I suppose two other issues are that
1) for flying in the circuit, cruise speed isn’t important so if you were to cut it from 85 to 60 knots you could fly on much less power but unlike when you’re trying to get somewhere, it doesn’t matter.
2) aircraft that are too slippery can be difficult to land so you need an airbrake. But if you can use regenerative braking from the prop then you could get the best of both worlds.
Do you get all the same advantages from a variable pitch prop, or will it be possible just to fit a cruise prop and have good take-off performance nonetheless?
You’ll run in aerodynamic limits if you exaggerate the coarse pitch. Aerodynamic efficiency is what will drive the use of variable pitch props.
Plane and Pilot magazine says the Virus has a L/D of 24 at 59 kts. Max endurance will be slightly below this speed, depending on how well the fuselage and winds are set up.
Electric motors have linear power output, which recipe engines do not. Therefore they can run at a lot less percentage of max power and still produce thrust, which an IC engine can’t. A gas engine is actually only fully semi-efficient from a power-to-weight point of view at absolute max power. Anything else and you’re carrying a lot of dead weight around. Not so with electric motors – they’re as efficient at 1rpm as at 10000rpm. This is actually what makes electric hybrid propulsion not as bad an idea as people think for aviation. It is much better to have a smaller IC or turbine engine running a generator at max power continuously, then lug around a bigger one that you use variably. Have electric motors be the prime mover and variable power unit, not the IC.
In aviation we rarely cruise below 55% of rated power at altitude. Maybe 45% at very long range cruise settings. That¨s because of two reasons: the IC engine loses steam up there, so the 55% could be close to wide open throttle at altitude. Electric doesn’t have that problem, so as air resistance goes down, you can reduce your power output considerably at altitude. The second reason is of what I described above. When aviation moves towards electric propulsion, you’ll probably see long range cruising power settings of 20-30%, which would be the equiv of 45-55% in an IC engine. I personally can’t wait. I’m done with the IC engine. Such a pain in the a*se that thing.
Elon Musk has talked a little bit about his idea for a supersonic electric jet. He said he would basically design it so it cruised above FL600 where air resistance is almost nothing and achieve high speed that way. TAS goes up about 2kts/1000ft, so at FL600 even a Cessna 152 would have GS of 220kts. How he’s going to achieve anything that needs fanjets or props to achieve supersonic speeds is however beyond me. That will be hard to do. Last time someone tried, USAF’s supersonic prop plane, the Thunderscreech, it made so much noise that ground personnel got sick.
Adam, I would translate what you’ve written into saying that electric motors are designed as constant torque machines, which is often true, but you can also design the motor system to produce constant power versus speed (i.e increasing torque with decreasing speed). The required motor system spec depends on the load characteristics and you are correct in saying that there is a lot of flexibility and predictability in the motor design.
Motor efficiency is not constant with speed, but may be closer than for an IC motor. Percent power depends only on how much power (and weight) you provide, and of course the demand.
The real issue is not the motor, it is the reliability and power density of the motor drive and the energy storage. The motor is simple until you really start pushing the state of the art in terms on rpm and power density – which driving an aircraft propeller does not require. I think the motor is the easy part for this application.
Aerodynamic efficiency is what will drive the use of variable pitch props.
But for an electric aircraft you wouldn’t be so worried about aerodynamic efficiency on take off and landing – apart perhaps from this one which is designed to do little else. What interests you is the amount of thrust you get out of the propeller. Even if the overall efficiency was hugely lower for the duration of your takeoff run, the savings in weight and complexity would probably more than make up for that.
On the other hand, if the propeller is partly stalled for half the takeoff run, putting more power into it may not help much.
But for an electric aircraft you wouldn’t be so worried about aerodynamic efficiency on take off and landing – apart perhaps from this one which is designed to do little else.
On the contrary. If you have limited energy available, you will tend to optimize the usage of it. And if you want to recuperate energy through the prop, you should optimize it for that, too. So I’d say there are more reasons to look into prop efficiency at different flight stages, than with an ICE, where recuperation is but a dream.
What interests you is the amount of thrust you get out of the propeller.
That basically is the efficiency of the prop.
On the other hand, if the propeller is partly stalled for half the takeoff run, putting more power into it may not help much.
That’s why you don’t find many fast aircraft with fixed pitch props :-)
I have no idea why anyone is designing an electrical powered training aeroplane.
As what the industry needs is a conventional powered replacement for the C150/2 PA28.
And if someone gets it right then the market it large.
So, according to this power chart:
http://www.flyrotax.com/enginesImpressum/product-rangeImpressum/carburetedImpressum/912-80hpImpressum/engine-data-performance.aspx
For rotax 912, 60kW:
55% rpm produces 30kW
45% rpm might produce 20kW (extrapolating the curve)
I don’t know much about the Pipistrel… but would 45% be enough to keep it straight and level in cruise? Even so 20kW will not last 1.5hrs on a 17kWh battery right? even if you assume the electric motor is 100% efficient…
My question is… are they stretching the figures?
QuoteOn the contrary. If you have limited energy available, you will tend to optimize the usage of it.
Not necessarily – you only spend a small amount of time at low speeds. And whilst there may be a definition of propeller efficiency as thrust at a given RPM the sense in which I meant it was thrust for a given power input.
Let’s take an electric aircraft cruising at 20% power on a 90 minute flight. For the sake of argument you run at 100% power for 1 minute during the take-off run.
A friend has a similar aircraft with a variable pitch propeller – he only needs 50% power to get the same performance so installs a smaller motor and runs at 40% power for most of the flight.
That 1 minute at full power will use the same amount of energy as 5 minutes in the cruise. The more efficient variable pitch propeller will use the same energy as 1.2 minutes in the cruise. So you lose 4 minutes of endurance but sidestep all the expense and complexity of a variable pitch propeller. You lose the weight of the variable pitch mechanism but need a slightly heavier motor. It sounds like quite a good deal to me.
What I don’t know is what the figures should be – or if you can even get the same peak thrust from a coarse pitch propeller just by spinning it faster, less efficiently, with a motor that is far less picky about its optimal RPM.
What I don’t know is what the figures should be – or if you can even get the same peak thrust from a coarse pitch propeller just by spinning it faster, less efficiently, with a motor that is far less picky about its optimal RPM.
That depends on the speed range of the aircraft. I would expect that for a trainer, you could design the motor system to produce greater torque at lower rpm, to account for the increased blade angle of attack during climb, and deliver 100% power. For an aircraft with a wider speed range, the efficient range of propeller angle of attack maybe become a limit.
Electric aircraft will undoubtedly be a niche player in the future, but I don’t think they will be as practical or inexpensive as gasoline powered aircraft in the foreseeable future. Their principal advantage is greater efficiency in burning fuel at the power plant, but the conversion losses and (particularly) energy storage issues more than offset that advantage, in my mind.
