplane). There are not many measuring systems that can resolve the little error I inject into my calcs.

Dale
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Old Jul 11, 2011, 08:31 PM
Dale
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Hp and Watts

Just to be clear as to what I refer to in my Hp and Watt numbers.

When I talk of gasoline motor Hp I mean mechanical shaft Hp.

The KW I refer to for my electric motors is input power to the speed controller and to actually covert to mechanical shaft Hp we would need to know the controller and motor efficiency at each and every temperature/voltage/current/rpm/etc to be tested. That is beyond my measuring capabilities.

One thing I have in the past done is to measure motor torque and rpm. I wish I had this setup now so I could get a true power conversion efficiency (at least in static thrust conditions).

Dale
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