Props and pitch: Used on an aircraft type different to the drawing specifications

[b3rn]

The "multiple" pitches Bob Gardner shows in his books for "Monosoupape" could be for different monosoupapes engines...

Possibly. But Bob's table for Bristol shows 100 hp alongside each Mono prop, suggesting the B-2. My assumption is that "100 hp" was used to distinguish the engine type.

This is from Wikipedia, and I haven't cross-checked.

7 Type A - 80 hp
9 Type B-2 - 100 hp
11 Type C - ?
9 Type N - 150 or 160 hp
Type R - 180 hp

[pmdec]

Hi,

.../... So PM - my question: Is the pitch relevant only to the engine, or to any characteristics of the planes or fuselage? .../...

Yes, the pitch, but also the diameter and the blade width depend of many parameters, but I dont't know how all this was computed in WW1 era, neither in France nor in England.

Since the 20's (and perhaps before) it is known that the propeller efficiency depends of its diameter: larger is the prop, higher the efficiency could be... but the prop tips have to stay away from sound speed*** and from the ground!
Blade width is more difficult: it depends of the choosen profile of the cross-sections. You have to look at NACA history, but I think all about profiles of cross-sections was empirical in WW1 era.
To compute the pitch, propellers manufacturers could use only max power and regime of the engine AND the speed the aircraft is presumed to reach. This last data is very difficult to know by the aircraft manufacturer and was also, I presume, empirical in this era. It depends of the engine power, the weight of the aircraft, its wing area, its running resistance (size and number of struts, ...).
And then, you can also choose between an aircraft which would have a high speed OR a high climb rate. But for speedy aircraft you have to verify that the aircraft could take-off! At low speed, a propeller designed to reach high speed has a pour efficiency: it is why variable pitch propeller are used (pitch can be changed in flight). The low pitch is computed for a maximised efficiency at take-off speed and high pitch computed for max efficiency at cruise speed. The difference between them could be in the order of 10 to 20 degrees for aircraft with a max speed in the order of 200 km/h.

So, yes, there are different propellers for different aircraft using the same engine, even for the same aircraft for different uses: Dave told you could have a "cruise prop" and and a "for performance" one and he is right, but the difference would be low. It why there were adjustable pitch propellers (that is, only on the ground with stopped engine).

*** To compute the true speed of blade tips, forward speed vector has to be added to tangential speed vector. IMHO, if only max speed is considered, this could be ignored until 200 km/h (1000 tangential + 200 forward > ~1020 true speed).

Regards,
PM

EDIT: Forgot the sound speed limit! The blade tips speed has to be kept subsonic. It is why "modern" props for small aircraft have a smaller size than WW1 ones: engine rotation speed is higher but gears are costly, so on "small" aircraft of nowdays (which have about the same power), the propellers are smaller. And with more efficient shape than during WW1, the prop efficiency is good even with a smaller used air surface.

[b3rn]

A recap. Bob's book has drawing numbers, and for the 100 hp Mono-engined Bristol Scout D, a pitch of 1800 is shown in that table. (In comparison, the 100 hp Mono-engined Pup - same drawing - has a pitch of 2640 mm.)

I followed PM's formula for the pitch of 1800 mm, and the maximum speed is 78.5 mph.

That seems too low?

Bob has checked his data and confirmed what's in the book. My guess is human error in the original documents, an early/transition period that was superceded, or it was practice (just not recorded here) to supply the prop specified for the 100 hp Mono Pup for the 100 hp Mono Scout D. I'm going to note the anomaly in my article but assume that the prop in question (P1620) is from the Bristol Scout D.

Here's my working in case I got it wrong:

0.9 * 1800 mm (pitch) = 1620 mm (allowing 10% skidding)

At maximum power, 115 hp at 1,300 rpm:

1300 rpm * 1620 = 2106000 mm = 2106 metres = 2.106 km = 1.3086077 miles

60 * 2.106 km = 126.36 kmh
60 * 1.3086077 miles = 78.516462 mph

Thanks for your help!

[pmdec]

Hi,

I think you are right. Perhaps there was a typo in "old papers"... Or the pitch measured was not the true one. I don't really know... but maths are a compulsory base!

The joined pic came from a Ratier workshop notebook. It is about test runs for an automatic variable pitch propeller which had only two positions: one for take off, one for max speed at level flight. The "trouble" for racer was to be able to take off with high pitch propeller which could permit high speed. What is of interest for you, IMHO, are the simple maths used:

- "V" is speed in kilometers per hour ("sol" is the French for "ground": those racers flew at low altitude, so there was no need to compensate for air pressure),
- "N" is rotation speed of the engine in rpm,
- "T" is the power delevered by the engine (note that the power at 2400 rpm is very higher than at 2200 rpm).
- "H" is the pitch in meters. H=2m30 à 0m60 means the pitch is 2,30 meters at 0,60 meter from the prop center (those props had not a constant pitch along the blade, but higher near the tip),
- ° and ' are angular degrees and minutes
- gr is another angular unit frequently used by Ratier (400 "grades" = 360 degrees = 1 rotation), I don't know which word is right in English,
- "PF" is for French "Point Fixe" (ground run full throttle).

You can see that 360 x 1000 [speed in meters by hour] / 60 [speed in meters by minute] / 2400 [advance by engine rotation] x 1.10 [10% slip] = 2.75 meters = pitch near the tip (the prop is 1,90 meter long).
This will be the higher speed the plane can do at 2400 rpm with the blades blocked at 31°20'.
But the prop is constructed with best efficiency when the pitch is 2.60 meters at 0.60 meter from prop center, that is 34°35', which gives 410 km/h at 2500 rpm for a plane with a 240HP engine. So they diminished the pitch to use it with the 160 HP engine by 3°15' (from 34°35' to 31°20').

And that 180 x 1000 x 1.1 / (60 x 2200) = 1.50 meter = pitch when the blades are blocked at 21°40' (the automatic system rotates the blades 31°20' - 21°40' = 9°40' after take off).

To be noted : the word "variable pitch" is a poor term, because the blade torsion doesn't change, only the global incidence changes. A true variable pitch propeller would have a variable torsion : This has been tried with blades made of "articulated" sections. Seems it didn't work...

We are far from WW1 props... Sorry for the digression!

Regards,
PM