Need for constant speed if you have constant torque?

[stratobee]

Help me understand some theory here.

The reason for gearboxes in cars, or constant speed props (which is a form of gearbox), is to be able to maximise the power by delivering the optimal rpm to the combustion engines sweet spot. As we know, they only produce their rated power and torque at a very narrow rpm band, usually at the top. With electric propulsion, the torque and HP is linear, so there's no need for a gearbox necessarily, as is the case with the Tesla Model S and many other electric cars.

Is it safe to assume that if aircraft had electric propulsion, the need for a CS prop would also be eliminated? Ignoring the feathering needs for now, or critical tip speeds (we'll assume max power is below transsonic) and just looking at the propulsive effects. That would be the correct assumption? Logic would seem to say so, but I want to make sure I'm not overlooking anything.

 

[Dan Thomas]

The propeller has its own limitations that demand a fairly constant RPM. It works best when the tip speed is around Mach 0.8 or so; much below that and you're not getting the performance out of it that it can produce, and above that the drag mounts as it approaches the speed of sound and horsepower is wasted. A fixed-pitch prop therefore works best at one forward speed, where the propeller is at its ideal performance. To go faster, one needs more RPM and then the prop's noise and drag increase and the ultimate top speed isn't much higher anyway. So we end up increasing the pitch of the blades to get what we want out of it, and on a combustion engine this also allows a lower pitch to get redline RPM and therefore full rated horsepower in the takeoff and climb, too.

Dan

 

[stratobee]

Thanks.

So you're saying that in a reduced cruise, like at 55% power, you'd still want the rpm to be close to the 0.8 mach tip speed number (i.e. geared)? That must mean that a fixed pitch prop running at low rpms is more inefficient than running at higher rpms closer to optimal blade speed?

I'm trying to deduct if in an electric plane there's any need for the complexity and weight of a CS prop.

 

[Dan Thomas]

Thanks.

So you're saying that in a reduced cruise, like at 55% power, you'd still want the rpm to be close to the 0.8 mach tip speed number (i.e. geared)? That must mean that a fixed pitch prop running at low rpms is more inefficient than running at higher rpms closer to optimal blade speed?

I'm trying to deduct if in an electric plane there's any need for the complexity and weight of a CS prop.

The fixed-pitch prop will be at about 0.8 Mach at redline, and at 55% power at sea level it'll be a lot lower than that and cruise speed won't be as good as it might be.

The typical 150-hp Lyc-powered 172 uses a 75" prop that redlines at 2700, and that translates to .793 Mach. Add a bit for the forward speed (the prop tips describe a spiral and so the travel is more than just circumferential) and we end up at .811 Mach at 135 MPH (at full throttle) and the noise is getting pretty bad. If we had a CS prop we could reduce the RPM by adding pitch and maybe still go that fast by converting some of that noise and drag into thrust, and reduce the power a bit, too.

In the electric airplanes we could build now, the greater weight and cost of a CS prop probably isn't worthwhile.

I should point out that horsepower is a function of both torque and RPM.

Dan

 

[DGlaeser]

The fixed-pitch prop will be at about 0.8 Mach at redline, and at 55% power at sea level it'll be a lot lower than that and cruise speed won't be as good as it might be.

The typical 150-hp Lyc-powered 172 uses a 75" prop that redlines at 2700, and that translates to .793 Mach. Add a bit for the forward speed (the prop tips describe a spiral and so the travel is more than just circumferential) and we end up at .811 Mach at 135 MPH (at full throttle) and the noise is getting pretty bad. If we had a CS prop we could reduce the RPM by adding pitch and maybe still go that fast by converting some of that noise and drag into thrust, and reduce the power a bit, too.

In the electric airplanes we could build now, the greater weight and cost of a CS prop probably isn't worthwhile.

I should point out that horsepower is a function of both torque and RPM.

Dan

It isn't just RPM, don't forget about prop AOA. As you move forward faster, the prop AOA decreases, which is one of the reasons RPM increases. It is a 3D environment so simple explanations are tough. So even with a constant torque motor, a CS prop will provide more thrust and efficiency by maintaining a better AOA at different air speeds.

 

[ElPaso Pilot]

Ever flown the same model of plane with a fixed pitch climb prop vs. a cruise prop?

The cruise prop sacrifices climb rate and TO distance for top end speed; the climb prop handles the first two while flogging along in cruise at redline with available airspeed it can't take advantage of.

A fixed pitch prop sacrifices top end speed, low end climb/TO performance, or both, due to the limited RPM range we can operate a propeller in.

Meanwhile, the wheels on the Tesla can just spin faster and faster, without having to worry about silly things like propeller tips going supersonic.

 

[Dan Thomas]

It isn't just RPM, don't forget about prop AOA. As you move forward faster, the prop AOA decreases, which is one of the reasons RPM increases. It is a 3D environment so simple explanations are tough. So even with a constant torque motor, a CS prop will provide more thrust and efficiency by maintaining a better AOA at different air speeds.

That's one thing I should have mentioned, too. The prop's angle of attack is most efficient at around 2 to 4 degrees, and that drops off as forward speed increases. The only way to increase the AoA on a fixed-pitch prop is to spin it faster, which, as we have seen, is a poor way to accomplish anything.

Dan

 

[Capt. Geoffrey Thorpe]

With electric propulsion, the torque and HP is linear, so there's no need for a gearbox necessarily, as is the case with the Tesla Model S and many other electric cars.

If torque is linear, HP can't be. Plus, the torque / speed curve depends on the type of motor used. For permanent magnet DC motors, the torque curve tends to be a straight line that is maximum at 0 RPM and reaches 0 at some RPM that depends on the voltage supplied.

Determining the value of a variable pitch prop would take some effort, but the speed / torque demand for an aircraft is a lot different than an automobile.