Subsonic airbreathing engines and inlet blade angle

[Benjies]

TL;DR Summary

I've been struggling to wrap my brain around blade angle with relation to ducted airbreathing engines, as we attempt to avoid the speed of sound at all points across the blades.

Hello! My background in propulsion is extremely lackluster with airbreathing engines, so any help is appreciated with the following questions. I understand blade pitch is highly variable for turbofans which operate near the speed of sound, to avoid any localities on the blades reaching the speed of sound. I suppose my confusion is with how blade angle relates to thrust. I have multiple questions, please feel free to bite off a specific question, or answer all of my questions holistically if a key concept is being missed:

1. I believe turboprop engines vary blade pitch based on Mach number. At starting conditions, do turboprops minimize blade angle relative to the shaft to maximize the centripetal acceleration of flow? (e.g., on the flight line, for liftoff, are blade angles at close to zero degrees with respect to the shaft).

2. Turbofans vary blade angle such that the blade angle is maximized near the blade tips (so that the acceleration imparted to air at the blade tips is minimized to avoid the speed of sound). Somewhat related to question 1, does increasing the blade pitch decrease thrust in general? I picture a blade at zero angle of attack relative to the shaft imparting extremely high changes in pressure (which would then expand and give great thrust), whereas a blade that is highly pitched (such as the end of a turbofan blade) imparts very little work to air that flows around it, thus it contributes very little to thrust.

I see a dichotomy: Avoiding the speed of sound seems to pidgeon-hole us into designing engines which are limited in performance. Ironically turbofans obviously have terrific specific impulses and efficiencies. Maybe I'm babbling. But the general rule of thumb seems to be "work the air as hard as you can until you hit the speed of sound on the blades- then, chill out and increase your blade pitch"

Thanks all and have a good weekend (after tomorrow).

 

[jrmichler]

The propeller blades are airfoils. Try searching airfoil angle of attack. Then consider that the airplane is moving forward with a velocity, and the propeller is spinning such that the blade has a velocity perpendicular to the direction of airplane travel. Show those two velocities in a vector sketch of the airfoil. The result is the angle of attack.

The propeller angle is changed so as to put it at the best angle of attack at all aircraft speeds from zero to maximum. To see how this is done, search constant speed propeller. Small, low speed airplanes can use fixed pitch propellers. Faster airplanes need constant speed propellers.

All propeller blades have a twist. That's because the tangential (circumferential?) speed is slow towards the hub, and fast at the tip. Repeat the above vector diagram at a point near the hub to see how the blade needs to be twisted to maintain a desired angle of attack.

 

[Benjies]

The propeller angle is changed so as to put it at the best angle of attack at all aircraft speeds from zero to maximum.

That sums it up nicely. AoA is zeroed relative to the incoming airflow at all speeds, and pitch changes towards to tip. to compensate for the higher velocity of the blade relative to the hub. Thanks!

 

[russ_watters]

Note though that it isn't twisted to avoid supersonic flow, it is twisted to reduce the angle of attack vs relative wind direction for more efficient flow/avoid stalling the tips.

 

[Averagesupernova]

I know the YouTube channel AgentJayZ has some info on on this sort of thing. Have a look.

 

[cjl]

I'm a bit unclear on your questions here - are you trying to imply that turbofans actually vary fan blade angle in flight? If so, this is false - blade angle is fixed. It would be more efficient to vary it, but it also would pose a huge problem in structure, weight, complexity, reliability, cost, etc, and since cruise conditions are pretty consistent, it makes more sense to just design them around an expected operating condition. This is in contrast to propeller planes, which do vary blade angle with flight condition.

Secondly, as russ said, the twist is to ensure consistent angle of attack across the blade. At the tip, the rotation causes a much higher velocity than at the root, but the incoming air is always at the same velocity, so at the blade tip, the blade velocity is much faster than the air (requiring a shallow angle), but at the root the air is much faster than the blade (requiring a steeper angle). The actual angle of attack (the angle of the airflow relative to the blade) should be fairly consistent across the blade, and the great majority of the actual work is done by the tips (mostly just because they sweep out a much larger area).

Finally, your premise that they avoid supersonic speeds is wrong. Propeller airplanes do try to avoid this, because it's awful for noise (to such an extent that one experimental plane with a supersonic prop was known for causing headaches and nausea to ground crews, and reportedly even a seizure), but basically every turbofan on a modern jet actually is running the blade tips at around mach 1.4-1.7 when at full power. The ducting and tight clearances mean this doesn't cause nearly the noise issue that it does for a free-air propeller, but it's why you hear kind of a "buzzsaw" noise if you're ever seated in front of the engine on a passenger airliner during takeoff. This is also why the leading edges of turbofan blades are fairly sharp compared to the rounded leading edges on propellers.

 

[berkeman]

Propeller airplanes do try to avoid this, because it's awful for noise (to such an extent that one experimental plane with a supersonic prop was known for causing headaches and nausea to ground crews, and reportedly even a seizure),

Noise

The XF-84H was almost certainly the loudest aircraft ever built, earning the nickname "Thunderscreech" as well as the "Mighty Ear Banger".[16] On the ground "run ups", the prototypes could reportedly be heard 25 miles (40 km) away.[17] Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches (61–76 cm) of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down;

 

[Benjies]

Finally, your premise that they avoid supersonic speeds is wrong. Propeller airplanes do try to avoid this, because it's awful for noise (to such an extent that one experimental plane with a supersonic prop was known for causing headaches and nausea to ground crews, and reportedly even a seizure), but basically every turbofan on a modern jet actually is running the blade tips at around mach 1.4-1.7 when at full power.

That I hadn't known. I thought the blades actively avoided Mach 1 at cruise. Honestly these Mach numbers at the tips sound much more feasible. Thanks!