Dumb Question

[JohnO]

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.

[Steve L.]

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.

[JohnO]

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?

[JohnO]

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!

[Stefan Lörchner]

>> 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.

[Beryl]

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.

[Maxwell]

"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.
>

[Maxwell]

"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.

[Stefan Lörchner]

>> 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.

[Maxwell]

> 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?

[Stefan Lörchner]

>> 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

Straight down can be a possibility. But if you can fly the thing, you
can also tilt the thrust vectors. So just tilt it so that the airflow
matches airspeed like in a heli.

>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?

That's a design issue since it is not free fall. Otherwise it wouldn't
be autorotation. The rotors/fans would of course have a windmilling
braking effect. And certainly, the inertia has to be sufficient to allow
smooth and preferably save touchdown as should be the case in every
helicopter.

Smaller and faster rotors are less effective and therefore need more
descent rate (more power). That's fact, too. Compare the sink rate in
autorotation of a Hughes 500 and a R44 (or the HV diagrams).

[Linc]

Stefan Lörchner wrote:
> 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.

Retreating blade stall.

Linc