Need for constant speed if you have constant torque?

[AdamFrisch]

Help me understand some theory here.

The reason for gearboxes in cars, or constant speed props (which is a form of gearing), 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 a correct assumption? Logic would seem to say so, but I want to make sure I'm not overlooking anything.

[TURIN]

Nope.
Variable airspeed.
The prop needs to be efficient at slow forward a/c speed, during acceleration and also at high speed cruise.

[Wizofoz]

Exactly- the prop is essentially a wing, and is most efficient at a particular angle of attack. The blade angle to achieve this will vary with airspeed.

COULD be that the pay-off is less than with a piston (or turbine for that matter) and a cruise-optimised prop may be sufficient in most cases as, as you say, there will still be maximum torque available at lower speeds, and thus not worth the cost/weight/complexity.

[43Inches]

You can not compare a road vehicle gearbox to a CSU on an aircraft. Part of what you say is correct regarding optimal RPM of the engine, but you also need to achieve the optimal aerodynamic benefit from the propeller. This is achieved from the best prop speed at the most efficient angle of attack. Too much angle (fixed pitch prop at low forward speed) and the prop effectively has stalled sections and increased drag. Too little angle (prop at high forward speed) and the prop can not generate any more useful forward thrust without increase rpm, you then get into further issues of tip speeds etc...

The only way you could replicate this on a road vehicle is to have variable tyre size so that the optimal wheel speed is achieved at the same time, ie as speed increases the tyre grows and wheel rpm remains the same. This would be the equivalent of an aircraft CSU. However this is just not required for a road vehicle as the tyres road speed does not impact greatly on the vehicle, where as propeller speed and angle is essential for efficient aircraft operation.

Even with an engine that has constant torque at all RPM a fixed pitch prop will still only have one rpm/forward speed combination that will provide maximum acceleration. This means you can set it to perform well on take-off and climb or achieve high speed flight, or, a compromise somewhere in the middle that doesn't really do either well.

[barit1]

To complete the aircraft - automobile analogy, consider the auto on a hilly road.

You're driving at first on level ground, then start up a hill. In physical terms, you are putting potential energy into the vehicle; but if you are producing constant torque, some of that torque now goes into lifting the vehicle, leaving less available to propel the auto. So the auto slows involuntarily.

Rounding over the crest of the hill, now that stored potential energy (a redundant ezpression, :O ) is helping propel the auto, so it speeds up even faster than when on level ground.

The same thing (almost) happens in aircraft.

In an auto, the cruise control will adjust throttle position to vary torque to maintain approximately constant rpm and vehicle speed. In an aircraft the CSU varies the load on the engine by adjusting prop blade angle, and if the throttle is not moved, torque will remain constant as a by-product. (Analogous to a variable-ratio gearbox in a car).

[AdamFrisch]

Thanks guys.

[Volume]

Nope. They are simple speed governors just like the ones on old (stationary) steam engines. Flywheel sets blade angle (more blade angle = more torque = less speed) without any high sophisticated optimisation.
The clever part of the CS governor is the guy at the controls selcting the speed (via the governor) and the torque (via manifold pressure).

[phiggsbroadband]

A lot of these questions can be answered by anyone that has a good basic knowledge of aero-modelling, where you can alter things quite cheaply. (i.e. props cost less than £4..)
Lets say you have a model glider that has a brushless electric motor, you can try several different props, all of which will all load the motor to exactly the same extent.


If you try a 12x4 (diameter 12 inches, pitch 4 inches) then the glider will climb vertically, but will only fly very slowly.
An 11x6 would give a 45 degree angle of climb and a moderate cruising speed.
However a 10x8 would be very poor on the climb, say just 10 degrees, but the top speed would be ballistic.


You can choose whatever suits your flying style. The VP full size prop can give you the benefits of a variable pitch but obviously cannot change it's diameter, which is needed to give the motor a similar load.

[Chu Chu]

If I remember right, a propeller blade makes lift (thrust) just the same way a wing does. Lift is proportional to the square of velocity. If you slow down propeller RPM for takeoff, the velocity isn't there, and neither is the thrust. And trying to run a high-pitch prop at high RPM at low airspeed will just cause stalling due to excessive angle of attack.

A fixed-low pitch prop would have to run at excessive RPM to generate a positive angle of attack at high speed. Even though the motor might be able to handle the RPM, the propeller tips would go supersonic, or perhaps the prop would just fly apart from the centrifugal force.

[Volume]

Optimizing a propeller for wide speed range is practically impossible. As long as you can not vary the blade twist or the blade chord, there is only one combination of airspeed and propeller speed at which the full blade operates at its optimum angle of attack for the selected airfoil at that radial position. At the tip the blairfoil has to be very thin for mach critical number reasons, at the root it has to be very thick for structural reasons. Accordingly the best l/d for each radial position is at a different lift coefficient and hence at a different angle of attack requiring a specific local blade chord.
The next restrictive parameter is the torque curve of the engine, it does not help to have a lot of torque available at low speed, if the propeller can not accept that torque, as it is approximately increasing with the square of propeller speed. Almost no engine has such a speed ^2 torque curve. So if shown in a torque over speed diagram, the intersection between the propeller curve and the engine curve happens with a lot of angle (up to perpendicular), hence you need to change the curves a lot to move the intersection point on the speed scale, this means you can only change propeller speed while significantly changing power. This restricts you to a very small speed range in which you can reasonably operate the propeller.

[Goldenrivett]

Hi AdamFrisch,
Is it safe to assume that if aircraft had electric propulsion, the need for a CS prop would also be eliminated?
Is this not what we have with the fan on a high bypass gas turbine engine?
The fan blades are set at a fixed angle. The engine fan RPM is varied from about 20% to 100% throughout the aircraft speed range.

[43Inches]

Is this not what we have with the fan on a high bypass gas turbine engine?
The fan blades are set at a fixed angle. The engine fan RPM is varied from about 20% to 100% throughout the aircraft speed range.

Both Fan and Prop based engines will deliver power between 20% to 100% throughout the speed range. The key is how efficient is the engine producing power at certain stages of flight.

The turbo-fan engine is only efficient at high engine speeds, which is optimised for cruise flight at altitude. They burn a lot more fuel for take-off, climb and low altitude operations. You have to take this engine high to take advantage of its efficiency at altitude and speed.

A turbo-prop engine delivers more mass flow at lower speed, this allows more power to be transfered from the shaft to the airflow at greater efficiency. That is of course limited by the aerodynamics of the prop, which limits the effective speed of the aircraft.

This is why turboprops are generally more efficient for flights under about 2 hours and turbojets take over when going further.

With relevance to this conversation you would have to ask if an electric motor replacing the gas generator would be of benefit to either prop or fan.

[Brian Abraham]

The key is how efficient is the engine producing power at certain stages of flight.A practical demonstration of the prop/engine relationship is the P-51, when the Allison and Merlin versions are compared. In the low level interdiction role the Allison was the engine of choice, as it was able to operate below 1,600RPM (the Merlin was unable) and achieve a lower fuel burn (greater range).

[tdracer]

Turbofan engines have an inlet. This is a huge advantage, since with a properly performing inlet, the airspeed conditions at the fan face are pretty much independent of free stream airspeed.

Ignoring things like inlet separation and boundary layer effects, downstream of the inlet "throat" the inlet conditions are basically governed by the corrected airflow - which in turn is largely governed by the corrected N1.

Long story short, turbofan engines can get away with a fixed fan blade angle because at a given corrected N1, the fan will see a pretty much constant angle of attack independent of free stream airspeed thanks to a good inlet.