Propeller

[Cheif Operations Director]

If a prop blade is fed more air/and increase in air pressure will it have a higher power output.

To summarise if you have a P-51 operating at for whatever reasons a higher pressure environment/where their are more particles of air than normal) will the plane go faster or slower.

The increased "drag" / resistance would slow down the motor but the extra air being moved would speed it up because it has more force behind it  so which would win out. I'm inclinded to think that the propeller would move faster but I'm not that much

[Xd the great]

No, it will move slower due to drag, so the speed of the plane will remain the same. Or slower due to more drag.

Conservation of energy.

[p1t1o]

@Cheif Operations Director 

If you simply put the aircraft in a higher pressure environment, then extra air is balanced by increased drag, though there may be some small scale imbalances you can take advantage of.

However if you are asking how forward motion "feeds" air into the prop, then there is a more interesting answer.

The answer is yes and no.

A stationary prop creates zone of low pressure in front of it, air mass flow is restricted by air's ability to flow into the space fast enough.

At this point, moving the prop forwards, and increasing its speed, increases the amount of air that can pass through the prop disc, which does increase power output.

At some speed, this will reach a limit, where further forwards speed cannot force more air through the blade and at this point the prop itself is producing drag.

The effect is more pronounced in jets I think (which have blades just like prop engines). A Jet produces FAR more power at speed than stationary, simply because the forward motion is ramming more air into the intake and stationary, pressure differential will force only so much air into the engine.

But yes, peak power output will not be at zero speed but at some positive forwards velocity due to the very effect you mention.

Edited July 20, 2018 by p1t1o

[Cunjo Carl]

7 hours ago, Cheif Operations Director said:

If a prop blade is fed more air/and increase in air pressure will it have a higher power output.

To summarise if you have a P-51 operating at for whatever reasons a higher pressure environment/where their are more particles of air than normal) will the plane go faster or slower.

The increased "drag" / resistance would slow down the motor but the extra air being moved would speed it up because it has more force behind it  so which would win out. I'm inclinded to think that the propeller would move faster but I'm not that much

 

Oh, this is a fun one! Your intuition on what's important is good, but it's very hard to guess how all the different effects play off eachother. For that very reason, some quite sensible engineers made charts!

The interplay between air pressure (normally described through 'altitude') and available min/max flight speeds is called the plane's flight envelope. Real world ones tend to be extremely nuanced, but in general the graphs I've seen (I'm not a pilot) show jet engine top speeds being ever so slightly higher at soupy sea level pressures than at the thinner pressures of increasing altitudes. That said, efficiency (payload miles per fuel burned, style thing) improves drastically up at the traditional ~30,000ft = 10km cruising altitude jets fly at.

(example jet plane flight envelope pg. 42, 43) https://www.princeton.edu/~stengel/MAE331Lecture6.pdf

Prop planes are another story, and here we're quite lucky that The P-51 is old enough we can just pull up the user manual and check! Briefly though, The P-51 can fly fastest and farthest (within engine spec conditions) in the much thinner atmosphere of 30,000ft (precise values model/engine/configuration pending).

(P-51 user manual on "pg. 54" shown as the 58th page of the PDF, see "T.A.S." = airspeed column on the bottom left when viewed landscape) http://www.wwiiaircraftperformance.org/mustang/P-51D-manual-5april44.pdf

Just for reference, up at this altitude, the pressure would be ~.3atm, so that's pretty thin! https://www.avs.org/AVS/files/c7/c7edaedb-95b2-438f-adfb-36de54f87b9e.pdf

The many and important nuances of all this stuff in real world planes rather immediately goes above my head, and I assume things like modern variable pitch props may have a different story to tell in terms of flight envelope! I hope you find some of this stuff useful, let me know if you have questions. Cheers.

[Cheif Operations Director]

The P-51 could move faster due too less air restiance right? If just the propellor got sea level pressure but the rest of the plane got the 30,000 feet pressure would th plane be able to give off more or less energy in your opinion @Cunjo Carl.

Thanks for th additions P-51 Materials

[magnemoe]

2 minutes ago, Cheif Operations Director said:

The P-51 could move faster due too less air restiance right? If just the propellor got sea level pressure but the rest of the plane got the 30,000 feet pressure would th plane be able to give off more or less energy in your opinion @Cunjo Carl.

Thanks for th additions P-51 Materials

Turoprop planes always go up to attitude even for fairly short flights as in 30 minutes as it increases speed and reduce fuel use. 
Almost feels ballistic as they climb then decent. 
You can change the angle of attack on the blades to increase trust at high attitude on modern planes. You can not on cheap single engine personal planes but they like staying lower for view and fun. I assume you could change the angle of attack on a p-51 they was doing bomber escort who involved long distance high attitude flights. 

[Cunjo Carl]

11 minutes ago, Cheif Operations Director said:

The P-51 could move faster due too less air restiance right? If just the propellor got sea level pressure but the rest of the plane got the 30,000 feet pressure would th plane be able to give off more or less energy in your opinion @Cunjo Carl.

Thanks for th additions P-51 Materials

That's interesting... My impression is that yes, it would go faster. Especially for a propeller whose thrust output is limited by its prop RPMs (in turn partially limited by manifold pressure) like the Spitfire's. I think energy is a slightly less important aspect than thrust. The two are closely related, but thrust winds up really being the ability of the system to propel the plane.

ED: Now I'm curious, did something in particular bring on this question? It's a nice one.

Edited July 20, 2018 by Cunjo Carl

[Cheif Operations Director]

12 minutes ago, Cunjo Carl said:

That's interesting... My impression is that yes, it would go faster. Especially for a propeller whose thrust output is limited by its prop RPMs (in turn partially limited by manifold pressure) like the Spitfire's. I think energy is a slightly less important aspect than thrust. The two are closely related, but thrust winds up really being the ability of the system to propel the plane.

ED: Now I'm curious, did something in particular bring on this question? It's a nice one.

Yes I was I looking into wind tunnels and thought about, when I make my own, increasing the pressure in the blades/fan area. It would be hard but perhaps it may work. The P-51 was just an easy example.

[Cheif Operations Director]

One other thing does anyone know why some wings a swept backyard. A flat surface would theoretically create more lift right?

[Cunjo Carl]

5 minutes ago, Cheif Operations Director said:

One other thing does anyone know why some wings a swept backyard. A flat surface would theoretically create more lift right?

Hopefully someone will be able to give a more thorough explanation, but the quick version is that sweeping the wings backwards decreases L/D (lift to drag) for subsonic flight, which is bad,  but it increases L/D for supersonic flight. So if you want to be going supersonic a lot, it's a worthwhile trade. The mechanism is from a special kind of drag in supersonic flight called 'bow shock' where the plane is going so fast that molecules can't flow around the plane nicely and instead ricochet off the nose (and other leading surfaces) creating high pressure shocks waves going backwards in a cone shape. If the edges of that cone line up on another part of the plane, you get silly amounts of drag (as I understand it), and the cone gets narrower the faster you go. So! If you have a more triangularly shaped plane that can fit entirely within the cone, you have less drag = better flight performance. And, the narrower (more swept) you can get it, the faster you can go before those shockwaves cause you trouble. The same applies to the wings themselves, that they should be more swept back than this bowshock cone starting where they attach to the plane. Here's some pictures:

(Lecture notes, "pg 27, 28" shown as pages 31,32 in the power point) https://mae.usu.edu/faculty/stephen-whitmore/files/mae-5420-compressible-fluid-flow/section7.3.pdf

Oh, I like the idea with the wind tunnel! Making a high pressure stream sounds tricky... As I understand it, pressure is just a fill-in for density anyways! Maybe you could use a light gas like He for the wings, or a heavy gas like (not the best for the environment) SF6 for the propeller?

[Cheif Operations Director]

1 minute ago, Cunjo Carl said:

Hopefully someone will be able to give a more thorough explanation, but the quick version is that sweeping the wings backwards decreases L/D (lift to drag) for subsonic flight, which is bad,  but it increases L/D for supersonic flight. So if you want to be going supersonic a lot, it's a worthwhile trade. The mechanism is from a special kind of drag in supersonic flight called 'bow shock' where the plane is going so fast that molecules can't flow around the plane nicely and instead ricochet off the nose (and other leading surfaces) creating high pressure shocks waves going backwards in a cone shape. If the edges of that cone line up on another part of the plane, you get silly amounts of drag (as I understand it), and the cone gets narrower the faster you go. So! If you have a more triangularly shaped plane that can fit entirely within the cone, you have less drag = better flight performance. And, the narrower (more swept) you can get it, the faster you can go before those shockwaves cause you trouble. The same applies to the wings themselves, that they should be more swept back than this bowshock cone starting where they attach to the plane. Here's some pictures:

(Lecture notes, "pg 27, 28" shown as pages 31,32 in the power point) https://mae.usu.edu/faculty/stephen-whitmore/files/mae-5420-compressible-fluid-flow/section7.3.pdf

Oh, I like the idea with the wind tunnel! Making a high pressure stream sounds tricky... As I understand it, pressure is just a fill-in for density anyways! Maybe you could use a light gas like He for the wings, or a heavy gas like (not the best for the environment) SF6 for the propeller?

Then why does the 737 has swept wings if it's sub sonic

[linuxgurugamer]

Because the jets to go fast enough for it to make a difference. Also, makes the plane more stable.

[Cheif Operations Director]

Thanks for the clarification