By: Beermat
- 15th October 2017 at 11:32Permalink- Edited 1st January 1970 at 01:00
A potential problem, but not so critical in the climb at around 200 mph TAS, so not the altitude issue. Unless the problem occurred at the 'bottom end' of the range.
By: Schneiderman
- 15th October 2017 at 11:46Permalink- Edited 1st January 1970 at 01:00
All well and good climbing smartly to altitude but not if there is b****r-all you can actually do in combat when you get there. Strangely the pitch angle data does seem to suggest that the prop. was working most efficiently at the fine end.
By: Beermat
- 15th October 2017 at 11:53Permalink- Edited 1st January 1970 at 01:00
True, but we are back to the issue that you couldn't possibly get near the speeds you'd need to start hitting the coarse stops etc. at altitude because you'd have 10 times the usual drag or more on the blades - which would fine themselves in response.
By: Beermat
- 15th October 2017 at 11:58Permalink- Edited 1st January 1970 at 01:00
What sort of speed would these blades be better for? By which I mean at the 'point-of-focus' as it were, minus 5 degrees, at 3,000 rpm x 0.477, what does the maths produce?
I wonder whether in fact I have it wrong, and making fine pitch more efficient was all about power delivery in take off and climb, prioritised over maximum speed. One could argue 'who needs acceleration when you are already fast enough.. and who doesn't need it at 150 mph in combat or when taking off'?
Having said that, I have just done a rough calculation and it looks like max efficiency of the 5868 would be at 50 mph and off the end of the fine pitch range, so I will need to revisit the DH data more carefully. Spitfire checks out at 150 mph and near enough right on the fine pitch stop.
By: Beermat
- 15th October 2017 at 13:51Permalink- Edited 1st January 1970 at 01:00
..and measuring the 'Lancaster' 6353 in the same way as the RAE measured the spit prop - from 0.3r to 0.7r, it has in fact got the same twist as the Spit blade, around 18.5 degrees. I cannot account for the 'flat spot', unless it was to aid feathering. Something to do with the 'exposed area' being different with an in-line engine instead of the usual US radial on anything multi-engined?
By: powerandpassion
- 16th October 2017 at 06:49Permalink- Edited 1st January 1970 at 01:00
I have been sitting here in the corner of the pool with my floaties on while you two Tarzans have been performing backflips off the highboard, triple somersaults through propeller arcana and theory, stopping every once in a while to jab a pipe into the others chest to make a point. At some stage you need to stop and turn your gaze to the flabby, useless fat kid in the water and slowly explain some of this material. Dumb it down.
How does a propeller work ? What were the bright people doing back then to chase more performance ? Chopping blades into bits and re-sticking them together? Twisting blades more ? Can you please do a youtube : get a tub of butter and twist up a butter knife to show how the different ideas worked to move butter !
Don't forget a lot of the yankee RHT blades were for air cooled radials, and some of the blade design was for engine cylinder cooling. The Mosquito - Lancaster blade was designed for US RHT radials, then used for the RHT Merlin. I wonder if dH, in chopping and resticking blades, was just getting rid of the 'unnecessary' engine cooling function?
I wonder if any part of the blade was designed to force air into an engine carb intake ? RR and Bristols did so much work on engine fairing. I cannot believe that there was not some kind of correspondence between the fairing designers and the prop designers.
By: Schneiderman
- 16th October 2017 at 07:39Permalink- Edited 1st January 1970 at 01:00
Believe me, I wish I knew. This is a case of learning on the job with a large amount of key information lacking. Re: your penultimate paragraph regarding the HS props starting life for use with raduials, I was beginning to think along similar lines but tending to veer towards the opposite conclusions. So it goes.
By: Beermat
- 16th October 2017 at 09:14Permalink- Edited 1st January 1970 at 01:00
Ah, Ralph's the real Tarzan. Mine's an inflatable fancy dress Tarzan suit. I am more like a duck - looks good on the surface but underneath little legs going like the clappers to make any progress at all.
I like the butter idea. Different blade twists cut the block different ways.. put the butter on some kind of low-friction bed, anchor the blade on a pivot beside the butter representing a hub, move the blade down and the more twisted one moves the butter more slowly as it cuts.
The engine cooling thing is interesting, but if there was some thrust intended for the cylinders then there would be little reason to not use it for propulsion if possible on an in-line engine. It looks to have been the other way around, in that there are quite narrow shanks to the Hamilton blades until they get outside the radius of the cowling of an air-cooled radial, while Rotol, who were designing for the UK market from the outset, had plenty of 'proper' aerofoil right up to the spinner.
When DH went on to design their own blades, and stated doing 'paddles' etc, then they also added some extra chord at the root end. Funnily enough this first happened when moving the de Havilland Mosquito on from prototype to production. There weren't really the conversations one imagines happened between the aircraft designer and prop designer if not in-house - it seems on the evidence that the main conversation was between the procurement office and the prop salesman.
I have to hold up my hand and admit that these later DH-designed blades don't (or shouldn't) follow on in my blade-hacking-about series - I am really only looking at the early 'needle' blades.
On the other thing, by the way, I have asked around and all we have is the Peregrine AP. I can get that to you - it has some nice schematics - or just read it for what it is you needed. Was it con rod material?
By: Schneiderman
- 16th October 2017 at 10:13Permalink- Edited 1st January 1970 at 01:00
I wish that were true, truth is I'm mathematically inept and floundering. That is the strength of these forums, it allows us to pool our knowledge and hence fill gaps in our understanding.
By: Beermat
- 18th October 2017 at 12:26Permalink- Edited 1st January 1970 at 01:00
OK, so far I have managed to glean, largely from that NACA doc, that a 'constant' or 'flat' pitch set considerably less than the 'on paper' optimum is actually more efficient in real life, for reasons which are unexplained. In other words, design your twist to 'cut the butter' at 200 mph with 1,500 prop RPM if you want it to be efficient at 350 mph. Then you have the compromise between take off and max speed efficiency, described in 'Airscrews - Part One' (Malloy, 1938), which might make you want to introduce even more twist.
This was discovered in the late 1930's, without doubt after the 6105 was designed. The apparently under-twisted 5,000 series blades are the theoretical 'state of the art' up until NACA 658 in 1938, with a twist designed (under an understandable misapprehension) to match airspeeds of 200-300mph. It was only understood later that you want 'flat pitch distribution' at 15 degrees to get max efficiency at 30 degrees. The 6127, the basis for the 4,000 series, already had this.
Shortly afterwards, DH 'twisted down' the 6105 to make the Spitfire's 55400 basic blade, maximising its efficiency but in an unexpected direction. Then, at their own pace, DH undertook the redesign of the twist of it's other, regular 5,000 series basic blades - thus taking the sub-series beyond the 100-400 range into the realms of the Lancaster's 800 etc.
As a vast number of blades had been manufactured and were flying on thousands of well-known British aircraft while under the earlier and a it turned out surprisingly wrong school of thought, it is no mystery that this wasn't well documented.
Efficiency curve for the 5868-9 / 6127-9 / 54409, from NACA 642, also 1938.
V/nD of 2.0 = 350 mph for the Whirlwind at 3,000rpm.
45 degrees at 0.75r = 47 degrees at 0.7r - the coarse pitch stop for the WW was at 48 degrees.
V/nD of 1.2 is 210 mph, climbing speed of the WW and near max efficiency for the blade at a 30 degree blade angle.
Minimum pitch stop of the WW was 28 degrees.
[ATTACH=CONFIG]256419[/ATTACH]
Note that this blade had a 'flat' or constant pitch distribution at 15 degrees - nowhere near the operating angle of peak efficiency.
From this it's clear that the propeller was the right choice for the Whirlwind's envelope (ignoring compressibility!) - but with very little margin given by the 20 degree range of the prop.
By: powerandpassion
- 19th October 2017 at 02:32Permalink- Edited 1st January 1970 at 01:00
I would just like to say that I find this thread most enjoyable and please continue to do dives and somersaults off the high board. For the fat kid floating in the water the odd ripple of logic penetrates and the more ripples the better. Good show !
As with most interesting things in science and industry, you get to realize that 'they were making it up as they went along'.
I would be very happy to receive any info on the Peregrine, all in good time.
I am specifically after detail on the additional cylinder hold down bolts which I believe came in with the Kestrel XVI, traveled into the Peregrine and early Merlin up to Merlin III one piece cylinder bank. These bolts held the cylinder 'up' against the top, to manage coolant leaks into the top of the cylinder. The giveway are removable blanking covers on the side of the bank that allow you to insert a spanner to tighten the end of the bolt. Most ideally I am after the remains, just a handful with at least one intact bolt, of any Kestrel XVI, Peregrine or early one piece Merlin bank to observe how the bolt was integrated into the casting and the metallurgy of the bolts. Sorry for going off topic. Pictures of the feature attached.
By: Schneiderman
- 19th October 2017 at 10:44Permalink- Edited 1st January 1970 at 01:00
To push this thread further off-topic I've often wondered at the detailed relationship between late marks of Kestrel, the Goshawk and Peregrine. Supermarine's tender to compete against Westland's Whirlwind is clearly indicated on the GA drawings to be powered by Goshawks, yet R-R had abandoned evaporative cooling by then and the Goshawk waas surely dead in the water. Is the Peregrine, therefore, little more than a Goshawk with 'conventional' cooling. I kind of suspect that there is some kind of parallel to the convoluted evolution of the PV12 - Merlin B&C - Merlin I - Merlin II
By: Beermat
- 19th October 2017 at 12:40Permalink- Edited 1st January 1970 at 01:00
I'll keep digging. Maybe an appeal in a different thread on here might yield something? There are a few Miles Masters with remains still visible on our hillsides, not to mention a few Fury-types. Maybe someone has a relic from when it wasn't seen as such a bad thing to take them, which they might wish to put to good use?
Edit - the key large scale change between Kestrel XXX and the Peregrine was the down-draft carb. Was the Goshawk up-draft?
By: Beermat
- 19th October 2017 at 13:02Permalink- Edited 1st January 1970 at 01:00
Back on topic (ish).. I have had a bit of a lightbulb moment which I should have had a week ago. The Hamilton Standard tables that I have talk of a reference station - normally at 42". The angle at this station is the angle at which the pitch change with distance is constant along the blade - ie. a flat distribution. This is the angle at 42" which would enable butter-cutting (but as above isn't the best angle for air-cutting, as it turns out).
So, the 6353 seems to have a 'reference angle' of 19.9 degrees.
I couldn't work out why HS didn't just use the UK method of putting a zero at 0.7 radius, and working back from there. I realise now that this doesn't tell you enough about the geometry of the blade without complex maths, while the US version puts it on a plate.
Doing the complex maths (actually not, cheating by drawing it on a piece of paper) shows the Spit blade has a reference angle at 0.7 radius (about 48 inches) of 25 degrees inboard and 15 degrees outboard - it has two foci. This is because DH twisted it when they telescoped it. The average is around 20 degrees - which works out more or less the same as the later Lancaster blade, but done less elegantly by simply twisting half the blade twice as much.
All that remains for me is to untwist the Spitfire blade to get an original 5,000 series twist - and then work out where it fits on the 'hundred' sub-series. Are we having fun yet?
By: Trak-Tor
- 19th October 2017 at 13:11Permalink- Edited 1st January 1970 at 01:00
It's really interesting, though most of the time I have only distant idea what are you talking about...
But I like it.
Is there some book/manual to study this?
Thanks.
By: Beermat
- 19th October 2017 at 13:29Permalink- Edited 1st January 1970 at 01:00
Propeller theory yes - https://babel.hathitrust.org/cgi/pt?id=uc1.$b80327 is one of them, also try Aircraft Propeller Design (New York and London: McGraw-Hill Book Co. 1930), by Fred E. Weick, which is also available online. They both get into pretty heavy maths, but are useful to check a lot of the web-based info against.
What blades DH and Hamilton actually made and why - no. Not yet, anyway :-)
Posts: 3,447
By: Beermat - 15th October 2017 at 11:32 Permalink - Edited 1st January 1970 at 01:00
A potential problem, but not so critical in the climb at around 200 mph TAS, so not the altitude issue. Unless the problem occurred at the 'bottom end' of the range.
Posts: 823
By: Schneiderman - 15th October 2017 at 11:46 Permalink - Edited 1st January 1970 at 01:00
All well and good climbing smartly to altitude but not if there is b****r-all you can actually do in combat when you get there. Strangely the pitch angle data does seem to suggest that the prop. was working most efficiently at the fine end.
Posts: 3,447
By: Beermat - 15th October 2017 at 11:53 Permalink - Edited 1st January 1970 at 01:00
True, but we are back to the issue that you couldn't possibly get near the speeds you'd need to start hitting the coarse stops etc. at altitude because you'd have 10 times the usual drag or more on the blades - which would fine themselves in response.
Posts: 3,447
By: Beermat - 15th October 2017 at 11:58 Permalink - Edited 1st January 1970 at 01:00
What sort of speed would these blades be better for? By which I mean at the 'point-of-focus' as it were, minus 5 degrees, at 3,000 rpm x 0.477, what does the maths produce?
I wonder whether in fact I have it wrong, and making fine pitch more efficient was all about power delivery in take off and climb, prioritised over maximum speed. One could argue 'who needs acceleration when you are already fast enough.. and who doesn't need it at 150 mph in combat or when taking off'?
Having said that, I have just done a rough calculation and it looks like max efficiency of the 5868 would be at 50 mph and off the end of the fine pitch range, so I will need to revisit the DH data more carefully. Spitfire checks out at 150 mph and near enough right on the fine pitch stop.
Posts: 3,447
By: Beermat - 15th October 2017 at 13:51 Permalink - Edited 1st January 1970 at 01:00
..and measuring the 'Lancaster' 6353 in the same way as the RAE measured the spit prop - from 0.3r to 0.7r, it has in fact got the same twist as the Spit blade, around 18.5 degrees. I cannot account for the 'flat spot', unless it was to aid feathering. Something to do with the 'exposed area' being different with an in-line engine instead of the usual US radial on anything multi-engined?
Posts: 3,447
By: Beermat - 15th October 2017 at 15:39 Permalink - Edited 1st January 1970 at 01:00
Want to see something that turns it all upside down?
From NACA full-size tests on our very own 5868, report 658, published in April 1938:
[ATTACH=CONFIG]256326[/ATTACH]
So much for theory. Looks like you need that twist to be 'wrong' for best results..
..and I could hazard an educated guess as to why.
Posts: 1,354
By: powerandpassion - 16th October 2017 at 06:49 Permalink - Edited 1st January 1970 at 01:00
I have been sitting here in the corner of the pool with my floaties on while you two Tarzans have been performing backflips off the highboard, triple somersaults through propeller arcana and theory, stopping every once in a while to jab a pipe into the others chest to make a point. At some stage you need to stop and turn your gaze to the flabby, useless fat kid in the water and slowly explain some of this material. Dumb it down.
How does a propeller work ? What were the bright people doing back then to chase more performance ? Chopping blades into bits and re-sticking them together? Twisting blades more ? Can you please do a youtube : get a tub of butter and twist up a butter knife to show how the different ideas worked to move butter !
Don't forget a lot of the yankee RHT blades were for air cooled radials, and some of the blade design was for engine cylinder cooling. The Mosquito - Lancaster blade was designed for US RHT radials, then used for the RHT Merlin. I wonder if dH, in chopping and resticking blades, was just getting rid of the 'unnecessary' engine cooling function?
I wonder if any part of the blade was designed to force air into an engine carb intake ? RR and Bristols did so much work on engine fairing. I cannot believe that there was not some kind of correspondence between the fairing designers and the prop designers.
Posts: 823
By: Schneiderman - 16th October 2017 at 07:39 Permalink - Edited 1st January 1970 at 01:00
Believe me, I wish I knew. This is a case of learning on the job with a large amount of key information lacking. Re: your penultimate paragraph regarding the HS props starting life for use with raduials, I was beginning to think along similar lines but tending to veer towards the opposite conclusions. So it goes.
Posts: 3,447
By: Beermat - 16th October 2017 at 09:14 Permalink - Edited 1st January 1970 at 01:00
Ah, Ralph's the real Tarzan. Mine's an inflatable fancy dress Tarzan suit. I am more like a duck - looks good on the surface but underneath little legs going like the clappers to make any progress at all.
I like the butter idea. Different blade twists cut the block different ways.. put the butter on some kind of low-friction bed, anchor the blade on a pivot beside the butter representing a hub, move the blade down and the more twisted one moves the butter more slowly as it cuts.
The engine cooling thing is interesting, but if there was some thrust intended for the cylinders then there would be little reason to not use it for propulsion if possible on an in-line engine. It looks to have been the other way around, in that there are quite narrow shanks to the Hamilton blades until they get outside the radius of the cowling of an air-cooled radial, while Rotol, who were designing for the UK market from the outset, had plenty of 'proper' aerofoil right up to the spinner.
When DH went on to design their own blades, and stated doing 'paddles' etc, then they also added some extra chord at the root end. Funnily enough this first happened when moving the de Havilland Mosquito on from prototype to production. There weren't really the conversations one imagines happened between the aircraft designer and prop designer if not in-house - it seems on the evidence that the main conversation was between the procurement office and the prop salesman.
I have to hold up my hand and admit that these later DH-designed blades don't (or shouldn't) follow on in my blade-hacking-about series - I am really only looking at the early 'needle' blades.
On the other thing, by the way, I have asked around and all we have is the Peregrine AP. I can get that to you - it has some nice schematics - or just read it for what it is you needed. Was it con rod material?
Posts: 823
By: Schneiderman - 16th October 2017 at 10:13 Permalink - Edited 1st January 1970 at 01:00
I wish that were true, truth is I'm mathematically inept and floundering. That is the strength of these forums, it allows us to pool our knowledge and hence fill gaps in our understanding.
Posts: 3,447
By: Beermat - 18th October 2017 at 12:26 Permalink - Edited 1st January 1970 at 01:00
OK, so far I have managed to glean, largely from that NACA doc, that a 'constant' or 'flat' pitch set considerably less than the 'on paper' optimum is actually more efficient in real life, for reasons which are unexplained. In other words, design your twist to 'cut the butter' at 200 mph with 1,500 prop RPM if you want it to be efficient at 350 mph. Then you have the compromise between take off and max speed efficiency, described in 'Airscrews - Part One' (Malloy, 1938), which might make you want to introduce even more twist.
This was discovered in the late 1930's, without doubt after the 6105 was designed. The apparently under-twisted 5,000 series blades are the theoretical 'state of the art' up until NACA 658 in 1938, with a twist designed (under an understandable misapprehension) to match airspeeds of 200-300mph. It was only understood later that you want 'flat pitch distribution' at 15 degrees to get max efficiency at 30 degrees. The 6127, the basis for the 4,000 series, already had this.
Shortly afterwards, DH 'twisted down' the 6105 to make the Spitfire's 55400 basic blade, maximising its efficiency but in an unexpected direction. Then, at their own pace, DH undertook the redesign of the twist of it's other, regular 5,000 series basic blades - thus taking the sub-series beyond the 100-400 range into the realms of the Lancaster's 800 etc.
As a vast number of blades had been manufactured and were flying on thousands of well-known British aircraft while under the earlier and a it turned out surprisingly wrong school of thought, it is no mystery that this wasn't well documented.
Efficiency curve for the 5868-9 / 6127-9 / 54409, from NACA 642, also 1938.
V/nD of 2.0 = 350 mph for the Whirlwind at 3,000rpm.
45 degrees at 0.75r = 47 degrees at 0.7r - the coarse pitch stop for the WW was at 48 degrees.
V/nD of 1.2 is 210 mph, climbing speed of the WW and near max efficiency for the blade at a 30 degree blade angle.
Minimum pitch stop of the WW was 28 degrees.
[ATTACH=CONFIG]256419[/ATTACH]
Note that this blade had a 'flat' or constant pitch distribution at 15 degrees - nowhere near the operating angle of peak efficiency.
From this it's clear that the propeller was the right choice for the Whirlwind's envelope (ignoring compressibility!) - but with very little margin given by the 20 degree range of the prop.
Posts: 1,354
By: powerandpassion - 19th October 2017 at 02:32 Permalink - Edited 1st January 1970 at 01:00
I would just like to say that I find this thread most enjoyable and please continue to do dives and somersaults off the high board. For the fat kid floating in the water the odd ripple of logic penetrates and the more ripples the better. Good show !
As with most interesting things in science and industry, you get to realize that 'they were making it up as they went along'.
I would be very happy to receive any info on the Peregrine, all in good time.
I am specifically after detail on the additional cylinder hold down bolts which I believe came in with the Kestrel XVI, traveled into the Peregrine and early Merlin up to Merlin III one piece cylinder bank. These bolts held the cylinder 'up' against the top, to manage coolant leaks into the top of the cylinder. The giveway are removable blanking covers on the side of the bank that allow you to insert a spanner to tighten the end of the bolt. Most ideally I am after the remains, just a handful with at least one intact bolt, of any Kestrel XVI, Peregrine or early one piece Merlin bank to observe how the bolt was integrated into the casting and the metallurgy of the bolts. Sorry for going off topic. Pictures of the feature attached.
Posts: 823
By: Schneiderman - 19th October 2017 at 10:44 Permalink - Edited 1st January 1970 at 01:00
To push this thread further off-topic I've often wondered at the detailed relationship between late marks of Kestrel, the Goshawk and Peregrine. Supermarine's tender to compete against Westland's Whirlwind is clearly indicated on the GA drawings to be powered by Goshawks, yet R-R had abandoned evaporative cooling by then and the Goshawk waas surely dead in the water. Is the Peregrine, therefore, little more than a Goshawk with 'conventional' cooling. I kind of suspect that there is some kind of parallel to the convoluted evolution of the PV12 - Merlin B&C - Merlin I - Merlin II
Posts: 3,447
By: Beermat - 19th October 2017 at 12:40 Permalink - Edited 1st January 1970 at 01:00
I'll keep digging. Maybe an appeal in a different thread on here might yield something? There are a few Miles Masters with remains still visible on our hillsides, not to mention a few Fury-types. Maybe someone has a relic from when it wasn't seen as such a bad thing to take them, which they might wish to put to good use?
Edit - the key large scale change between Kestrel XXX and the Peregrine was the down-draft carb. Was the Goshawk up-draft?
Posts: 3,447
By: Beermat - 19th October 2017 at 13:02 Permalink - Edited 1st January 1970 at 01:00
Back on topic (ish).. I have had a bit of a lightbulb moment which I should have had a week ago. The Hamilton Standard tables that I have talk of a reference station - normally at 42". The angle at this station is the angle at which the pitch change with distance is constant along the blade - ie. a flat distribution. This is the angle at 42" which would enable butter-cutting (but as above isn't the best angle for air-cutting, as it turns out).
So, the 6353 seems to have a 'reference angle' of 19.9 degrees.
I couldn't work out why HS didn't just use the UK method of putting a zero at 0.7 radius, and working back from there. I realise now that this doesn't tell you enough about the geometry of the blade without complex maths, while the US version puts it on a plate.
Doing the complex maths (actually not, cheating by drawing it on a piece of paper) shows the Spit blade has a reference angle at 0.7 radius (about 48 inches) of 25 degrees inboard and 15 degrees outboard - it has two foci. This is because DH twisted it when they telescoped it. The average is around 20 degrees - which works out more or less the same as the later Lancaster blade, but done less elegantly by simply twisting half the blade twice as much.
All that remains for me is to untwist the Spitfire blade to get an original 5,000 series twist - and then work out where it fits on the 'hundred' sub-series. Are we having fun yet?
Posts: 146
By: Trak-Tor - 19th October 2017 at 13:11 Permalink - Edited 1st January 1970 at 01:00
It's really interesting, though most of the time I have only distant idea what are you talking about...
But I like it.
Is there some book/manual to study this?
Thanks.
Posts: 3,447
By: Beermat - 19th October 2017 at 13:29 Permalink - Edited 1st January 1970 at 01:00
Propeller theory yes - https://babel.hathitrust.org/cgi/pt?id=uc1.$b80327 is one of them, also try Aircraft Propeller Design (New York and London: McGraw-Hill Book Co. 1930), by Fred E. Weick, which is also available online. They both get into pretty heavy maths, but are useful to check a lot of the web-based info against.
What blades DH and Hamilton actually made and why - no. Not yet, anyway :-)
Posts: 146
By: Trak-Tor - 19th October 2017 at 13:58 Permalink - Edited 1st January 1970 at 01:00
Thanks a lot. That's my kind of reading before falling asleep...
And I have a degree in maths, so it should make some sense.
;)
Posts: 823
By: Schneiderman - 19th October 2017 at 14:10 Permalink - Edited 1st January 1970 at 01:00
Yes, he's lost me now to. Pictures, I need pictures and diagrams, my brain works better that way.
Goshawk was updraught
Posts: 3,447
By: Beermat - 19th October 2017 at 14:29 Permalink - Edited 1st January 1970 at 01:00
So, Goshawk = evaporatively-cooled Kestrel, Peregrine = downdraft - oops, downdraught Kestrel?
I'll do some diagrams at some point - I want to check the maths first - or even better get Trak-tor to do it!