Fixed pitch prop formula

[Ronald Sutton]

The formula for fixed pitch propellers is
propeller pitch in inches X RPM X 60 minutes
12 inches X Ft. per mile 5280 feet
RV4 180 Lycoming aluminum sensenich prop model 72 FM 85 9-1-85

85 X 2750 X 60 = 14025000
12 X 5280 = 63360 = 221.354 miles per hour

85 X 650 X 60 = 3315000
!2 X 5280 = 63360 = 52.32 miles per hour
Ever wonder why you float so far down the runway?: your stall speed and landing speed that your prop is pulling you are the same.

[rv7boy]

I had a friend (he's dead now) who got into a writing argument a long time ago (before email and the Internet) with a contributor to a prominent model airplane magazine over the practical application of the same calculations you have done. There is a term you have omitted, sometimes called "slippage," sometimes called "frictional losses," that accounts for inefficiencies as the propeller turns through the air, especially at high speed. It can only be determined empirically for each propeller installation, although some modeling programs come very close with their predictions. And some manufacturers include that "inefficiency" factor in their pitch term while others do not.

Thanks for reminding me of my good friend, John Brownlee.

P.S. Stated another way, calculating forward speed of a propeller is not an exact science as is calculating speed of a car with a manual transmission. Using calculations such as the above can get you in the ballpark however.

[gasman]

Originally Posted by Ronald Sutton

Propeller pitch in inches X RPM X 60 minutes
12 inches X feet per mile 5280
RV4 sensenich aluminum prop model 72FM859-1-85
85 pitch X 2750 RPM X 60 =14025000
12 inches X 5280 Feet per mile =63360
equals 221.35 miles per hour

85 pitch X 650 RPM X 60 = 3315000
12 inches X 5280 feet per mile =63360
Equals 52.32 miles per hour
Ever wonder why you float so far down the runway
your stall speed and landing speed are the same -plus ground effect.

Due to the fact that this is an aluminum prop, and makes a great flywheel, the idle should be set at 550 rpm. With the above math, this would make the prop settle on 44.2 mph...... well under the stall speed.

Does not apply to a light weight prop.

[CATPart]

yea but....a prop does not move forward the distance of 1 pitch per revolution. You are stating it like it is a nut spinning on a bolt. There are efficiencies, airfoils, angles of attack, P factor, blah blah, all affecting thrust. The aircraft forward speed depends on the balance of forces, thrust vs drag simply put.

[Ronald Sutton]

The propeller formula can be used to figure out your aircraft speed with your propeller pitch and engine speed. This is just a formula. Any time your aircaft is in tractor mod, IE The propeller is pulling the aircraft through the air in level flight there is certainly going to be slippage do to the aircraft weight and the apparent drag of the aircraft.
When the aircraft is in a glide and the air speed exceeds the prop pitch at that RPM, the prop becomes a disc, IE An aircraft will glide further with the prop stil, or not rotating, than when the prop is wind milling creating a disc.
On final, just over the fence, your air speed is say,70 MPH and your rpm is650 RPM. You are pushing a disc of 15 miles per hour. As you transition from 70 MPH to landing speed/stall speed, about 55 MPH, the aircraft slows down, the prop will catch up to its tractor mod of pulling the aircraft across the ground/runway at 55 MPH. In this transition, close to the ground, you are in ground effect, which eliminates a great deal of the weight of the aircraft. At this point the prop is pulling an almost weightless aircraft though the air at almost 100% prop efficiency. this is why the RV4 with a fixed pitch propeller floats on landing. Some aeronautical engineers will have better numbers , but the idea is the same.

[CATPart]

..........

[rv7boy]

Your concept is correct but your prop math needs work. Other than that, you make a good point.

[Ronald Sutton]

what math mistsakes were made?

[rv7boy]

Your calculation treats an airplane propeller like a threaded nut on a bolt, which it is not.

Air is a compressible fluid. The study of the dynamics of air (aerodynamics) flowing over an airfoil moving in a circle (a propeller) is quite complex. It would take more than this post to explain it. Some call the aerodynamic energy losses experienced by a propeller "aerodynamic inefficiencies" or "slippage," which has nothing to do with the "slippage" you refer to as "slippage do (sic) to the aircraft weight and the apparent drag of the aircraft," which I have never heard of.

Your point is well made that an engine at idle on approach to landing affects the landing speed. A rotating propeller produces more drag than a stopped propeller. But trust me, the calculation you have put forth for determining air speed based solely on rpm and pitch and then converting the units to mph does not begin to mathematically describe the aerodynamics of an airplane propeller.

Every propeller manufacturer has his own way of defining pitch. Pitch numbers are valuable when comparing propellers from a single manufacturer but become somewhat frustrating when comparing propellers from different manufacturers. That's why I said earlier that the performance of a propeller should be determined empirically, i.e., bolt it on and go fly. Pitch numbers from propeller manufacturers are usually derived from a particular airspeed, not the other way around, as you have attempted to do.

I wish Paul Lipps were still around. He could explain this much better than I can.

Peace...I'm out of this discussion.

[SvingenB]

Originally Posted by rv7boy

Your calculation treats an airplane propeller like a threaded nut on a bolt, which it is not.

The point is that at 55 mph it acts as it was a threaded nut on a bolt. The aoa is zero, or very close. The prop creates no drag or no thrust. If the speed decreases further it starts creating thrust.