If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
Dumb Question
When designing rotor systems, you have to keep the tip speed subsonic
to prevent all sorts of problems and losses that would otherwise occur at the sound barrier, right? What about large turbofan engines such as on modern airliners? The fan tips must be travelling at many orders of magnitude faster than the speed of sound? Why are they not subject to the same limitation? Just wondering. |
#2
|
|||
|
|||
Dumb Question
Good question!
Hey, since here aren't a lot of people, I dare to write something for discussion although I don't know much. When designing rotor systems, you have to keep the tip speed subsonic to prevent all sorts of problems and losses that would otherwise occur at the sound barrier, right? Well, the main reason is noise and then there are strength issues. What about large turbofan engines such as on modern airliners? The fan tips must be travelling at many orders of magnitude faster than the speed of sound? Why are they not subject to the same limitation? Why do you think, they must be travelling at Mach 1? AFAIK it's not true at all. First there is no advancing blade, so the TAS does not add to the fan blade tip speed because it is perpendicular. Second, even Jets capable of supersonic speed have most parts of their engines working with subsonic velocities. Especially the air intake is designed to be at subsonic. Good to see at the Concorde. It sounds unlogical, that the air intake can be subsonic when the whole aircraft is supersonic, but I cannot recall the explanation. Supersonic aerodynamics are rather simple, but the transition is extremely weird. Having said that, the fans must be rotating at a reasonable speed compared to a rotor. They may rotate a bit faster because their diameter is smaller and they do not need a margin for an advancing blade. |
#3
|
|||
|
|||
Dumb Question
Steve L. wrote: Good question! Hey, since here aren't a lot of people, I dare to write something for discussion although I don't know much. That's the spirit! When designing rotor systems, you have to keep the tip speed subsonic to prevent all sorts of problems and losses that would otherwise occur at the sound barrier, right? Well, the main reason is noise and then there are strength issues. What about large turbofan engines such as on modern airliners? The fan tips must be travelling at many orders of magnitude faster than the speed of sound? Why are they not subject to the same limitation? Why do you think, they must be travelling at Mach 1? AFAIK it's not true at all. First there is no advancing blade, so the TAS does not add to the fan blade tip speed because it is perpendicular. Second, even Jets capable of supersonic speed have most parts of their engines working with subsonic velocities. Especially the air intake is designed to be at subsonic. Good to see at the Concorde. It sounds unlogical, that the air intake can be subsonic when the whole aircraft is supersonic, but I cannot recall the explanation. Supersonic aerodynamics are rather simple, but the transition is extremely weird. Having said that, the fans must be rotating at a reasonable speed compared to a rotor. They may rotate a bit faster because their diameter is smaller and they do not need a margin for an advancing blade. Hmm, I'm thinking about say 60,000rpm or 10,000 rps with a 3 metre diameter fan.(Making these numbers up but I expect they are in the ballpark) therefore the tip is travelling approx 9m * 10,000m every second or 36km/sec or 1500km/hr or about mach1.2? That's in stationary air - ignoring any advancing blade scenario. My guess is that because the tips are enclosed inside a duct that there is no vortex or boundary effects at the tips? |
#4
|
|||
|
|||
Dumb Question
JohnO wrote: Steve L. wrote: Good question! Hey, since here aren't a lot of people, I dare to write something for discussion although I don't know much. That's the spirit! When designing rotor systems, you have to keep the tip speed subsonic to prevent all sorts of problems and losses that would otherwise occur at the sound barrier, right? Well, the main reason is noise and then there are strength issues. What about large turbofan engines such as on modern airliners? The fan tips must be travelling at many orders of magnitude faster than the speed of sound? Why are they not subject to the same limitation? Why do you think, they must be travelling at Mach 1? AFAIK it's not true at all. First there is no advancing blade, so the TAS does not add to the fan blade tip speed because it is perpendicular. Second, even Jets capable of supersonic speed have most parts of their engines working with subsonic velocities. Especially the air intake is designed to be at subsonic. Good to see at the Concorde. It sounds unlogical, that the air intake can be subsonic when the whole aircraft is supersonic, but I cannot recall the explanation. Supersonic aerodynamics are rather simple, but the transition is extremely weird. Having said that, the fans must be rotating at a reasonable speed compared to a rotor. They may rotate a bit faster because their diameter is smaller and they do not need a margin for an advancing blade. Hmm, I'm thinking about say 60,000rpm or 10,000 rps with a 3 metre diameter fan.(Making these numbers up but I expect they are in the ballpark) therefore the tip is travelling approx 9m * 10,000m every second or 36km/sec or 1500km/hr or about mach1.2? That's in stationary air - ignoring any advancing blade scenario. My guess is that because the tips are enclosed inside a duct that there is no vortex or boundary effects at the tips? D'oh! Only 1000 rps! |
#5
|
|||
|
|||
Dumb Question
Hmm, I'm thinking about say 60,000rpm or 10,000 rps with a 3 metre
diameter fan.(Making these numbers up but I expect they are in the ballpark) therefore the tip is travelling approx 9m * 10,000m every second or 36km/sec or 1500km/hr or about mach1.2? That's in stationary air - ignoring any advancing blade scenario. My guess is that because the tips are enclosed inside a duct that there is no vortex or boundary effects at the tips? D'oh! Only 1000 rps! Yeah, you were thinking of the inner stages. Your RPM range of ten thousands is realistic on the inner shaft for last compressor and first turbine stages. Their diameters are below 0.5m. The outer stages get bigger and have lower RPM. For the fans there might even be a gear. The border between (turbo)propeller and (turbo)fan is quite fluent. But an interesting question is why nobody ever tried a supersonic rotor. Maybe it's easier to fold or stop the rotor than to make it strong enough and still efficient for sub- and supersonic speeds. |
#6
|
|||
|
|||
Dumb Question
JohnO wrote:
JohnO wrote: Steve L. wrote: Good question! Hey, since here aren't a lot of people, I dare to write something for discussion although I don't know much. That's the spirit! When designing rotor systems, you have to keep the tip speed subsonic to prevent all sorts of problems and losses that would otherwise occur at the sound barrier, right? Well, the main reason is noise and then there are strength issues. What about large turbofan engines such as on modern airliners? The fan tips must be travelling at many orders of magnitude faster than the speed of sound? Why are they not subject to the same limitation? Fan blades are relatively short, rigid, fastened to an outer ring, and often titanium too. Rotor blades are so long and flexible they curl like a whip at some points around their path. There's an interesting video of that on the net somewhere. Why do you think, they must be travelling at Mach 1? AFAIK it's not true at all. First there is no advancing blade, so the TAS does not add to the fan blade tip speed because it is perpendicular. Second, even Jets capable of supersonic speed have most parts of their engines working with subsonic velocities. Especially the air intake is designed to be at subsonic. Good to see at the Concorde. It sounds unlogical, that the air intake can be subsonic when the whole aircraft is supersonic, but I cannot recall the explanation. Supersonic aerodynamics are rather simple, but the transition is extremely weird. The intake duct expands behind the mouth, slowing the airflow. And this sounds illogical, but because the air is slower moving it has higher pressure (even though expanded!) than it would if the duct didn't expand behind the mouth. There's another expansion area, the "diffuser", after the compressor stages. Compressed air expands, slows, and gains more pressure there too. |
#7
|
|||
|
|||
Dumb Question
"JohnO" wrote in message ps.com... When designing rotor systems, you have to keep the tip speed subsonic to prevent all sorts of problems and losses that would otherwise occur at the sound barrier, right? When props (or rotors I would suppose) exceed the sound barrier at about 1100 fps tip speed, they have a strong tendancy to self distruct. I assume due to acoustic vibrations at the supersonic tip. I do know they get very, very loud. What about large turbofan engines such as on modern airliners? The fan tips must be travelling at many orders of magnitude faster than the speed of sound? Why are they not subject to the same limitation? I think it is the ducting. It greatly limits (or directs) the acoustic vibrations or waves from the tips. In a jet engine, the blades usually run very close to the duct, even touching occasionally in some cases. And you are correct, they often run 2 or 3 times the speed of sound. Just wondering. |
#8
|
|||
|
|||
Dumb Question
"JohnO" wrote in message oups.com... Steve L. wrote: Good question! Hey, since here aren't a lot of people, I dare to write something for discussion although I don't know much. That's the spirit! When designing rotor systems, you have to keep the tip speed subsonic to prevent all sorts of problems and losses that would otherwise occur at the sound barrier, right? Well, the main reason is noise and then there are strength issues. What about large turbofan engines such as on modern airliners? The fan tips must be travelling at many orders of magnitude faster than the speed of sound? Why are they not subject to the same limitation? Why do you think, they must be travelling at Mach 1? AFAIK it's not true at all. First there is no advancing blade, so the TAS does not add to the fan blade tip speed because it is perpendicular. Second, even Jets capable of supersonic speed have most parts of their engines working with subsonic velocities. Especially the air intake is designed to be at subsonic. Good to see at the Concorde. It sounds unlogical, that the air intake can be subsonic when the whole aircraft is supersonic, but I cannot recall the explanation. Supersonic aerodynamics are rather simple, but the transition is extremely weird. Having said that, the fans must be rotating at a reasonable speed compared to a rotor. They may rotate a bit faster because their diameter is smaller and they do not need a margin for an advancing blade. Hmm, I'm thinking about say 60,000rpm or 10,000 rps with a 3 metre diameter fan.(Making these numbers up but I expect they are in the ballpark) therefore the tip is travelling approx 9m * 10,000m every second or 36km/sec or 1500km/hr or about mach1.2? That's in stationary air - ignoring any advancing blade scenario. My guess is that because the tips are enclosed inside a duct that there is no vortex or boundary effects at the tips? Although some VTOL aircraft have been designed with ducted fans, I think the biggest reason for a lack of interest in that area is that you loose the ability to perform emergency landings by autorotation. And that's a biggie in most peoples book. |
#9
|
|||
|
|||
Dumb Question
My guess is that because the tips are enclosed inside a duct that there
is no vortex or boundary effects at the tips? Although some VTOL aircraft have been designed with ducted fans, I think the biggest reason for a lack of interest in that area is that you loose the ability to perform emergency landings by autorotation. And that's a biggie Why should a ducted fan/rotor not be capable of working in autorotation? I never heard about that and cannot find a reason. |
#10
|
|||
|
|||
Dumb Question
Why should a ducted fan/rotor not be capable of working in autorotation?
I never heard about that and cannot find a reason. How could it? With a duct around it, would you not have to descend straight down? And if you did, would the rotational mass of a rotor that small in diameter, and designed to work at such a high rpm, store enough energy in free fall, to stop your descent when you applied collective to land? |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
DUMB ROOKIE QUESTION | Travis Beach | Soaring | 10 | November 7th 06 07:28 PM |
stupid question | JaKoB | General Aviation | 2 | November 9th 04 09:48 AM |
A question on Airworthiness Inspection | Dave S | Home Built | 1 | August 10th 04 05:07 AM |
Dumb Transponder Question! | John P | Owning | 2 | March 30th 04 01:26 AM |
Partnership Question | Harry Gordon | Owning | 4 | August 16th 03 11:23 PM |