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HomeAircraftHelicopterMain Discussion › RC breaks sound barrier
03-11-2012 03:30 AM  6 years agoPost 1
DarkSide41

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Sylacauga Alabama USA

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LOL, thought some might get a kick outta this . Great day of flying at the field this afternoon.

" The biggest threat to our freedom is not from foreign terrorist , but domestic morons"

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03-11-2012 03:43 AM  6 years agoPost 2
Justin Stuart (RIP)

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Plano, Texas

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Maybe not breaking the sound barrier, but it certainly does look cool.

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03-11-2012 05:21 AM  6 years agoPost 3
Dood

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Wescanson

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Pretty cool. Did you do that in Photoshop?

Strangely enough, the helicopter is MUCH closer to breaking the sound barrier than that plane is. And by much closer, I mean not close at all. A typical heli with 600mm blades needs a headspeed of about 4860rpms to go supersonic at sea level.

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03-11-2012 05:33 AM  6 years agoPost 4
jgunpilot

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Pollock, LA

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If you are going to consider the helicopter rotor, then you have to consider the pusher airplanes prop. It's the law.

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03-11-2012 06:31 AM  6 years agoPost 5
Dood

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Wescanson

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I don't know how big the prop is nor do I know the RPM, but I'll make some guesses.

Lets say it's a 7" prop turning at 15,000 RPM's (or 250 revs per second).

7" * pi=@21.99"
21.99*250=5497.5"
5497.5" / 12=458' per second.

Speed of sound @ sea level = approx. 1126' per second.

The 7" prop at 15,000 RPM's is travelling at roughly 40% the speed of sound.

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03-11-2012 06:33 AM  6 years agoPost 6
jgunpilot

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Pollock, LA

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I guess I'm the asshole.

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03-11-2012 06:38 AM  6 years agoPost 7
rudyy

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E. Amherst, NY

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This is my breaking the sound barrier photo.. taken last year's spring with puddles of water at the flying field's runway.

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03-11-2012 06:45 AM  6 years agoPost 8
Dood

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Wescanson

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Plug in any realistic number and you'll never come close to the sound barrier with a propeller.

Lets use a 6'3" prop on a Cessna 172 turning at 2400 RPM's.
With that, the tip speed is travelling at 785 feet per second, only 70% the speed of sound.

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03-11-2012 06:47 AM  6 years agoPost 9
Dood

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Cool picture, Rudyy. Would I be wrong to assume that landing ended with a nose-over?

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03-11-2012 06:53 AM  6 years agoPost 10
Justin Stuart (RIP)

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Plano, Texas

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I believe that the propellers on the XF-84H would regularly go supersonic even at idle and as a consequence came to be known as the "loudest aircraft ever". I have been searching for years for an audio recording but never found one.

http://en.wikipedia.org/wiki/Republic_XF-84H

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03-11-2012 07:00 AM  6 years agoPost 11
Dood

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Wescanson

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Well no wonder, it has a 12 foot prop!!

12' prop at 2100rpm's is 1319 feet per second.

I wonder why they only built two of them?

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03-11-2012 02:20 PM  6 years agoPost 12
DarkSide41

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Sylacauga Alabama USA

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Pretty cool. Did you do that in Photoshop?

Strangely enough, the helicopter is MUCH closer to breaking the sound barrier than that plane is.
Yeah a quick job in Photoshop just for giggles . There are so many sites on the net that wrongly attribute the vapor cones to breaking the sound barrier that I figured I would have some fun with them

The canopy for the Foamy F18 is a raptor 30 shield folded over in the fuselage .

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03-11-2012 02:49 PM  6 years agoPost 13
jschenck

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La Vista, NE.

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Waco's get some nice prop sound in an airshow by cracking the sound barrier

random YT vid of a Waco

Watch at YouTube

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03-11-2012 02:57 PM  6 years agoPost 14
Sillyness

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Little Rock AR

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Geeking out here.

Gotta keep in mind that the speed a prop is experiencing is the vector sum of rotational speed plus the speed of forward flight, and it needs to stay subsonic at all operating altitudes. Here's a little example:

Speed of sound in knots (SL): 661 knots
Speed of sound at 25k feet: 601 knots (avg)
Tip speed of a C-130's 13.5' Prop at 1021 RPM: 430 knots
True airspeed in typical descent starting from 25000': 350knots

Vector sum of 350 forward + 430 rotational: 554 knots
MACH Number: .92 MACH

That's the net flow. At high points over the prop's airfoil the local flow will be faster, right in the transonic range.

It's a bad thing if the C-130 prop overspeeds... it really is close to supersonic all the time in normal flight. It doesn't take much to push it over. It's so critical that at 103 percent the prop dumps internal pressure to keep blade angle from decreasing any more, and at 103.5% the motor starts aggressively dumping fuel-flow to prevent any further overspeed.

Another neat fact... the vector math for blade angle. In the same scenario, at idle in the descent, the prop blade angle will be over 40 degrees. That's to produce zero power... just to align with the slip-stream over the blade. Add power and you are in the 50+ degree range.

Only reason I bring this last bit up is that I see people bringing up limiting tail blade angle to an angle below blade stall. The heli tail is subject to the same vector math effects. If you are going 100 mph sideways, your tail blades will need to be at around 23-25 degrees to produce NO thrust, more to keep the tail from weather-vaning, and more still to counter torque. Point... there's no point to limiting tail blade travel. Get as much as you can. You can prevent stalling by ensuring your tail is spinning fast enough and your blades are long enough. If you are actually stalling the tail in flight, your blades are too short or your tail is not up to speed (low head speed flying?).

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03-11-2012 03:33 PM  6 years agoPost 15
rudyy

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E. Amherst, NY

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Cool picture, Rudyy. Would I be wrong to assume that landing ended with a nose-over?
Dood, how do you know that? You have 6th sense?

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03-11-2012 03:47 PM  6 years agoPost 16
Justin Stuart (RIP)

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Plano, Texas

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Geeking out here.
Gotta keep in mind that the speed a prop is experiencing is the vector sum of rotational speed plus the speed of forward flight, and it needs to stay subsonic at all operating altitudes. Here's a little example:
Speed of sound in knots (SL): 661 knots
Speed of sound at 25k feet: 601 knots (avg)
Tip speed of a C-130's 13.5' Prop at 1021 RPM: 430 knots
True airspeed in typical descent starting from 25000': 350knots
Vector sum of 350 forward + 430 rotational: 554 knots
MACH Number: .92 MACH
That's the net flow. At high points over the prop's airfoil the local flow will be faster, right in the transonic range.
It's a bad thing if the C-130 prop overspeeds... it really is close to supersonic all the time in normal flight. It doesn't take much to push it over. It's so critical that at 103 percent the prop dumps internal pressure to keep blade angle from decreasing any more, and at 103.5% the motor starts aggressively dumping fuel-flow to prevent any further overspeed.
Another neat fact... the vector math for blade angle. In the same scenario, at idle in the descent, the prop blade angle will be over 40 degrees. That's to produce zero power... just to align with the slip-stream over the blade. Add power and you are in the 50+ degree range.
Only reason I bring this last bit up is that I see people bringing up limiting tail blade angle to an angle below blade stall. The heli tail is subject to the same vector math effects. If you are going 100 mph sideways, your tail blades will need to be at around 23-25 degrees to produce NO thrust, more to keep the tail from weather-vaning, and more still to counter torque. Point... there's no point to limiting tail blade travel. Get as much as you can. You can prevent stalling by ensuring your tail is spinning fast enough and your blades are long enough. If you are actually stalling the tail in flight, your blades are too short or your tail is not up to speed (low head speed flying?).
It's nice when the math nerds come on here to comment. Much appreciated.

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03-11-2012 05:16 PM  6 years agoPost 17
rexxigpilot

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Florida

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Good job explaining that Sillyness.

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03-12-2012 11:19 AM  6 years agoPost 18
Sillyness

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Little Rock AR

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Well, it's part the math nerd in me and part the pilot in me. You have to be a geek to some extent to be a pilot. The saving grace of this particular variety of geekiness is that chicks still think you're cool

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03-12-2012 11:28 AM  6 years agoPost 19
Dood

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Wescanson

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Dood, how do you know that? You have 6th sense?
Just a lucky guess. I could tell by the picture you came in really hot. Wet grass is "stickier" than dry grass. Add all that to the fact that your plane is tail dragger, it makes for a guaranteed nose over a prop stuck in the mud.

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03-12-2012 09:39 PM  6 years agoPost 20
Rogman88

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West Monroe, LA

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I love Doods' responses. I've never seen him wrong, and he's usually really entertaining too. I a sense..."Dood is the man"...

Rock on Dood!

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