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Prop/Engine question. 2700 RPM max?

Capt_CrashN_Burn
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Capt_CrashN_Burn Community Regular

Capt_CrashN_Burn

Posted
Posted

Now, I've been told that the reason that direct drive engines operate at a max of 2700 rpm has something to do with the prop tips approaching the speed of sound.


Now if you take the speed of sound at sea level and figure for a prop tip speed of .9 mach on a 72" prop, you get an RPM of 3193.


Obviously, the speed of sound is much lower @ 30,000 ft, and you can probably push the prop tips to .95 mach. Does anyone have a chart of the speed of sound at different altitudes??


Is there a reason the industry settled on the 2700 figure??

Immelman Mentor

Immelman

Posted
Posted 

The speed of sound in the atmosphere varies inversely with temperature.

From Dr. Ed Williams' avaiation formulary:

---

Mach Number (M) = TAS/CS

CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius.

TAS is true airspeed in knots.

---

Let's take an example:

1. 15C (std day at sea level): 678 knots

2. -40C (you're flying your Mooney up high, up north, on a cold winter day): 595 knots.

I haven't done the math on the prop circumference vs. rpm to get its true airspeed, but that should be easy.

Capt_CrashN_Burn Community Regular

Capt_CrashN_Burn

Posted
  • Author
Posted

Quote: Immelman

The speed of sound in the atmosphere varies inversely with temperature. From Dr. Ed Williams' avaiation formulary:

Mach Number (M) = TAS/CS

CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius.

TAS is true airspeed in knots.

N601RX Grand Master

N601RX

Posted
Posted

I have an experimental with a 6 cylinder Jabiru.  The red line is at 3300 rpm's, but they use a 64" prop with a special shape tip on it.


 

MARZ Mentor

MARZ

Posted
Posted

Quote: Capt_CrashN_Burn

 I don't see where that compensates for altitude though. Temperature is a minor variable compared to elevation.

MARZ Mentor

MARZ

Posted
Posted

 


Some calculations based on prop and speed as well as some tables on Speed of sound - basically it looks like the tips of prop at 2700 rpm don't start to approch the speed of sound in our operating environment.



RPM      2,700.00
Prop diameter          72.00
Prop Radius        226.19
tip travel in inches at 2700 RPM  610,725.61
tip travel in feet at 2700 RPM    50,893.80
Feet in a mile

     5,280.00


Tip Travel - miles in a minute             9.64
Tip Travel - miles in an hour        578.34
   
Tip travel Meters in a Minute    15,512.43
Tip travel Meter per second        258.54


 


Effect of temperature
Temperature Speed of sound Density of air Acoustic impedance
\vartheta in °C c in m·s−1 ρ in kg·m−3 Z in N·s·m−3
+35 351.96 1.1455 403.2
+30 349.08 1.1644 406.5
+25 346.18 1.1839 409.4
+20 343.26 1.2041 413.3
+15 340.31 1.2250 416.9
+10 337.33 1.2466 420.5
 +5 334.33 1.2690 424.3
 ±0 331.30 1.2920 428.0
 -5 328.24 1.3163 432.1
-10 325.16 1.3413 436.1
-15 322.04 1.3673 440.3
-20 318.89 1.3943 444.6
-25 315.72 1.4224 449.1

Altitude Temperature m·s−1 km·h−1 mph knots
Sea level 15 °C (59 °F) 340 1225 761 661
11 000 m−20 000 m
(Cruising altitude of commercial jets,
and first supersonic flight)		 −57 °C (−70 °F) 295 1062 660 573
29 000 m (Flight of X-43A) −48 °C (−53 °F) 301 1083 673 585
Immelman Mentor

Immelman

Posted
Posted

Quote: maropers

 

Some calculations based on prop and speed as well as some tables on Speed of sound - basically it looks like the tips of prop at 2700 rpm don't start to approch the speed of sound in our operating environment.

carusoam Grand Master

carusoam

Posted
Posted

Reality strikes...


Crash, I think what you have been told about prop speed and speed of sound (although correct) has been simplified to explain why the RPM range is chosen for a particular prop.  It sounds more mysterious than the more mundane actual reasons.


So my guess is.... somebody at the airplane factory noted prop speed vs. effectiveness (angle of attack too).  Engineers like to test this kind of stuff and write long reports that go with that.


They (engineers) need to answer stuff like:  How much pull can I get out of this prop from take-off through cruise?  and How is this best accomplished?


At take-off, with no airspeed, through rotation and climb, 2,700 rpm must be the most pull available to get the plane off the ground quickly and safely (including safely for the engine). 


Realisticly  was it 2705 rpm or 2690.32 rpm that gave the best performance?  Again somebody simplified the system to help the pilot not need to be a flight engineer.  Another factor of safety is added to the system because of the accuracy of the rpm gauge in the aircraft may be off 10s of rpm or worse.


During cruise, the same prop selects an RPM that is comfortable(lower vibration), an efficient match to the airframe (speed vs. mpg), economical (to make, sell, operate).


Ordinary propellers are clearly not efficient at anywhere near supersonic speeds (prop or airplane).  Compressibility of air makes it far from ideal, and gets further away from bernoulli's principles as the speed of the system increases.


We want air molecules to flow smoothly over the airfoil in an ideal manner.  compressibility, mass of air molecules, momentum, and friction all work against efficiency.


It would be my guess that aircraft engineers stay a fair percentage (25% ???) away from the speed of sound (at any condition)  The parts of the propeller that reach this plateau would not be worth the added weight, stress, or other costs.


Clearly the point is well taken, the faster the tip speed, the more noise is generated.  You don't need to go to supersonic speeds to generate a lot of noise.  A whistle or horn intentionally makes a lot of noise with very little air speed.  Mooneys are not quiet beasts on take-off and neither are Bonanza's.


Our European brothers are seeing new propeller designs that achieve quiet operation and still strive for good pull through the flight regimes.


Question for N601X, On the Jabiru is that direct drive?  Often, a high rpm engine, rotax and auto engines included, a gear box (or belt) will be used to bring the prop speed back down into the 2,700 rpm range.


a 64" prop spinning at 3,300 rpm gives a 514 mph tip speed (using maropers method above) 


prop circumference = Pi X D   = tip distance traveled per revolution.


Still 30+% short of the speed of sound at any altitude or temperature.


Respectfully submitted


-a-

rob Experienced

rob

Posted
Posted

I once encountered a Bonanza with a four blade prop and what the pilot referred to as the Q-tip mods. It was explained to me that the prop tips were bent so as to avoid them reaching the speed of sound. Anyone know anything about this?

N201MKTurbo Grand Master

N201MKTurbo

Posted
Posted

I think you are all missing the point. I think the red line is set more by centrifugal forces on the prop hub and torsional vibration issues then efficiency.

Capt_CrashN_Burn Community Regular

Capt_CrashN_Burn

Posted
  • Author
Posted

Quote: carusoam

Reality strikes...

Crash, I think what you have been told about prop speed and speed of sound (although correct) has been simplified to explain why the RPM range is chosen for a particular prop.  It sounds more mysterious than the more mundane actual reasons.

So my guess is.... somebody at the airplane factory noted prop speed vs. effectiveness (angle of attack too).  Engineers like to test this kind of stuff and write long reports that go with that.

They (engineers) need to answer stuff like:  How much pull can I get out of this prop from take-off through cruise?  and How is this best accomplished?

At take-off, with no airspeed, through rotation and climb, 2,700 rpm must be the most pull available to get the plane off the ground quickly and safely (including safely for the engine). 

Realisticly  was it 2705 rpm or 2690.32 rpm that gave the best performance?  Again somebody simplified the system to help the pilot not need to be a flight engineer.  Another factor of safety is added to the system because of the accuracy of the rpm gauge in the aircraft may be off 10s of rpm or worse.

During cruise, the same prop selects an RPM that is comfortable(lower vibration), an efficient match to the airframe (speed vs. mpg), economical (to make, sell, operate).

Ordinary propellers are clearly not efficient at anywhere near supersonic speeds (prop or airplane).  Compressibility of air makes it far from ideal, and gets further away from bernoulli's principles as the speed of the system increases.

We want air molecules to flow smoothly over the airfoil in an ideal manner.  compressibility, mass of air molecules, momentum, and friction all work against efficiency.

It would be my guess that aircraft engineers stay a fair percentage (25% ???) away from the speed of sound (at any condition)  The parts of the propeller that reach this plateau would not be worth the added weight, stress, or other costs.

Clearly the point is well taken, the faster the tip speed, the more noise is generated.  You don't need to go to supersonic speeds to generate a lot of noise.  A whistle or horn intentionally makes a lot of noise with very little air speed.  Mooneys are not quiet beasts on take-off and neither are Bonanza's.

Our European brothers are seeing new propeller designs that achieve quiet operation and still strive for good pull through the flight regimes.

Question for N601X, On the Jabiru is that direct drive?  Often, a high rpm engine, rotax and auto engines included, a gear box (or belt) will be used to bring the prop speed back down into the 2,700 rpm range.

a 64" prop spinning at 3,300 rpm gives a 514 mph tip speed (using maropers method above) 

prop circumference = Pi X D   = tip distance traveled per revolution.

Still 30+% short of the speed of sound at any altitude or temperature.

Respectfully submitted

-a-

acronut Enthusiast

acronut

Posted
Posted

A lot of airshow pilots flying experimentals bump their max RPM for power (and noise). Engine experts tell us 3000 RPM is no big deal.  I know of plenty that run 3200-3300.  Almost all that run them that high rebuild their engines every 400hrs or so.  And yes at those speeds you do hear the difference.  Anyone listen to a T6 Texan take off?  Huge prop and right at the sound barrier speed I'm told.


Kendal

richardheitzman Rising Star

richardheitzman

Posted
Posted

Props exceed the speed of sound all the time. The limiting factor in how fast you run the engine is reliability. 2000TBO for most engines is a FAA mandated time limit based upon (expected) wear and tear. If you run the engine at higher RPMs you are inducing hight levels of wear and tear (so they say). The Mooney "screaming eagle" modification turns up the RPM on the engine from 2500 to 2700 for a higher level of HP. BUT you cannot run this continuously. This allows the STC to be added and keep the engine at 2000TBO. (don't try that at home please).

N601RX Grand Master

N601RX

Posted
Posted

Quote: carusoam

Question for N601X, On the Jabiru is that direct drive?  Often, a high rpm engine, rotax and auto engines included, a gear box (or belt) will be used to bring the prop speed back down into the 2,700 rpm range.

a 64" prop spinning at 3,300 rpm gives a 514 mph tip speed (using maropers method above) 

prop circumference = Pi X D   = tip distance traveled per revolution.

Still 30+% short of the speed of sound at any altitude or temperature.

Respectfully submitted

-a-

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