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zoltank8
04-05-2007, 02:58 AM
I've heard that in certain cases the propeller tip can go supersonic and this can cause trouble. Is this correct? Can someone give me some insight of this issue?

zoltank8
04-05-2007, 02:58 AM
I've heard that in certain cases the propeller tip can go supersonic and this can cause trouble. Is this correct? Can someone give me some insight of this issue?

stathem
04-05-2007, 03:33 AM
Yes, you're correct.

It was called a blue note. Acheiveable by diving the aircraft hard in the wrong pitch, fine I think, although I often get confused. Very detrimental to the health of the aircraft.

It can be seen in game or something like it (or used to able to be seen) in the Hurricane, where you need to drop the PP when in a hard dive with low throttle. Most true CSP or autoPP a/c in game don't seem to do it. Expect it to be a big issue when SoW:BoB arrives.

M_Gunz
04-05-2007, 04:04 AM
A prop with 2 meter diameter turning at 2500 rpm is already mach .77 when the plane is stopped.

1 meter radius * 2 * pi * 2500 rpms / 60 sec/min gives appx 262 m/s at the tip.

As plane moves forward the length of the true path is sqrt( plane TAS squared + tip speed squared).

600kph TAS is appx 167 m/s, the tip of the prop above would be going appx 310 m/s.

And here is fun; at sealevel 16C temperature sound is appx 340 m/s but at 10km alt it is appx 300
so just by flying about 5km alt (4km mach 1 at 324.6 m/s, 6km mach 1 at 316.5) with 2 meter prop
at 2500 rpm you should have prop tips physically at 1 mach. With TAS at 600 kph, well below top
high alt speed on so many fighters. I guess prop diameters really count -- and Corsair I am sure
has a much larger diameter prop... is it 9 foot?

stansdds
04-05-2007, 04:22 AM
Corsair, Thunderbolt and Hellcat props are approximately 13 feet in diameter.

Ugly_Kid
04-05-2007, 04:27 AM
It's not "very detrimental to the health of the aircraft" - it happens - like the previously given numbers show, although they do not account for possible reduction gear (BTW already Bf-109 had 3 m prop diameter, I'd think Corsair had much more). The prop starts loosing effectivity when this happens.

stathem
04-05-2007, 04:52 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Ugly_Kid:
It's not "very detrimental to the health of the aircraft" - it happens - like the previously given numbers show, although they do not account for possible reduction gear (BTW already Bf-109 had 3 m prop diameter, I'd think Corsair had much more). The prop starts loosing effectivity when this happens. </div></BLOCKQUOTE>

Well, I'm going off an account in "They gave me a Seafire" by Commander RM Crosley.

In there he describes a pupil doing it on a Fairy Battle (or possibly a Defiant). Popped all his oil seals and scrapped the engine, although he got it down safe.

A typical reduction gear (on a Spit for example) would be ~.43, so an engine turning at 3000 revs would be spinning the prop at about 1400.

F16_Neo
04-05-2007, 08:16 AM
Every time I've seen Texans on airshows, the speaker has been quick to tell that the sharp sound is occurring when prop-tips go past soundbarrier. And indeed there is a sort of sharp sound. Seems like it was no big deal, for them at least.

Ugly_Kid
04-05-2007, 11:25 AM
Yes, parachutists on our airfield also had somewhat awe inspiring choice of prop for their Cessna taildragger, blade tips went supersonic in each and every take-off, the sound is quite mean - that's all.

Viper2005_
04-05-2007, 12:32 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by stathem:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Ugly_Kid:
It's not "very detrimental to the health of the aircraft" - it happens - like the previously given numbers show, although they do not account for possible reduction gear (BTW already Bf-109 had 3 m prop diameter, I'd think Corsair had much more). The prop starts loosing effectivity when this happens. </div></BLOCKQUOTE>

Well, I'm going off an account in "They gave me a Seafire" by Commander RM Crosley.

In there he describes a pupil doing it on a Fairy Battle (or possibly a Defiant). Popped all his oil seals and scrapped the engine, although he got it down safe.

A typical reduction gear (on a Spit for example) would be ~.43, so an engine turning at 3000 revs would be spinning the prop at about 1400. </div></BLOCKQUOTE>

Almost all Merlin Spitfires had reduction gear ratios of 0.477:1.
Therefore running the engine at 3000 rpm would spin the prop at 1431 rpm, or 23.85 revolutions per second.

The Spitfire IX's Merlin 60 series swung a 10'9" prop. That's 3.2766 m, giving a circumference of 10.2937425 m, for a static tip speed of 245.51 m/s (2 d.p.), which is Mach 0.722 on a standard day.

At +25 psi, the Spitfire IX had a maximum speed of about 360 mph, which is about 161 m/s.

(161^2+246^2)^0.5 = 294 m/s, giving a tip Mach number of 0.865.

The HF.IX was capable of 416 mph, or 186 m/s at 27500 feet.

The speed of sound at 27500 feet on a standard day is a little over 306 m/s. The tip speed of the prop remains the same at 246 m/s.

Therefore the tip Mach number is (186^2+246^2)^0.5/306 = 1.01

Of course, the critical Mach number of the prop aerofoil will be rather less than 1, so there is likely to be shock development over the prop in high speed level flight.

Supersonic flow over the props of high performance aircraft is pretty routine.

Sergio_101
04-05-2007, 06:20 PM
Yep, prop tips can go supersonic easily in a fighter.
Shock waves and vibration can destroy the aircraft
when more of the prop is effected by shock waves.

One of the reasons the P-51B through P-51H
got such a thin prop is that the shock waves
set up later on the thinner blades.

Some Corsairs for a 3:1 reduction ratio to keep
the prop sub sonic as late as possible.

Convairs B-36 was powered by R-4360s and swung a 19 foot dia prop.
I am not aware of the gear ratio at this time
but it must have been 3:1 or lower.

Sergio

Ratsack
04-05-2007, 08:16 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by F16_Neo:
Every time I've seen Texans on airshows, the speaker has been quick to tell that the sharp sound is occurring when prop-tips go past soundbarrier. And indeed there is a sort of sharp sound. Seems like it was no big deal, for them at least. </div></BLOCKQUOTE>

Yes, I've heard this sound, too, on Wirraways and Boomerangs, which are based on the Texan. The sound is like the air is ripping.

The Fw 190 A-3 running at 1.42 ATA @ 2,700 rpm spun its 3.0 m prop at 1,290 rpm. So that's a reduction gear of 0.48. I guess that comes to about Mach 0.61 when the plane is stationary.

At its top speed of 680 km/h at 6,500 m, the speed of the prop tip is about Mach 0.88. This is calculated on an approximation of Mach 1.0 at 6,500 m being 314 m/s. The rest is just geometry.

I figure that if you climb above 6,500 m, and then at max power (i.e., 1.42 ATA at 2,700 rpm) dive to an indicated airspeed of 606 km/h at 6,500 m, then the prop tips would be ripping through the air at Mach 1.0. The plane itself would moving at Mach 0.76, which I suppose would be unpleasant.

cheers,
Ratsack

The-Pizza-Man
04-05-2007, 08:34 PM
I've also heard that said of Avengers at airshows. They seem to make a tearing sound as they go past.

M_Gunz
04-05-2007, 08:39 PM
So it makes sense to cut prop speed in dives?

Doubly so when you as pilot know the Vne, velocity not to exceed, of your plane... cutting power
gives you a little extra time before you might see the needle flash right past safe speed for
any much maneuver at all which really cuts the options down.
What kind of war would it have been if pilots could bail out knowing that 1 minute later they
would taking off in a new, perfect condition plane with no loss over the old one? How fast
might we have found out that any plane made could really go? Historic data won't get you that!

M_Gunz
04-05-2007, 08:43 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by The-Pizza-Man:
I've also heard that said of Avengers at airshows. They seem to make a tearing sound as they go past. </div></BLOCKQUOTE>

Where do the straight exhaust pipes point? The motor is running faster than the prop and makes
a loud ripping poppity noise itself!

Ratsack
04-05-2007, 10:16 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by The-Pizza-Man:
I've also heard that said of Avengers at airshows. They seem to make a tearing sound as they go past. </div></BLOCKQUOTE>

Where do the straight exhaust pipes point? The motor is running faster than the prop and makes
a loud ripping poppity noise itself! </div></BLOCKQUOTE>

There's a difference between the exhaust sound and the sound of a supersonic blade tip. If you get a chance to hear a Texan take off from a short strip (or from a higher alt strip), where they have to use max revs right at the start, you will definitely hear the distinctive sound of supersonic prop tips. You'll know it once you hear it.

cheers,
Ratsack

VW-IceFire
04-05-2007, 10:26 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Ratsack:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by M_Gunz:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by The-Pizza-Man:
I've also heard that said of Avengers at airshows. They seem to make a tearing sound as they go past. </div></BLOCKQUOTE>

Where do the straight exhaust pipes point? The motor is running faster than the prop and makes
a loud ripping poppity noise itself! </div></BLOCKQUOTE>

There's a difference between the exhaust sound and the sound of a supersonic blade tip. If you get a chance to hear a Texan take off from a short strip (or from a higher alt strip), where they have to use max revs right at the start, you will definitely hear the distinctive sound of supersonic prop tips. You'll know it once you hear it.

cheers,
Ratsack </div></BLOCKQUOTE>
Yeah its an odd sound...its almost never when they are flying at you or flying away from you but just at certain angles as they pass by there is this enormous roar and its allegedly from the prop tips according to every announcer that I've ever listened to.

The-Pizza-Man
04-06-2007, 12:34 AM
I can't say for certain what it was but the noise of the Avenger didn't sound like the engine.

stathem
04-06-2007, 12:36 AM
It's interesting to Google 'Havard Prop tips'

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">
Unlike the BC-1, the BC-1As had a direct-drive engine giving the propeller tips a high speed causing a distinctive rasping noise unique to all SNJs. </div></BLOCKQUOTE>

Link (http://www.microworks.net/PACIFIC/aviation/snj.htm)

zoltank8
04-06-2007, 03:04 AM
Thanks for all your answers.

FritzGryphon
04-06-2007, 03:31 AM
Also note that, like a wing, the airflow over the surface of the blade is traveling faster than the relative airflow.

Of course, the tip of a blade is very thin, but it still applies.