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

[b3rn]

Hello all,

I'd like to describe a scenario in general terms, and ask you whether you've encountered something similar.

I've interpreted a propeller's hub data using Bob Gardner's excellent books. The data narrows the prop down to a particular engine (100hp Mono), and two aircraft (Bristol Scout D and Sopwith Pup).

I think I know the aircraft from which the prop came, but confusingly, the pitch is not the pitch listed in the drawing for that type (Bristol Scout D). It corresponds with the other of the aircraft (Sopwith Pup).

It's possible - due to supply lines and availability, or a decision on the ground - that the prop with a pitch for one aircraft type was used on another.

Have you come across similar scenarios?

[Dbahnson]

Are you going by a listed drawing number or just stamped diameter and pitch?

Can you post photos of the hub data, and do you know the manufacturer?

[b3rn]

I'm looking at 'Known Bristol Drawing Numbers' (in Bob Gardner's book), specifically drawing number P3012. These are all for 100hp Mono engine.

D2550 P1800 BAT Bantam
D2550 P1800 Bristol Scout D
D2550 P2610 Sopwith Pup
D2550 P2640 Sopwith Pup
D2550 P2310 Sopwith Pup
D2550 P2300 Sopwith Pup
D2550 P1800 Sopwith Pup
D2550 P2610 Sopwith Scout

The hub data:

100 HP MONOSAUPAPE
D2550
P2640
B&C 2851

As you can see, this suggests the propeller comes from 100hp Mono Pup, but I'm reasonably sure it was from a 100hp Mono Bristol Scout D.

Have forum members seen instances where a prop for one type is used on another type?

In short, I'm trying to establish whether I must rule out the Bristol Scout D definitively.

[Bob Gardner]

Greetings Bernard,

I'm glad that Dave Bahnson has enrolled you in this forum.

But it would appear that my emails have not reached you. I wondered why you had not replied. I enclose a precis of my answer below.

The short answer is that it is quite feasible that the 2640mm pitch prop and the 1800mm was fitted to both the Scout D and the Sopwith Pup. Both the Scout D and the Sopwith Pup used the same engine and therefore the same hub. Both were used by the RNAS. But I do not wish to imply that it was officially authorised.

I am aware of lots of variations within published data of this era. There is evidence that British pilots during WW1 sought certain props from certain makers believing them to be better than others, even though both were made to a similar specification, a process which included wishful thinking and superstition, as well as comparative trials. Often a RFC pilot would sing the praises of a prop made by a specific maker, so his chums in his squadron would try to find a similar one.

The same thing happened with German flyers. The German Flying Troops document, the Propellermerkbuch der Prufenstalt und Werft der Fliegertruppen of 1916, published by P & W at Aldershof, emphasises that all propellers were rigorously tested by them and a prop made by one maker was just as good as those made by other makers. Variations in the performance of a propeller could depend, the Buch states, on fuel, the plugs and even on certain periods of the engine rev range which had to be avoided; for example the Mercedes 160hp engine at 1320-1340 rpm.

Considering these aspects of human behaviour it is quite likely that the RNAS at unit level tried out the 2640mm and the 1800mm pitch prop on both the Bristol Scout D and the Sop Pup. There might have been advantages at different parts of the envelope.

Also Mesopotamia, which I think is where your Scout D was shot down, is at the end of a long supply line and perhaps fitting the Pup prop to a Scout D was due to a shortage, either at the RNAS airfield or in the RN supply chain from the factory in GB by ship to Malta and hence to Mesopotamia.

Perhaps a forumite versed in the mathematics of flight could offer a view on the different flying characteristics of each prop on each aircraft?

But, the short answer is that there is no reason to rule out your Scout D.

With kind regards,

Bob


With kind regards,

[Dbahnson]

I might add that if a fighter airplane was to be transported a long distance in a non-combat setting it would make sense to install the propeller with the higher number pitch, even though it might not correspond to its "authorized" usage. It would serve as a "cruise" prop rather than a climb, or performance prop.

[pmdec]

Hi,

The 80 HP Le Rhône and the 100 HP Gnome Monosoupape were respectivly 84 and 105 HP at 1200 rpm at ground level (Sopwith Pup data).
The Bristol Pup was given to have a max speed at ground level of 100 mph, that is 161 km/h. The minimum propeller pitch to reach this speed at 1200 rpm is 2300 mm.
It is absolutly impossible to reach 100 mph with the 1800 mm pitch indicated above (the engine have to run at 1550 rpm!).

Just my two cents...
PM

EDIT: typo

[b3rn]

Many thanks for the information.

I must admit that I know nothing of the performance of propellers.

A question for PM: My interest is the Bristol Scout D. How would it affect that aircraft using a propeller of pitch 2640 rather than 1800?

[b3rn]

I might add that if a fighter airplane was to be transported a long distance in a non-combat setting it would make sense to install the propeller with the higher number pitch, even though it might not correspond to its "authorized" usage. It would serve as a "cruise" prop rather than a climb, or performance prop.

The aircraft were shipped by sea to the main island base in the eastern Mediterranean, then flown to the various stations. These could be from 65km to 100km away. Some missions look to have involved a 200km return journey. And of course, the aeroplanes were mostly flying over sea.

That might offer a reason for employing a "cruise" prop, if I understand you correctly.

[pmdec]

.../... How would it affect that aircraft using a propeller of pitch 2640 rather than 1800?

Hi,

IMHO, for a 2550 mm propeller, the incidence difference at ~2/3rd of the blade between 1800 and 2640 mm pitch is ~7.7 degrees. It is high enough to make the take-off impossible with 2640 mm pitch if 1800 mm pitch was the standard one. And also high enough to make the engine rotate past its maximum regime using 1800 mm pitch in place of a standard 2640 after take-off (and even during engine run-up).

To have an answer about your question, you need to know:
- which engine is used and what is its max torque regime AND its max power regime AND its highest possible regime,
- what was the standard approved pitch used.

It seems that Scout D were fitted with 80HP Le Rhône but some were intended to be fitted with 100 HP monosoupape. Does it exist a picture of WW1 era showing a Scout D with this 100 HP engine?

Regards,
PM

[Bob Gardner]

Thank Goodness we have Pierre-Michel!

With kind regards,

Bob

[b3rn]

Hello Pierre-Michel,

It seems that Scout D were fitted with 80HP Le Rhone but some were intended to be fitted with 100 HP monosoupape. Does it exist a picture of WW1 era showing a Scout D with this 100 HP engine?

Yes, I think there is enough evidence (documentary and photographic) to be sure that the Bristol Scout D was operated with the 100 hp Gnome Monosoupape B.2 engine during WW1.

Jack Bruce's Windsock Datafile 44 says:

The 100-hp Gnome Monosoupape was fitted to some of the later Scouts D delivered to the RNAS. This engine had a full-circular cowling with a large frontal opening and, on its starboard side, a peculiar bulge to improve exhaust outflow.

The 'peculiar bulge' in the cowling (also described as a gumboil) is a distinctive feature, and shown in several photographs, including one I have when the Mono engine itself is clearly seen.

The Bristol Scout D that I am researching is described as having a 100 H.P. Mono[saupape] Gnome engine, No. 5812, in the RNAS weekly operations report for that period.

The RNAS serials book by Sturtivant and Page lists the contract for that batch of aircraft, and notes that it was ordered with 110 hp Clerget but delivered with 100 hp Gnome Monosoupape.

To have an answer about your question, you need to know:
- which engine is used and what is its max torque regime AND its max power regime AND its highest possible regime,
- what was the standard approved pitch used.

The first question: I think we can be sure it was the 100 hp Gnome Monosoupape engine. According to Wikipedia -
Power output: 86 kW (115 hp) at 1,300 rpm (Maximum power)
Compression ratio: 4.85:1

Unfortunately, Jack Bruce's Datafile doesn't give performance data for the Bristol Scout D with the Mono engine. But he does have the following, not sure if it helps:

Weight and performance
Bristol Scout D
Engine: 110-hp Clerget
Weight fully loaded: 1,442 lb
Max. speed (mph)
- ground level 107.3
- 3,000ft 109
- 7,000ft 108
- 10,000ft 100 (est.)

The second question: The standard approved pitch used was 1800. That is based on drawing number P3012, as given in Bob Gardner's book. A possibility is that the source data is incorrect. I'm not sure why (but then I know nothing about this subject!) a Bristol Scout and a Sopwith Pup fitted with the same engine would require such different pitches.

When I look up the Bristol Scout D in Bob's book for the Mono and other engines, I see the following pitches:

Bristol Scout D with 110 hp Le Rhone - P2710
Bristol Scout D with 100 hp Gnome Monosoupape - P1800
Bristol Scout D with 110 hp Gnome - pitch not listed
Bristol Scout D with 80 hp L Rhone - P2400
Bristol Scout with 80 hp Clerget - P3300

[pmdec]

Hi,

I have quite not to very approximative knowledge about English aircraft and have to refer to some credible documentation. The joined pic is from Flight, 1958-1, p0697 PDF. Does it seem right?
Are we talking about the Scout n°8976?

Anyway, there is a measure which can give some information: the central hole diameter and the thickness of the hub of your prop. The "80HP" and the "100HP" Gnome had not the same metallic hub. I have to search about the Le Rhône 80 and 110.
BTW, it would be pleasant for forumers if you post some pics of the markings!

Regards,
PM

[b3rn]

PM, I'm writing an article, and don't want to post too much specific detail because Google will spoil the surprise!

I am trying to connect an aircraft (RNAS Bristol Scout D with 100 hp Gnome Monosoupape engine) with a prop in a museum.

All the evidence links the two, except that the hub data on the prop shows P2640, whereas the published information in Bob's book (based on drawing numbers) shows that the Bristol Scout D prop as P1800.

This is the only discrepancy in the argument I'm trying to build.

So far, it seems you think P1800 is unlikely for a 100 hp Mono engine? (For me, I'm also not sure why there would be such a difference between the Bristol Scout and the Pup with the same engine.)

The joined pic is from Flight, 1958-1, p0697 PDF. Does it seem right?

Yes, but unfortunately, that table doesn't include the Bristol Scout D with 100 hp Mono engine. Only a relatively small number (8951-9000 and perhaps a handful more) were ordered by the Admiralty in this configuration for the RNAS.

Anyway, there is a measure which can give some information: the central hole diameter and the thickness of the hub of your prop. The "80HP" and the "100HP" Gnome had not the same metallic hub. I have to search about the Le Rhône 80 and 110.

The prop is in a museum. I took a number of photos but unfortunately did not measure the thickness of the hub. Also the central hole is covered over by a plaque!

BTW, it would be pleasant for forumers if you post some pics of the markings!

Attached is a good image of Bristol Scout D N5393 with 100 Mono engine and distinctive gumboil on the cowling.

Also attached is my photo of the hub in the museum.

[pmdec]

Hi,

Also the central hole is covered over by a plaque! It seems that the plaque bolt had marks of (un)screwing... It would be the easier way to have, perhaps, an answer...

[boring writing ON]

About pitch, "in general". Airscrews are exactly what means their name: a screw for air! At the rotating speed they are used (around 1000 km/h or 600 mph at the tips of the blades), the air is as hard as wood for a screw. There is very little "slip" or "skid" (I don't know the right word): the "skidding" of a good airscrew is no more than 10% at max or cruising speed (depends of usage). For a very good one, 6%, even 5% can be reached. But, sometimes, you need to have more skidding to prevent the engine running to high, using a "too high" pitch and wide blades: you will never reach the max speed.

So, the maths are very simple:
- the pitch marked on the hub is the geometric pitch: It is the distance the airscrew would travel forward when it turn 360° if it was not "skidding". Say, for example, a prop with 1800 millimeters pitch (quite all English propellers of the WW1 era are marked using millimeters, for the diameter and for the pitch).
- For each complete rotation of the prop, the plane travel forward about 0.9 x pitch, say ~0.9 x 1800 mm = ~1620 mm with 10% "skidding".
- With a Gnome monosoupape engine running at 1300 rpm, each minute the propeller attached to the engine made 1300 rotations.
- So, each minute the propeller travels forward 1300 x 1620 mm = 2,106,000 mm = 2106 meters = 2.106 kilometers = ~ 1.31 mile.
- Each hour, the propeller, and the aircraft!, travels 60 x 2.106 km = 126 km. It is flying at 126 km/h. Or 78 mph.
It is impossible without running the engine faster to exceed that speed at level flight. You can't screw the prop in the air more that the pitch for each rotation. And if you run a Gnome past, say, 1400 rpm, I think it will be destroyed.

And don't forget that Gnome monosoupape did not have any air intake command: there is no carburator. The only way to slow the engine is to cut the magnetos (or some of them on some engines) and/or close the gazoline line. So, if you fit a too low pitch prop on the engine, it will exceed its max rotation speed.

With 2640 mm pitch, the prop you show could reach 185 km/h (115 mph) at 1300 rpm.
It could be a prop for a 9N monosoupape, used on Sopwith Camel. It is strange it has neither serial number nor aircraft indicated.

[boring OFF (I hope!)]

The "multiple" pitches Bob Gardner shows in his books for "Monosoupape" could be for different monosoupapes engines, because there was the 7 cylinders ("A"), and at least 3 engines with 9 cylinders : 9B, 9N and 9R... I don't know if it is linked to or it is by chance, but the 7A had in France a "type A" metallic hub (60 mm central hole and around 100 mm thick) and the 9B a "type B" metallic hub (70 mm central hole, around 140 mm thick). But there could be exceptions... and in England it was perhaps different!

Regards,
PM

[b3rn]

That's very interesting and not boring at all, PM. I appreciate it!

I think we can assume this is a British-built Gnome Monosoupape Type B-2. All references in contemporary reports use the designation 100-hp Mono.

For that particular Mono, here is some data gleaned from Google!

Power rating: 75 kW (100 hp) at 1,200 rpm
Source: https://airandspace.si.edu/collectio...-rotary-engine

Power output: 86 kW (115 hp) at 1,300 rpm (Maximum power)
Compression ratio: 4.85:1
Source: https://en.wikipedia.org/wiki/Gnome_...ape_9_Type_B-2)

Applying PM's method:

0.9 * 2640 mm (pitch) = 2376 mm (allowing 10% skidding)

100 hp, 1,200 rpm

1200 rpm * 2376 = 2,851,200 mm = 2851.2 metres = 2.8512 km = 1.77165354 miles

60 * 2.8512 km = 171.07 kmh
60 * 1.77165354 miles = 106.23 mph

At maximum power, 115 hp at 1,300 rpm

1300 rpm * 2376 = 3088800 mm = 3088.8 metres = 3.0888 km = 1.91929134 miles

60 * 3.0888 km = 185.33 kmh
60 * 1.91929134 miles = 115.16 mph

These speeds seem reasonable, but perhaps all I have done is confirm the propeller's hub data! And the manufacturers weren't resorting to Google.

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

Why would the drawings show such a difference in pitch between two aircraft using the same engine?