Engine design as related to airplane power : with particular reference to performance at varying alt

[Deleted member 68059]

Well it sounds to me like you need to find someone with access to millions of pages of WW2 aviation technical reports, access to Rolls-Royce Archives , who has interviewed the son of the Chief Designer at Daimler-Benz AG has a current career in high performance engine design up to and including for Formula One Engine constructors - and a proven track record to write you a book about engines?

Since it wont be published until August 2018, I`m planning on making a series of videos on aero-engine design to help promote the book before its published. If anyone has specific suggestions I`ll be happy to start on those first.

 

[MIflyer]

Here is some info on Intercoolers, if anyone is interested.

 

[MIflyer]

"If anyone has specific suggestions I`ll be happy to start on those first."

One thing that at least some people need explained is the interplay between supercharging and power. At a certain website I no longer visit (J-aircraft) I was told by two non-pilot non-engineers that use of supercharging below a certain altitude actually costs power rather than adds it. If that were the case ground vehicles would not ever be supercharged.

It is true that with a fixed single speed supercharger you can find yourself overboosting the engine at low altitude, I recall reading where a P-40E pilot jumped in his airplane when the PI was attacked and shoved the throttle all the way forward, desperate to get out from under the Japanese air raid and build up some speed and altitude. After a couple of minutes he looked at his instruments and was very dismayed to see he was only pulling 10 inches of MP. Obviously his engine had some serious problems and he was doomed. But gradually he realized that the MP gauge had gone past the 60 inch mark, all the way around until it was reading 10 inches. In reality he was pulling more than 70 inches. I have been told that on the test stand at the factory they were able to pull well over 80 inches MP on the V-1710 without hurting anything, but obviously it was not a recommended daily practice.

When the P-51's began escorting the B-29's to Japan they were distressed to find that the Japanese were smart enough not to climb to 20,000 ft and tangle with the Mustangs. They refused to go over 15,000 ft, which no doubt represented their best performance altitude. But at 15,000 ft the Merlin was reaching the limit of its low speed supercharger performance, which switched at about 18,000 ft. A P-51 squadron commander on Iwo told the maintenance guys to replace the spring loaded switch that enabled a momentary test of the high speed supercharger function with a toggle switch, so he could switch to high speed supercharger manually at an altitude below 18,000 ft and get some extra performance to catch Japanese aircraft. The Packard tech rep replied (insert Scottish accent), "You canna do that Captain! It'll blooow up the engine!" They installed the new toggle switch and it worked fine for momentary busts of power. The idiots at J-aircraft insisted that going to high speed supercharger below 18,000 ft would have made the airplane go SLOWER. So I think it would be good to explain this, Some power versus altitude charts would be useful...

 

[Deleted member 68059]

Ah I see - well it gets a bit complicated so its easy for people to add six oranges to six apples and say *Twelve !" with respect to multi-speed geared superchargers. I`d say you are BOTH correct .....within certain mis-applied boundaries.!

That is a very convenient topic as its basically the first third of a presentation I did in 2016 in Ohio for the "Aircraft Engine Historical Society" chaps, I`ll gladly make a short video which will clear all this up.



Thank you for being kind enough to take the time to make a suggestion. Its important as its very good material for me for the upcoming book, as it shows me areas which are very important to make sure I have provided material on, which I might well otherwise have forgotten about/missed. If I make the book based purely on my own interests, that`s fine but since I might like people to actually buy one, at some point - feedback on interesting aspects that remain a bit blurry is valuable.

 

[tomo pauk]

"If anyone has specific suggestions I`ll be happy to start on those first."

One thing that at least some people need explained is the interplay between supercharging and power. At a certain website I no longer visit (J-aircraft) I was told by two non-pilot non-engineers that use of supercharging below a certain altitude actually costs power rather than adds it. If that were the case ground vehicles would not ever be supercharged.

I could not resist - why aren't you visiting the J-aircraft any more?
An un-supercharged engine will no be using any boost. No boost = low power. Take the V-1710 or Merlin - those be lucky to do 700 HP at 3000 rpm at sea level. As it is explained in the 1st PDF you've posted in this thread. (obviously you know that for decades, but still need to be pointed out)

It is true that with a fixed single speed supercharger you can find yourself overboosting the engine at low altitude, I recall reading where a P-40E pilot jumped in his airplane when the PI was attacked and shoved the throttle all the way forward, desperate to get out from under the Japanese air raid and build up some speed and altitude. After a couple of minutes he looked at his instruments and was very dismayed to see he was only pulling 10 inches of MP. Obviously his engine had some serious problems and he was doomed. But gradually he realized that the MP gauge had gone past the 60 inch mark, all the way around until it was reading 10 inches. In reality he was pulling more than 70 inches. I have been told that on the test stand at the factory they were able to pull well over 80 inches MP on the V-1710 without hurting anything, but obviously it was not a recommended daily practice.

Pilots were pushing early V-1710s ( F3R, F4R ) beyond 60 in Hg. link

When the P-51's began escorting the B-29's to Japan they were distressed to find that the Japanese were smart enough not to climb to 20,000 ft and tangle with the Mustangs. They refused to go over 15,000 ft, which no doubt represented their best performance altitude. But at 15,000 ft the Merlin was reaching the limit of its low speed supercharger performance, which switched at about 18,000 ft. A P-51 squadron commander on Iwo told the maintenance guys to replace the spring loaded switch that enabled a momentary test of the high speed supercharger function with a toggle switch, so he could switch to high speed supercharger manually at an altitude below 18,000 ft and get some extra performance to catch Japanese aircraft. The Packard tech rep replied (insert Scottish accent), "You canna do that Captain! It'll blooow up the engine!" They installed the new toggle switch and it worked fine for momentary busts of power. The idiots at J-aircraft insisted that going to high speed supercharger below 18,000 ft would have made the airplane go SLOWER. So I think it would be good to explain this, Some power versus altitude charts would be useful...

Sure enough there is plenty of power vs. altitude charts on the net. Eg. here for the Merlin Mustangs: link
1600 HP available above 15000 ft for the V-1650-7 even with plenty of ram.

 

[Shortround6]

Actually they were sort of/maybe/half right...........

An awful lot depends on the exact supercharger set up.

Very few non-racing cars use the amount of boost aircraft engines use as they are set up for sea level or near sea level. 15lbs of boost at sea level means the supercharger is only compressing the air at a 2:1 ratio.

The Merlin III hit it's peak power at 16,250 ft. using 87 octane fuel. but it was compressing the air just under 2.6 times just to get 6lbs boost.
IF they had been able to keep the throttle open at sea level the supercharger would have delivered enough air to make around 1600-1700hp.

Instead due to the limits of the fuel the throttle had to be closed down to the point where the engine was only delivering 880hp to the propeller,
Still more than it would have made with no supercharger but obviously the plane was limited in speed at low altitude by the supercharger set up which was addressed by both the Merin VIII used in the Early Fulmar and in the Merlin X used in early bombers. Using an identical supercharger and, from a power handling standpoint, identical engine block, crank/pistons, etc several hundred HP were added near sea level simply by spinning the impeller slower (using less supercharging?)
Once better fuel came along the throttle could be opened and the extra power could be used.

This is why it is to hard to "simplify" things. What is "true" in a particular set of circumstances, is NOT true in another set of circumstances and unless all the details are presented you get different camps believing different things.

 

[MIflyer]

No, there was no half right or both right. I am not talking about extremes like trying to run an engine on kerosene or switching to high speed supercharger in a P-51D at 5000 ft. I used the example of the P-51's manually kicking into high speed supercharger at below the nominal 18,000ft switch point and the Clipped Clapped and Cropped Spitfire V's being modified to get full boost at a lower altitude - and those guys said it was impossible.

 

[pbehn]

The notion that turbo or supercharging has no effect at sea level is strange. Without forced aspiration you chase higher power by higher revs and improved breathing by polishing and straightening inlet and outlets with ever wilder cam timing.

 

[Shortround6]

No, there was no half right or both right. I am not talking about extremes like trying to run an engine on kerosene or switching to high speed supercharger in a P-51D at 5000 ft. I used the example of the P-51's manually kicking into high speed supercharger at below the nominal 18,000ft switch point and the Clipped Clapped and Cropped Spitfire V's being modified to get full boost at a lower altitude - and those guys said it was impossible.

Given those examples they were certainly wrong.

 

[wuzak]

Instead due to the limits of the fuel the throttle had to be closed down to the point where the engine was only delivering 880hp to the propeller,
Still more than it would have made with no supercharger but obviously the plane was limited in speed at low altitude by the supercharger set up which was addressed by both the Merin VIII used in the Early Fulmar and in the Merlin X used in early bombers. Using an identical supercharger and, from a power handling standpoint, identical engine block, crank/pistons, etc several hundred HP were added near sea level simply by spinning the impeller slower (using less supercharging?)
Once better fuel came along the throttle could be opened and the extra power could be used.

In the case of the lower altitude engines, the gearing spun the impeller at a lower speed, meaning lower pressure ratio and less power to drive it. It also meant less throttling, which also meant less power loss.

So, yes, it was less supercharging. The pressure ratio that could be achieved was less.

In a 2 speed engine, such as the X, the low gear was rated as medium supercharged and the high gear as fully supercharged.

The compromise, in a single speed engine, is the altitude band where the engine performs best. Drop the gearing to get better low altitude power and you lose out at higher altitudes, and vice versa.

 

[wuzak]

When the P-51's began escorting the B-29's to Japan they were distressed to find that the Japanese were smart enough not to climb to 20,000 ft and tangle with the Mustangs. They refused to go over 15,000 ft, which no doubt represented their best performance altitude. But at 15,000 ft the Merlin was reaching the limit of its low speed supercharger performance, which switched at about 18,000 ft. A P-51 squadron commander on Iwo told the maintenance guys to replace the spring loaded switch that enabled a momentary test of the high speed supercharger function with a toggle switch, so he could switch to high speed supercharger manually at an altitude below 18,000 ft and get some extra performance to catch Japanese aircraft. The Packard tech rep replied (insert Scottish accent), "You canna do that Captain! It'll blooow up the engine!" They installed the new toggle switch and it worked fine for momentary busts of power. The idiots at J-aircraft insisted that going to high speed supercharger below 18,000 ft would have made the airplane go SLOWER. So I think it would be good to explain this, Some power versus altitude charts would be useful...

To get more power in the higher gear, before the changeover point, would most likely require overboosting, which would require adjustment of the throttle controls as well. Otherwise the engine would make less power and the plane would fly slower.

Not sure which engine was being used in the Mustang cited, the following chart is for the Merlin 66/V-1650-7.

http://www.spitfireperformance.com/merlin66hpchart.jpg

At +18psi boost (67inHg MAP) the changeover point was ~13,000ft. Looking at the chart the lines that increase from left to right are lines of constant boost. The boost is maintained by throttle control (automatic). When the throttle is full open peak power (in that gear and boost) is achieved, and then the boost (and power) falls away. The lines that decrease from left to right are the full throttle lines (and are so labelled).

You can see from that chart that the power between ~9,750ft and 13,000ft in low gear is still more than it is in high gear even though it is at lower boost.

S/Hi gear produces more power in that altitude band at +25psi (80inHg ?). Continuing the M gear/+18psi power line in S gear would be around +21psi, I suspect.

 

[Zipper730]

If I may, there are also several other books that people might find interesting.
By A.W. Judge: Elementary handbook of aircraft engines
Aircraft engines

I like the table of contents with all the formulas available, I've started reading through a book titled "Aerodynamics for Naval Aviators". Even though it's got a lot of math in it, everything's well explained in context so far.

'Development of aircraft engines' and 'Development of aviation fuels'

I've actually been reading through that one...

It is an idea, but who would be the final editor/arbiter.

No idea, but I figure it would be a consensus thing. I figure some of the people I listed would be highly valuable as they know a lot.

Just look at the still live discussions about best turning performance between Spitfire and Bf 109.

If I recall the Spitfire out-turned it at all speeds though it required pilots to really manhandle the plane and get uncomfortably close to stalling (which many pilots obviously were uncomfortable with, but test pilots were), whereas the Me-109's slats popping out acted as a stall-warning and gave them a clear warning, though the slats might come out unevenly and wobble the plane all over the place and make it hard to hold gunsight on target.

 

[Zipper730]

Regarding what other people were saying, in terms of additions made absent of WWII era classification: What I was thinking of amending was the following

1. Adding naturally aspirated engines: They were used at small scales and very primitive engines
2. Explaining the advantages the turbochargers had at altitudes: It takes little horsepower off the engine so it provides doesn't take away horsepower off the shaft, but adds horsepower through compression; the increasing RPM of the turbo due to altitude changes allows very high critical altitudes in theories (all things being equal); drawbacks come in terms of a reduced amount of available thrust.
3. Hydraulic Clutching #1: Evidently, the power was transferred through a fluid-coupling and differing amounts of oil were used to vary the lubricity of the system allowing the supercharger to spin faster or slower, producing a similar effect to a turbocharger. The drawback was the gear-ratio was more limited than a turbocharger, and horsepower was progressively taken off as one went higher and higher.
4. Hydraulic Clutching #2: The Pratt & Whitney R-2800's used some type of hydraulic clutching that seemed to be fixed gears, but used fluid to transfer the force over traditional means, so less friction was needed for the same RPM, so one could lose less horsepower to drive the supercharger, or drive it harder for the same horsepower loss.

Sound good?
As for the subject of sea-level blowers vs high altitude blowers

1. As I understand it the sea-level blower is designed to augment horsepower by taking a small amount of shaft-horsepower; then using that to drive a compressor which produces more horsepower than is lost; thus producing a more powerful engine for the same size
2. The higher altitude superchargers take more power off the shaft: I would assume this means that they would add more horsepower than they took off if you could push the engine right to full RPM, and had no manifold pressure limit?

 

[Shortround6]

Regarding what other people were saying, in terms of additions made absent of WWII era classification: What I was thinking of amending was the following

1. Adding naturally aspirated engines: They were used at small scales and very primitive engines

Regarding the book/pamphlet listed in the original post/s, it already uses a naturally aspirated engine as a baseline, see page 25 for graph, and several pages around it for basic discussion, to illustrate the need for supercharger of some sort for altitude performance. The alternative was used by the Germans in WW I and by the Jacobs company in the US, build oversize engine and don't allow it to use full throttle at sea level. Open the throttle as the plane climbs to keep a relatively constant power output up to design altitude.

2.
Explaining the advantages the turbochargers had at altitudes: It takes little horsepower off the engine so it provides doesn't take away horsepower off the shaft, but adds horsepower through compression; the increasing RPM of the turbo due to altitude changes allows very high critical altitudes in theories (all things being equal); drawbacks come in terms of a reduced amount of available thrust.

This is already pretty well explained in the booklet. Drawback is much more in the areas of increased weight, bulk and drag. Please remember that a lot of aircraft did not use exhaust thrust or at least did not use it well. If you really read the booklet there are other things that come up, like in the section of "interesting facts and definitions" at the end. Under "back pressure" where they figure that a 1000hp sea level engine would make 1080hp at 20,000ft due to less atmospheric pressure at the exhaust ports. Assuming of course that you could deliver sea level pressure and temperature air to the engine inlet.

3.
Hydraulic Clutching #1: Evidently, the power was transferred through a fluid-coupling and differing amounts of oil were used to vary the lubricity of the system allowing the supercharger to spin faster or slower, producing a similar effect to a turbocharger. The drawback was the gear-ratio was more limited than a turbocharger, and horsepower was progressively taken off as one went higher and higher.

actually to power was closer to constant.

In the input turbine/impeller always revolved at the same speed in regards to the engine. the output pump/impeller was slower and then picked up speed as more oil filled the housing. At low altitudes the the input was turning the high rpm but was doing a lot of 'churning' of the oil, which also didn't do much for the oil temperature. (that is it got hot).

1. Hydraulic Clutching #2: The Pratt & Whitney R-2800's used some type of hydraulic clutching that seemed to be fixed gears, but used fluid to transfer the force over traditional means, so less friction was needed for the same RPM, so one could lose less horsepower to drive the supercharger, or drive it harder for the same horsepower loss.

They only lost about 1-2% of the power needed to drive the impellers in the drive gears. Changing the type of clutch wasn't going to change that. Different drive set-ups (clutches/gear changes) had different problems with sludge build up and/or sticking. Some pilot's manuals advised changing the supercharger gears periodically while cruising in order to keep the clutches/gear change free. Different companies used different designs.

As for the subject of sea-level blowers vs high altitude blowers

1. As I understand it the sea-level blower is designed to augment horsepower by taking a small amount of shaft-horsepower; then using that to drive a compressor which produces more horsepower than is lost; thus producing a more powerful engine for the same size
2. The higher altitude superchargers take more power off the shaft: I would assume this means that they would add more horsepower than they took off if you could push the engine right to full RPM, and had no manifold pressure limit?

I believe you are using the wrong term. Once engines got constant speed props they pretty much took off at full engine rpm. They limited the power to a safe level by limiting the boost. Usually by not fully opening the throttle (limiting the amount of air reaching the supercharger on most engines, French, Russian and Italian inline engines living in a world of their own).

A big limit on the amount of boost that could be used was the fuel available. Next came the actual strength of the engine.

 

[Zipper730]

Regarding the book/pamphlet listed in the original post/s, it already uses a naturally aspirated engine as a baseline, see page 25 for graph, and several pages around it for basic discussion, to illustrate the need for supercharger of some sort for altitude performance.

I'm surprised I missed that...

The alternative was used by the Germans in WW I and by the Jacobs company in the US, build oversize engine and don't allow it to use full throttle at sea level.

The Germans seemed to overcome this obstacle in WWII, though they did use fairly large displacement engines.

Please remember that a lot of aircraft did not use exhaust thrust or at least did not use it well.

I know this is going to sound stupid, but when did people realize the thrust was useful in the US, Europe, and Japan?

 

[Zipper730]

In the input turbine/impeller always revolved at the same speed in regards to the engine.

So the driven member comes off the engine and the torque converter which is essentially a hydraulic turbine, varies it?

At low altitudes the the input was turning the high rpm but was doing a lot of 'churning' of the oil, which also didn't do much for the oil temperature.

I'm not sure what you mean by churning, but it seems like there'd be less lubrication...

 

[pbehn]

I'm not sure what you mean by churning, but it seems like there'd be less lubrication...

Churning is what happens in a churn.

 

[Zipper730]

Churning is what happens in a churn.

Great definition

 

[pbehn]

Great definition

Well if you just google "churn" it is obvious what churning is. Basically severe turbulence in a fluid (milk) caused by vanes rotating in a drum to produce butter.

 

[Shortround6]

This is a picture of a car torque converter

The "impeller" is pretty much a mirror image of the turbine. A bowl with a lot of fins and some bracing to keep them from bending (aircraft ones might be better made?).
In any case on the 109 the Impeller will always be turning at a bit over 10 times the crankshaft speed or whatever the exact gear ratio was. There are two ways to vary the speed of the "turbine" (the diven part, on the 109 attached to the supercharger). One is have the distance between the two variable. One of them slides on it's shaft further away from the other and this allows the oil (or working medium) to slip in the empty space between the two and not transmit the full force to the turbine. The other is to vary the amount of oil in the housing. No moving part but full of oil or nearly so means as the Impeller rotates the oil, moved by centrifugal force moves to the outer rim and crosses over to hit the fins on the turbine, then flows to the center and crosses back to the impeller. Impeller is turing at 24,000rpm on a DB601 and higher on later engines. Less oil in the housing means less force is transmitted. However this oil is going to get hot and if you use the engine oil and not a separate supply you are going to get lots and lots of tiny air bubbles in the oil which need to be taken out before the oil is feed back into the engine.