"Gross" and "Net" rotaries power outputs.

Greybeard · 3 January 2008, 12:57 AM

From when Bletchley and other mates on this forum let me discover windage losses were an important factor about aircraft powered by that kind of engine, I got more and more "jammed" when reading about performances.

I try to explain by an example:

Air Data Board reports Sopwith Camel performances with various engine models, included an A.R.1 (Bentley rotary) and Clerget (not better specified). Actual respective horsepowers are 150 HP at 1250 RPM and 126 HP at 1250 RPM. Speed are almost the same, with a little advantage for Bentley powered one. Anyway, those 24 more HP are absolutely incoherent with difference in speed, which is negligible; also weight, that is higher by only 50 lbs for Bentley powered, can't explain its almost equal speed.

Only explanation could be Clerget horsepower was expressed "net" of windage losses (on the contrary of Bentley, on same document), so very close to 126.5 HP developed by A.R.1 in same circumstances. This implies "gross" Clerget power output should have been about 150 HP, but I never read such a datum.

If so, the question is: how to orientate into this labyrinth?

GB

Bletchley · 3 January 2008, 04:28 AM

GB, there were at least three different 130 hp Clerget's in use by the British and at least two different AR1/BR1 rotaries, each with a different 'real' hp output, but all nominally 130 hp (Clerget) and 150 hp (AR1/BR1):

Most of the RFC Clerget Camels were, I think, powered by the French-built Clerget 9B engines. These were nominally 130 hp at 1200/1250 rpm but they could develop up to 134/135 hp at this speed, and up to 150 hp for a short period at 1300 rpm.

RNAS Camels were, at first, powered I think by British built Clerget 9B engines produced under licence by Gwynnes (or a mixture of British and French built engines). These were poor copies of the original French 9B (Gwynnes was a producer of marine pumps before it acquired the Clerget licence, and the company does not appear to have been used to the fine engineering work needed to produce rotary engines), and these engines suffered in particular from rather badly made obdurator rings that rarely lasted longer than 10-15 hours and caused a loss of compression. I think the British Air Board figure of 126 hp is probably based on one of these licence built engines (although I do not think there is any indication of this on the data sheet). Bentley was sent by the Admiralty to Gwynnes to "sort them out" and introduced a number of improvements and modifications to the 9B (the main one being aluminium-alloy pistons) that not only improved the reliability of the engine but also raised output from a nominal 130 hp to 140 hp at normal max. speed, 1250 rpm, 160 hp for 'a few minutes only' (Gwynnes renamed it the 9BF and rated it, rather optimistically, at 150 hp). My reference for this is the Gwynnes Ltd manual, 'Clerget patent aero engines, 9b & 9BF', 1917 (reprinted by Camden Miniatures, 2001).

There was both an original lower compression and a later higher compression version of the AR1/BR1. The low compression version was nominally 150 hp at 1250 rpm (Air Board figure), but Andrew Nahum of the Science Museum has quoted 158 hp at 1250 (in his booklet on rotary engines) which I think may have been the output of the later high compression version of this engine, but it could reach 1300 rpm 'for a few minutes only' (Air Board), although I have no figure for the hp at this engine speed. In his memoirs W.O. Bentley referes to this later high compression version, without giving and performance details, and also states that up to a further 11 hp could be gained by drilling a small hole in one of the induction pipes of the BR1, and "We sent the word round and fitters on every B.R. squadron were soon busy with little 2-mm drills" (Bentley, W.O. The autobiography of W O Bentley, Hutchinson, 1958).

So, to sum up:

Gwynnes licence built 9B: 126 hp (?) (rated at 130 hp)
French built 9B: 134/135 hp full, 150 hp max (rated at 130 hp)
Gwynnes/Bentley 9BF: 140 hp full, 160 hp max (rated at 150 hp)
Bentley AR1/BR1 (low compression): 150-161 hp full, max ? (rated at 150 hp)
Bentley AR1/BR1 (high compression): 158 hp (?) full, max ? (rated at 150 hp)

I would assume that these figures are without any reduction for windage.

Bletchley

Greybeard · 3 January 2008, 05:19 AM

Thanks Bletchley, as always you've a lot of info and can provide detailed explanations.

BTW, did you see this about Clerget rotaries?
http://www.hydroretro.net/etudegh/mo...erget130ch.pdf

The point that confuses me is indeed with or without windage?

Sources of that time use loosely rotaries' horsepower without specifying if "gross" or "net", not to mention absolute negligence about british and metric units. But windage can't be neglected; the case of Camel is significant: if its british Clerget 9B really developed 126 HP "gross" it couldn't reach same speed of its Naval counterpart and vice-versa... or I must believe that donkeys can fly!

9BF had also a longer stroke (172 against 160) and an higher compression ratio (5.14 vs. 4). Actually, it was credited with 148 BHP on same Data Board document. Nevertheless, Camel powered with it wasn't the fastest, being so for Le Rhone powered one, developing "only" 137 BHP (perhaps "net"). Do you see what I mean?

GB

paolomiana · 3 January 2008, 07:16 AM

Hi Greybeard and Bletchley,
I found same problems with a comparison between Ni.11 and Caproni Ca.20 performances.
The “lose” of 25 Hp maybe is due to a fitting of different kind of propeller (with different efficiency) in the two versions of Camel.
If I understand it right at Vmax Drag = Trusth, with the
Drag Coefficient as = (550*nu*P)/(0,5*ro*S*(1,47*Vmax)^3), with

nu = propeller efficiency
P = available power
ro = air density
S = wing surface
And 550 and 1,47 constant valid only for UK system

By this (if I understand it right) you can see that for a unit increase of Vmax you need to give ^3 more power to the system.

In my comparison I use a nu=0,68. With this value (that feels a little bit low) my ceiling rates calculations match with test datas.
So if you have the ceiling times of Camel with both engines is possible to verify my hypothesis of two different spinners……. Maybe.

Paolo

Brad · 3 January 2008, 08:31 AM

There are a lot of variables involved here.

To begin with, top speed is limited by several factors. Power of course, but the effective "gearing" of the prop- in other words, the maximum speed that the prop could theoretically pull an aircraft. There are "climb props" and "cruise props" which for the same hp, a "climb prop" will provide more thrust (better takeoff and climb performance) and a "cruise prop" will provide a higher top speed, but sacrifice acceleration and climb performance. And of course the drag of the airframe.

And as stated by one of the other posters... but I'll paraphrase. An increase in speed takes a much greater proportional increase in horsepower. An increase in climb rate is more linear. I suspect the power was applied to giving the aircraft a better climb rate instead of a very marginal increase in speed, which of course is determined by prop selection. It is all a trade-off.

paolomiana · 3 January 2008, 08:57 AM

everything true, but it generates in me another lot of doubts...
is it possible to gear a propeller of rotative engine?
another question is it really understood by WWI designers what is a climb/speed propeller?
I cosntantly hit this soft wall...... what is the real understendment of flight laws during WWI. Please could someone give me some references about the aerodinamic/structural technology level in 1914-18? and what is they way they use this informations

thanks

paolo

David_Layton · 3 January 2008, 09:04 AM

The forces that flight depends on are Lift, gravity, drag and THRUST (not horsepower). This is where many authors, armchair types and most importantly, flight sim designers fail in assessing the flight dynamics of an aircraft. This was a major issue with the first FM's for Red Baron II/3D.

As mentioned in this thread, the 'transmission' is the propellor and is fixed pitch. I have a file that I compiled with known engine propellor combinations (diameter/effective pitch) to provide sim developers better knowledge.

The variable pitch propellor was invented during WWI, but was experimental.

Brad · 3 January 2008, 11:55 AM

Quote:

Originally Posted by paolomiana

everything true, but it generates in me another lot of doubts...
is it possible to gear a propeller of rotative engine?
another question is it really understood by WWI designers what is a climb/speed propeller?
I cosntantly hit this soft wall...... what is the real understendment of flight laws during WWI. Please could someone give me some references about the aerodinamic/structural technology level in 1914-18? and what is they way they use this informations

thanks

paolo

Paolo, while is IS possible to gear a rotary engine (Siemens did it) that is not what I am referring to. Pitch acts somewhat like a gear ratio- think of a prop as a screw. Each turn of the prop advances it through the air by a given amount. A climb prop will have less pitch than a cruise prop, (And they are usually longer) A cruise prop will have more pitch, but be shorter in length. The cruise prop, due to more pitch, will move the aircraft faster, because it moves further through the air with each rotation... but, will not have as much thrust. It is all a trade-off.

Bletchley · 3 January 2008, 11:14 PM

GB, I think that the Air Board data would have been recorded in a consistent way, and I am fairly sure that the figures for rotary engine hp would have been without any losses for windage. Andrew Nahum ('The rotary aero engine', HMSO, 1987) quotes S D Heron, who was with the RAE/AID at Farnborough, as saying that the 'rotary engine builders objected strenuously to the power of their engines being determined on the dynamometer. They insisted that torque stands should be used. On a torque stand, the very considerable windage loss due to rotating the cylinders was recorded as useful horsepower'. I think this must have become the standard way of recording output from rotary engines.

Bletchley

Greybeard · 4 January 2008, 12:40 AM

Quote:

Originally Posted by Bletchley

...I am fairly sure that the figures for rotary engine hp would have been without any losses for windage.

I fear it's not so. Report No.448 by Technical Department at the Air Ministry, May 1918, it's not clear about; B.R.1 is credited with 150 HP at 1250 RPM and then are sperimented losses for windage, leaving to think 150 HP is "gross" power output. Do you think, or have evidence that B.R.1 could otherwise develop about 180 HP inclusive of windage? In addition (and again): why B.R.1 powered Camel should have same speed of Clerget powered one, with 24 HP more?

Frenchs, OTOH, seem to have the habit to give "net" horsepowers. I think the guys of Air Board used loosely a mix of "net" and "gross" power outputs, reporting french test bench results literally, without converting to british units and, as mentioned, "net" of windage, while B.R.1, perhaps the sole british-built original rotary and tested in britain, retained its "gross" horsepower.

And what about germans?

Quote:

Andrew Nahum ('The rotary aero engine', HMSO, 1987) quotes S D Heron, who was with the RAE/AID at Farnborough, as saying that the 'rotary engine builders objected strenuously to the power of their engines being determined on the dynamometer. They insisted that torque stands should be used. On a torque stand, the very considerable windage loss due to rotating the cylinders was recorded as useful horsepower'. I think this must have become the standard way of recording output from rotary engines.

Bletchley

I would be explained what a "dynamometer" is intended in previous quote: as a mechanical engineer, I don't know any device to measure power different from "torque*RPM" system, where torque is measured by just a dynamometer (more precisely a brake, a water brake, I think, at that time).

Thanks as always for your big help.
GB