Author Topic: IAS and Altitude (Read 427 times)

whgates3 · « **Reply #15 on:** August 12, 2002, 01:56:44 AM »

if the prop diameter of a F4U is 13'
and it spins at 2750 RPM,
then arent the tips of the prop rotating at 112311'/minute
which = 1276 MPH?

Holden McGroin · « **Reply #16 on:** August 12, 2002, 02:44:08 AM »

The P & W R 2800 had a 2:1 reduction gear, so at 2750 engine RPM, the prop spins at 1375 RPM. The prop tips are doing (using your math that I assume is correct) 638 mph around the circle: add 440+ mph forward speed, and do a little vector addition, and you get a prop tip speed of 775 mph.

Depending on air density, that's damn near mach.

Badboy · « **Reply #17 on:** August 12, 2002, 07:22:03 AM »

Just to expand a little...

The critical propeller tip speed was generally considered by designers in 1940 to be about 1000 feet per second (M0.9) and most designers wouldn't exceed that because beyond that point there is some reduction in propeller efficiency. Tip speed can be calculated from:

Vc = sqrt( (PI*D*N/60)^2 + V^2)

Where Vc is the tip speed, V is the aircraft speed, D is the diameter of the propeller and N is the RPM of the propeller, remembering that the propeller is geared to run at a lower speed than the crankshaft. So that with a reduction ration of 2:1 the propeller would run at 1/2 the speed of the crankshaft. For the F4U with a prop diameter of 13'-4" at say a top speed at sea level of about 350mph the calculation becomes...

Vc = sqrt( (3.142*13.3*1300/60)^2 + (350*1.467)^2)

Vc =1041 fps (M0.93)

Which is just above 1000fps and right at the point where the propeller might be expected to begin losing efficiency.

Badboy

whgates3 · « **Reply #18 on:** August 13, 2002, 03:20:03 AM »

thats good@sea level, but how about at 26000' where (according to AH charts) the F4U did 440 mph & sound speed is slower.
that gives 1154'/s or 787 MPH (i used RPM = 2750, rather than 2600, as thats what the dial shows in AH)...sound speed is lower at alt, is it not?

Holden McGroin · « **Reply #19 on:** August 13, 2002, 04:01:21 AM »

Did a little surfing and found this...

a = Square Root(g R T)

where

a = speed of sound
g = ratio of the specific heat at constant pressure to the specific heat at constant volume (1.4 for the atmosphere)
R = universal gas constant
T = Temperature (Kelvin or Rankin)

One method commonly used is to develop charts with ratios to mathematical equations. One chart available is the "Standard Atmosphere Table."

This table provides a "Speed of Sound ratio" column. This column provides the speed of sound (standard day data) ratio for any altitude based on the speed of sound at sea level of 761 MPH.

Altitude in feet Speed of Sound ratio

Sea Level 1.00

5,K ft 0.9827
10,K ft 0.9650
15,K ft 0.9470
20,K ft 0.9287
25,K ft 0.9100
30,K ft 0.8909
35,K ft 0.8714
40,K ft 0.8671
50,K ft 0.8671
60,K ft 0.8671

so at 60 k, .8671 * 761 = 659

At Black Rock, the Brits were hoping for cooler temperatures, the heat in the afternoon causing them to reach higher speeds for a Mach breaking land speed record.

I had the effect of altitude backwards for quite some time. Good thing this thread started, I learned something new.

whgates3 · « **Reply #20 on:** August 13, 2002, 05:13:05 AM »

horay - zoidberg is right!

Badboy · « **Reply #21 on:** August 13, 2002, 07:32:38 AM »

Quote

Originally posted by whgates3
thats good@sea level, but how about at 26000' where (according to AH charts) the F4U did 440 mph & sound speed is slower.
that gives 1154'/s or 787 MPH (i used RPM = 2750, rather than 2600, as thats what the dial shows in AH)...sound speed is lower at alt, is it not?

Yes it is, and of course as an aircraft's true airspeed increases so the problem gets worse. Just as aircraft suffer more from compressibility at high altitude, the propeller suffers also, and that means an increase in torque and decrease in thrust, in other words a loss in efficiency.

Some improvement is achieved with changes in the blade section near the tips, making them thinner, or of a laminar flow section, and by changing the degree of twist in the blade slightly to provide some washout. That can help to reduce the losses, but as the aircraft speed increases losses are inevitable. Once they become too serious, the blade diameter can be reduced, and the number of blades increased.

Unfortunately, once you begin to consider airspeeds much greater than 400mph and approaching 500mph it is no longer possible to keep the tip speeds below the speed of sound. When the tip speeds exceed the speed of sound, the loss in efficiency becomes more serious and spreads to a larger proportion of the propeller blade.

When you consider speeds as high as that, the losses either have to be accepted, or a designer must turn to other means of propulsion, as the Germans did with the Rocket and Jet powered aircraft they introduced in the later stages of WWII.

Badboy

whgates3 · « **Reply #22 on:** August 14, 2002, 01:35:20 AM »

i wonder if 'MACH jump' affects the prop

Holden McGroin · « **Reply #23 on:** August 14, 2002, 06:34:22 AM »

By ‘Mach Jump’, are you asking whether one can quickly pass through the transonic region and get to supersonic, then using the rules that govern SS flow?

I have been doing a little research, taking away from AH flying, and have found some interesting stuff.

Back when they were NACA, in the late 40’s Langley Flight research division researchers were trying to measure the pressure distributions on a spinning prop, and installed pressure taps in a hollow steel prop blade at the factory, the sent it to Langley’s 8’ high speed tunnel.

“Significant departures from two-dimensional airfoil data are evident in the outboard regions, chargeable to the combined effects of tip relief, Mach number gradients, radial flow of the boundary layers, and possibly to an induced-camber effect. The method successfully predicted the performance of the 4-foot propellers tested at airspeeds up to Mach 0.93 in the re-powered 8-foot tunnel program.”

They also toyed with Scimitar shaped propellers,

“Swept propellers show a delay in the onset of compressibility losses to higher tip speeds than those of the straight blades of equal thickness. However, the delay was only about a quarter of what might be expected from the simple sweep theory. Offsetting the beneficial high-speed effect were generally lower levels of efficiency and other aerodynamic problems for the swept propellers. But the major conclusion brought out in the analysis stated that an unswept blade of slightly reduced thickness could always be found which would have equally good high-speed performance, better overall performance, significantly lower blade stresses, and freedom from the other structural complications of the swept propellers. This emphatic and disillusioning result put an end to any further attempts to exploit swept propellers.”

Then some late, unnoticed successes.

“Three propellers were eventually tested at flight speeds up to slightly above Mach 1 on the XF-88B. (Turboprop powered research a/c) By the time the results were analyzed in 1957, the Subcommittee on Propellers for Aircraft had been disbanded, eliminating a main heading on this subject in the NACA Annual Report.”

edit> different photo: should have posted as a What's This?

whgates3 · « **Reply #24 on:** August 14, 2002, 08:22:12 AM »

what i mean by 'MACH jump' is the phenomenon encountered in the early supersonic test flights - the MACHmeter would read 0.98 one second and 1.02 the next - the explanation given is that the A/C gets a 'kick in the pants' when passing through it's own bow shock (the bow shock pushes the A/C as the A/C passes through)