Good High Altitude Performer

[plan_D]

It was Adolf Galland that asked Goering for a squadron of Spitfires . And the Spitfire would fit perfectly into the Bf-109 role, since they were both interceptors.

 

[Gemhorse]

And furthermore, after the War Ernst Heinkel, the famous German aircraft designer, said of the Mosquito: ''That is the aircraft I would have liked to have designed...'' and as a reminder, he kept a picture of one displayed in his boardroom....
They were testing nitrous oxide injection in Mosquitos in 1943 for higher altitude work, which improved performance by 50mph, but the Merlin was an exceptionally versatile engine when one can see in hindsight what was achieved with them collectively, especially when they got around to plopping one in the Mustang....

I personally felt the Fw.190 was the Luftwaffe's best aircraft, and it's development pretty much mirrors the Spitfire's....They wasted alot of Me.109's with that undercarriage, something Willi never rectified......

 

[delcyros]

Is this anything to do with compressability? If so, swept or eliptical wings will rule?

Swept and/or elliptical wings will indeed show an advanatge, as would any thin airfoil (this will exclude the Typhoon/Tempest wing at hi alts). The term "compressability" is more often related to buffeting aspects instead of pure drag but it is obvious that there is a strong relationship. However, this drag doesn´t excludely belong to wings but also to the fuselage and more worrisome to the propeller arcs (=huge drag factor) and -hubs.

Quote:
The next is cooling. In opposition to what You might think, cooling in thinner air (regardless of it´s temperature) is more difficult than in denser atmospheres. The cooling by cold air has indeed a higher theoretical cooling aspect but it is FAR MORE DIFFICULT FOR THE ENGINE to disperse its high engine temp. by interchanging with far fewer air molekules.

I found something similar to this out in my own research. A Laminova-type oil cooler would work wonders here? What about the weird rad on the He 100? (or was it 112?).

I will second Your thoughts about a Laminova-type cooler. The surface vapourating cooling of He-100 however wouldn´t (most probably, since not tried and we will never know with certainity) work properly because the surface has the disadvantage of very limited interchange hence it covered such a large aerea on the wing. At high altitudes this problem will become bigger and bigger with fewer and fewer air molekules...

 

[DerAdlerIstGelandet]

I consider the Spitfire and Bf-109 equals as most people do here. I personally would not consider either of them in the best high alltitude performance catagory though.

 

[Twitch]

Jabberwacky- Interesting to note what the reverse might have been- Spits attacking Luftwaffe over home territory in France returning on petrol fumes to jolly old England going down in the Channel and all that. Let's imagine the Spit pilot constantly worrying about the ubiquitous "red fuel light" blazing on the dash while escorting Blennheims as he attempted to break away for Dover from 109s bent on flaming his ass.

And that Galland quote about tell Göring he wanted Spits is only half the quote. Geez it's becoming a bogus urban legend. He did it simply to piss off The Fat One since in the next sentence relating the incident he says he'd had much rather had 109s.

 

[lesofprimus]

he wanted Spits is only half the quote. Geez it's becoming a bogus urban legend.

Very true Twitch...

 

[Jabberwocky]

Jabberwacky- Interesting to note what the reverse might have been- Spits attacking Luftwaffe over home territory in France returning on petrol fumes to jolly old England going down in the Channel and all that. Let's imagine the Spit pilot constantly worrying about the ubiquitous "red fuel light" blazing on the dash while escorting Blennheims as he attempted to break away for Dover from 109s bent on flaming his ass.

Actually, I was more interested to imagine what it would of been like after the Battle of Britain if the LUftWaffe had operated the Spitfire instead of the 109. Imagine Spitfire V/IX and XIVs intercepting British and American formations for the rest of the war, or supporting low level operations over the Russian steppe. I wonder if they would of been more or less sucessful?

If we swapped roles over southern England in mid-1940 you'd have Spitfires and Hurricanes escorting Wellingtons (22 squadrons), Hampdens (12 squadrons), Whitleys (6 squadrons) and Blenheims (7 squadrons). Costal Command may of used its 5 Beaufort squadrons and 3 Boston I squadrons as well, most likely on shipping strikes and port targets but probably in a conventional level bomber role as well.

Most likely would of been that the Wellingtons would of done the majority of the daylight bombing, with Hampdens, Whitleys and Blenheims switching more towards night bomber roles because of their inadequate defensive firepower. If the bombing continued in a blitz style of scenario, then the first Stirling and Manchester squadrons would of been active in very late 1940/ early 1941 and added a bit more spine to the Wellington squadrons.

It's an interesting intellectual exercise. Over France in 1941 the RAF repeated many of the mistakes that the LuftWaffe had over England in 1940. In fact, they probably made less of a fist of it, because the LuftWaffe generally engaged the RAF on its own terms in 1941 and 1942, much as it did in 1940. But, if the RAF could force the initiative, things become much better for them. Personally, I feel that a more concerted effort needed to be made against the LuftWaffe fighter, bomber and control bases, much like the LW did in July/August 1940 over England.

 

[Marshall_Stack]

Gentlemen (and Gentlewomen if any are out there),

I have a question that is related to this topic. The question is that I have heard / read two conflicting aspects of the P-47's high altitude performance - is it good at high altitude or not?

I have always thought that with the turbocharging system that it was a great fighter at 30,000 feet or above. I saw an episode on the military Channel about the P-47 and a pilot from the 15th Air Force said it was good at medium altitude but never above 28,000 ft. He said that a particular plane might be able to trim out to a higher altitude but formation flying was too difficult. He stated that the P-51 could go up to 35,000 ft with no problem.

I always thought that the P-47 was better at the higher altitudes...

 

[lesofprimus]

They were probably referring to the earlier models of the -47...

 

[Jabberwocky]

Gentlemen (and Gentlewomen if any are out there),

I have a question that is related to this topic. The question is that I have heard / read two conflicting aspects of the P-47's high altitude performance - is it good at high altitude or not?

I have always thought that with the turbocharging system that it was a great fighter at 30,000 feet or above. I saw an episode on the military Channel about the P-47 and a pilot from the 15th Air Force said it was good at medium altitude but never above 28,000 ft. He said that a particular plane might be able to trim out to a higher altitude but formation flying was too difficult. He stated that the P-51 could go up to 35,000 ft with no problem.

I always thought that the P-47 was better at the higher altitudes...

When the P-47 arrived in Europe in 1943, one of the first decisions made by the USAAF was to make a 'hard deck' of 18,000 feet. P-47 pilots were not supposed to engage German fighters below this height (although they did) because the P-47 was at a disadvantage to the lighter and more nimble LuftWaffe fighters below this altitude.

Eventually, as the P-47 became more and more powerful (patricularly with the addition of Water Injection and paddle blade propellors) the P-47 began to engage the LuftWaffe at lower and lower altitudes.

For comparison:

The FW-190A began to lose power and speed above 5,500-6,000 meters/ 18,000-19,500 feet.
The Bf-190G (without Mw-50) peaked at around 6.5-7 km/ 21,000-23,000 feet. With Mw-50 is another story though (and one I'm not fully versed on).
The P-47D produced its best speeds at around 8-9km/26,000-29,000 feet.

So, while its German opponents would start to flag in the thin air above 25,000 feet, the turbosupercharged P-47 could maintain its sea-level output above 27,000 feet and have a decided speed advantage at this height.

 

[schwarzpanzer]

Galland, of course it was! Thanks PlanD.

delcyros:

Swept and/or elliptical wings will indeed show an advanatge, as would any thin airfoil (this will exclude the Typhoon/Tempest wing at hi alts).

I understand the elliptical wing outperformed a later laminar-flow Supermarine wing design in certain instances.


So compressability includes the fuselage also, the other factor doesn't - Is that right?


I wonder how the cooling problem is solved?
- A huge pressure fan and/or massive pressure differential maybe?

And that Galland quote about tell Göring he wanted Spits is only half the quote. Geez it's becoming a bogus urban legend.

For the close-in defense that the Luftwaffe later insisted on, I'm sure the Spit would do much better?

I don't see any advantage after the BoB?


I think any turbo'ed craft would naturally be worse at low altitudes - the engine that is, not the frame.

 

[wmaxt]

delcyros:

Swept and/or elliptical wings will indeed show an advanatge, as would any thin airfoil (this will exclude the Typhoon/Tempest wing at hi alts).

So compressability includes the fuselage also, the other factor doesn't - Is that right?

Compressability occurs when the airflow which is accelerated accross the wing goes supersonic creating a shockwave/separation of the airflow. When this happens the aircraft gets hammered to some extent by the shockwave. The control surfaces often become inoperable because they are in that separation zone or imovable because the shockwave "traps" them. There is also a tendency for stick reversal and for the aircraft to steepen the dive because the lift is destroyed on the upper side of the wing.

The fuselage isn't much of an issue in compressability, except the drag/weight it produced which dictates to some extent the speed/altitude of the compressability zone.

Eliptical wings tend to hit this transition at different times depending on the chord length/lift created, this lessens and delayes the effect. Thin/Laminar flow wings create less acceleration of the airflow and develop their lift further back on the wing - again delaying and lessening the effects of compressability.

I think any turbo'ed craft would naturally be worse at low altitudes - the engine that is, not the frame.

Actually a turbo is still equal or a little better at low altitudes.
A mechanical supercharger is dependant on
1. air quality - density, humidity, temp etc.
2. Engine speed gearing
3. Capacity

Most M Superchargers were two stage the first being a lower capacity for lower altitudes (denser air) and shifted at about 14,000ft (this varei even on individual aircraft depending on the Air quality again.

A turbo acts this way
1. Air quality
2. Exaust volume (rpm)
3. Exaust heat (Load on engine)
4. Backpressure
5. Pressure limiting devices (waste gates)

The Turbo can maintain full boost at throttle positions and rpms less than full throttle. This allows for better efficency in the engine by increasing boost artificaly increases compression ratio. I don't remember the formula but an increase from 8:1 to 10:1 compression will result in about 12% more horse power at the same time reducing fuel consumption by ~10% by increasing efficincy of the combustion process. An example of this is the P-38L at a cruise the book calls for 1900rpm @ 25,000ft giving a range of 2200mi with 1010gal of fuel and a manifold pressure around 28". Lindberg showed the pilots that 1,700rpm and a coarser pitch on the prop (more load) gave a manifold pressure of ~34" anf range went to ~2600mi.

I hope this helps. I probably went into to much detail but I didn't want to leave gaps to cause missunderstandings.

wmaxt

 

[schwarzpanzer]

Thanks for the compressability info wmaxt, it's inline with what I'd heard before.

I think you are incorrect on some aspects of the turbo though. For what it's worth, this is how I know turbo's/blowers:

1. air quality - density, humidity, temp etc.

The blower is superior to the turbo here.

2. Engine speed gearing

They're usually geared up, but it's true.


A big point is the actual design, the turbo's were in their infancy, the blowers very advanced by comparison.

If run at a constant speed, turbo's would be more effective, but I doubt you'd be able to avoid going up and down the rev range if you were a fighter pilot? - especially at low alts?

an increase from 8:1 to 10:1 compression will result in about 12% more horse power at the same time reducing fuel consumption by ~10% by increasing efficincy of the combustion process.

The last part is only true on a Naturally Aspirated engine. What forced induction does is mainly increase power and give 'softer' power. This is what Lindberg is meaning.

If you think about it, say 12:1 air/fuel ratio, you increase the airflow and you must increase fuelling.

Of course you could increase the Compression Ratio (CR) of the engine, lower the boost, whilst leaving the compression ratio (FCR) the same as it was. This would improve fuel consumption and acceleration, but destroy high-alt performance I'd guess.


I've just had prop pitch explained to me and already I've forgotten, doh! I know it alters the way the plane 'swims' through the air though.


I suppose even at high alts, airspeed and airpressure are still 2 very important points?

Another problem I've had of late; trailing vortices - severely affects cooling on the ground, so it must be a major problem at high alts?

 

[wmaxt]

Thanks for the compressability info wmaxt, it's inline with what I'd heard before.

Thanks. One thing I didn't mention was that piston engines have one more factor that can influence there speed at/near the compressability range. The prop can be moved to a flat pitch and the rpms reduced to create an airbrake. This was actualy recomended for the P-38 in dives to avoid compressability.

I think you are incorrect on some aspects of the turbo though. For what it's worth, this is how I know turbo's/blowers:

1. air quality - density, humidity, temp etc.

The blower is superior to the turbo here.

Why? They both take in air through a fixed opening, compress it per their compression rates, cool it with a charge cooler. Air quality affects both equaly.

2. Engine speed gearing

They're usually geared up, but it's true.

The mechanical superchargers have a fixed ratio to the engine, some German fighters used a tourque converter arangement allowing a larger compressor and a wider range of operation.

Turbos run more to the work, heat the engine is turning out, in other words the boost is in proportion to the engines operating conditions, not just its operational speed.

A big point is the actual design, the turbo's were in their infancy, the blowers very advanced by comparison.

Actualy no, the first turbos were being experimented with in ~1905, serious turbo research in the USAC began in 1922 and effective turbos were in Air Core service in the early 30s in B-10s, and later in B17s. The US concentrated on turbos because of their effective range was from sea level to the compressor limits that could be over 30k ft. The Europeans concentrated on Mechanical superchargers which are simpler and don't need special alloys for the exaust side turbine used in Turbos.

If run at a constant speed, turbo's would be more effective, but I doubt you'd be able to avoid going up and down the rev range if you were a fighter pilot? - especially at low alts?

Again why? the Turbo supplies the boost in proportion to the engine needs, compressor speed is the direct result of Air quality and engine requirements controlled/ limited by the wastegate. AP-38J test shows that 60' boost could be maintained from SL to 23,800ft in the test I have:
http://www.spitfireperformance.com/p-38/p-38-67869.html
Whats the problem with that?

an increase from 8:1 to 10:1 compression will result in about 12% more horse power at the same time reducing fuel consumption by ~10% by increasing efficincy of the combustion process.

Maybe this will clarify this statement.
Boost or positive intake pressure is artificaly adding extra air to the cylinder changing the "Effective" or "Apparent", compression ratio. To simplify that think of this, A 10:1 mechanical (normaly asperated) compression ratio is when the piston is at the bottom of the stroke there is, as an example, 10 cu/in of air at ambient air pressure. When that same piston has reached the top of its stroke that 10cu/in has been compressed into 1cu/in = 10:1. Boost, adds air to that cylinder by outside pressure so when that same cylinder is at the botton of its stroke that 10cu/in space at 14.7" boost (+1 atmosphere at SL) now has the equivalent of ~20cu/in of air or twice as much as before. When the piston returns to the top it is still only has 1cu/in of space giving the cylinder an "Apparent" or "Effective" compression ratio of 20:1, boosting both power (at this level approximaly 50%) and Tourque which results in a smaller but measurable increase in efficency ie fuel economy through conservation of heat.

The last part is only true on a Naturally Aspirated engine. What forced induction does is mainly increase power and give 'softer' power. This is what Lindberg is meaning.

If you think about it, say 12:1 air/fuel ratio, you increase the airflow and you must increase fuelling.

Of course you could increase the Compression Ratio (CR) of the engine, lower the boost, whilst leaving the compression ratio (FCR) the same as it was. This would improve fuel consumption and acceleration, but destroy high-alt performance I'd guess.

Softer Power? What Lindbergs technics did was increase pitch of the prop increasing thrust for that rpm, this in turn added heat to the exaust increasing boost. A P-38 ran a 6.5:1 mechanical compression ratio the extra 5"-8" boost made that the equivalent of 9.5/10.5:1 increasing the efficency ie fuel economy.

Increasing mechanical ratio when you have a controllable pressure source available is kind of risky because as the mech ratio goes up so does detonation, boost in easier to control and still get max power where you need it.


[quote+swartzpanzer"]I've just had prop pitch explained to me and already I've forgotten, doh! I know it alters the way the plane 'swims' through the air though.


I suppose even at high alts, airspeed and airpressure are still 2 very important points?

Another problem I've had of late; trailing vortices - severely affects cooling on the ground, so it must be a major problem at high alts?[/quote]

Prop pitch simplified, is coarser produces more thrust and takes more power to operate. A finer pitch cruises better, and a flat pitch can be used to slow you down. To reduce drag with an engine out the Position is called feathered and is at right angles to the airflow (mostly props have twist).

Sure, Airspeed and airpressure are dependant. Also air quality varies one day the top speed of an aircraft will be 421 at 25,000ft, the next it could be 425 at 27,000ft because the density, temp, humidity (which affect airpressure) are different.

Vortices can be an issue anywhere but as a rule they increase with weight and speed. As to their effect on cooling - maybe Flyboy can help you more there. I do know that the placement of the radiator on the P-51 was in part to fill the area thad vortices from the wing/fusaage juncture would be eliminated to reduce drag.

Swartz, I've tried to simplify and be complete, your questions are cool and I hope I've helped.

wmaxt

 

[schwarzpanzer]

Hi wmaxt,

Swartz, I've tried to simplify and be complete, your questions are cool and I hope I've helped.

Thank you very much, helpfulness is always appreciated.

The prop can be moved to a flat pitch and the rpms reduced to create an airbrake. This was actualy recomended for the P-38 in dives to avoid compressability.

I don't know how, but I understand that. Thanks, very interesting.

- Did the P38's high wing loading cause problems? - From the looks of it, I mistakenly thought it had low wing-loading.


What I mean about the turbo being less efficient is thermal efficiency (the turbine heat transfers to the compressor), so air is less dense. This can be offset with better (hence bigger heavier) aftercooling, creating inefficiencies there.

The mechanical superchargers have a fixed ratio to the engine, some German fighters used a tourque converter arangement allowing a larger compressor and a wider range of operation.

I meant the 2-speed 'gearbox' ones, the viscous coupling - I've been looking for that for ages - thanks wmaxt!

Turbos run more to the work, heat the engine is turning out, in other words the boost is in proportion to the engines operating conditions, not just its operational speed.

True, but there are things that can alter this e.g. shutting the wastegate.


I know how old the turbo is, but the scroll, roller-bearing (or even 360 bearing??), VDT and ECU controlled and water-cooled etc design turbo's weren't properly used in WW2?

They were also very big and heavy.

- This would make the lag atrocious?

Again why? the Turbo supplies the boost in proportion to the engine needs, compressor speed is the direct result of Air quality and engine requirements controlled/ limited by the wastegate.

I was meaning turbo lag - though Lunatic explained that this is slightly less of a problem on planes?

Boost or positive intake pressure is artificaly adding extra air to the cylinder changing the "Effective" or "Apparent", compression ratio. To simplify that think of this, A 10:1 mechanical (normaly asperated) compression ratio is when the piston is at the bottom of the stroke there is, as an example, 10 cu/in of air at ambient air pressure. When that same piston has reached the top of its stroke that 10cu/in has been compressed into 1cu/in = 10:1. Boost, adds air to that cylinder by outside pressure so when that same cylinder is at the botton of its stroke that 10cu/in space at 14.7" boost (+1 atmosphere at SL) now has the equivalent of ~20cu/in of air or twice as much as before. When the piston returns to the top it is still only has 1cu/in of space giving the cylinder an "Apparent" or "Effective" compression ratio of 20:1, boosting both power (at this level approximaly 50%) and Tourque which results in a smaller but measurable increase in efficency ie fuel economy through conservation of heat.

Taking that example:

I don't know what air/fuel ratio piston planes need, but I'll guess circa 12.6:1?

So:

You're 10:1 mechanical CR engine is now producing twice the power, true. (~20cu/in vs 10 cu/in).

I forget the calculations and don't work in cu/in, but to make this easy, let's say 10 cu/in requires 1 lb of fuel, yes?

- So in order to go from 10 cu/in of intake air (requires 1lb of fuel) you now have a requirement of twice the weight of fuel in order to maintain the fuel/air ratio.

If you don't, the mixture wil be too weak.

- Add to this the extra heat generated by the increased boost and loading and you will need to add even more fuel.

If the fuel flow remains at the original rate, the engine will detonate, unless say Hydro/Methanol (Water Inj) is used constantly.

Softer Power?

As the piston moves to the bottom of the stroke, the boost pushes it down - giving a helping hand.

Boost also cushions the piston as it moves to the top of the stroke, but this is of little advantage below 6,000 rpm (low speed to me).

Although all these engines seem to be under-square and may be have run to 6,000rpm in some instances? so it might be an advantage after all?

Can anyone tell what are the average piston speeds on these engines?

A P-38 ran a 6.5:1 mechanical compression ratio the extra 5"-8" boost made that the equivalent of 9.5/10.5:1 increasing the efficency ie fuel economy.

10:1 CR is a good common average for me so great! 6.5:1 is impossibly sluggish, as is any less than 8:1.

@ alt a 10:1 mechanical CR would be more like, I dunno, 5:1?

Whereas a boosted motor can start @ 8:1 @ 4" boost, then switch to 8" @ high alt?

That way the the performance stays more-or-less exactly the same? (Same FCR of 10:1 on all examples - not accurate, just a random figure ).

Increasing mechanical ratio when you have a controllable pressure source available is kind of risky because as the mech ratio goes up so does detonation, boost in easier to control and still get max power where you need it.

Would more boost at high alt cause any of these problems?

Compressing air that's twice as thin twice would make it the same?

- Sorry, don't know how to speak properly on air quality - half as thick?


Thanks for the pitch explanation.


I meant airspeed in the supercharger(s) and ports, thanks anyway.

I suppose headwinds and high-pressure etc would be a knightmare, also the cunning tactic of slipstreaming would not be so cunning in an aeroplane, so I hear? - unless you can fly without lift?

Vortices can be an issue anywhere but as a rule they increase with weight and speed.

True, but it's usually bad design that starts it, looking at it I suppose the early Mustangs (Razorback) would suffer most, bubble canopies and Me109/Spitfire shapes shouldn't.

I dunno if weight would affect them?

I do know that the placement of the radiator on the P-51 was in part to fill the area thad vortices from the wing/fusaage juncture would be eliminated to reduce drag.

That makes sense, but wouldn't it reduce lift slightly?

Also the heat from the rad created forward thrust IIRC?


I wonder that the placement of the P38's rads would be good or this, but would take in the warm air of the engine's and turbo's?


Also is torque (lb/ft) of any importance to a prop-plane?

Thanks again,

Schwarz

 

[wmaxt]

Hi wmaxt,

Thank you very much, helpfulness is always appreciated.

Your welcome!

- Did the P38's high wing loading cause problems? - From the looks of it, I mistakenly thought it had low wing-loading.

Yes and no, the large span is decieving but because of the high taper ratio the wing also had a high aspect ratio increasing it efficency a lot. It was also a high lift design which was also a win/lose situation. Only a couple of fighters could climb with a P-38 the F4U-4, Spitfire, Ki-84 and the Bf-109K4 (if any actualy made it with c3 fuel), on the other hand it had the lowest entry speed into compressability of all the major fighters.

What I mean about the turbo being less efficient is thermal efficiency (the turbine heat transfers to the compressor), so air is less dense. This can be offset with better (hence bigger heavier) aftercooling, creating inefficiencies there.

The heat transfer from impellar to compressor is miniscule, the two are seperated with free air flowing between and the shaft is cooled by oil and occasionaly liquid. The heat in the air is from compression not the exaust.

Turbos run more to the work, heat the engine is turning out, in other words the boost is in proportion to the engines operating conditions, not just its operational speed.

True, but there are things that can alter this e.g. shutting the wastegate.

True, but why? The extra manifold pressure allows the engine to produce more power at a lower setting increasing efficency. By the way a wastegate closes to direct all the flow through the turbo and opens to relive that pressure allowing the turbo to slow down and produce less boost.

I know how old the turbo is, but the scroll, roller-bearing (or even 360 bearing??), VDT and ECU controlled and water-cooled etc design turbo's weren't properly used in WW2?

They were also very big and heavy.

- This would make the lag atrocious?

The scroll, and roller bearings certainly were and some were water cooled, then again WWII turbos on aircraft didn't really require VDT and ECUs. Turbos in aircraft applications are a lot more straight foward that hi perf autos.

They were big and heavy, they were attached to 1,710cu/in and 2800cu/in engines. 2800 valves are almost as big as the pistons in a Honda engine its the scale that makes them so big.
Lag is not really an issue in an aircraft the throttle isn't moved as often as auto applications.

Boost or positive intake pressure is artificaly adding extra air to the cylinder changing the "Effective" or "Apparent", compression ratio. To simplify that think of this, A 10:1 mechanical (normaly asperated) compression ratio is when the piston is at the bottom of the stroke there is, as an example, 10 cu/in of air at ambient air pressure. When that same piston has reached the top of its stroke that 10cu/in has been compressed into 1cu/in = 10:1. Boost, adds air to that cylinder by outside pressure so when that same cylinder is at the botton of its stroke that 10cu/in space at 29.92" boost (+1 atmosphere at SL) now has the equivalent of ~20cu/in of air or twice as much as before. When the piston returns to the top it is still only has 1cu/in of space giving the cylinder an "Apparent" or "Effective" compression ratio of 20:1, boosting both power (at this level approximaly 50%) and Tourque which results in a smaller but measurable increase in efficency ie fuel economy through conservation of heat.

Taking that example:

I don't know what air/fuel ratio piston planes need, but I'll guess circa 12.6:1?

So:

You're 10:1 mechanical CR engine is now producing twice the power, true. (~20cu/in vs 10 cu/in).

I forget the calculations and don't work in cu/in, but to make this easy, let's say 10 cu/in requires 1 lb of fuel, yes?

- So in order to go from 10 cu/in of intake air (requires 1lb of fuel) you now have a requirement of twice the weight of fuel in order to maintain the fuel/air ratio.

If you don't, the mixture wil be too weak.

- Add to this the extra heat generated by the increased boost and loading and you will need to add even more fuel.

If the fuel flow remains at the original rate, the engine will detonate, unless say Hydro/Methanol (Water Inj) is used constantly.

Basicaly your on the right track, and at some point you will need more fuel or a charge cooling device like water. The 2800 as an aircooled engine did use extra fuel in this way, after Lindberg the P-38L could go 300mi more than the P-47N with 256gal less and that is exactly why.

However before the point of detenation the added boost increases the "effective" compression ratio of the engine. This allows the engine to make a little more power in proportion to the fuel/air heat generated by concentrating that heat. Its the same as increasing the mechanical compression ratio to increase the concentration of the heat expansion driving the piston down twords the crank.

Softer Power?

As the piston moves to the bottom of the stroke, the boost pushes it down - giving a helping hand.

Boost also cushions the piston as it moves to the top of the stroke, but this is of little advantage below 6,000 rpm (low speed to me).

Although all these engines seem to be under-square and may be have run to 6,000rpm in some instances? so it might be an advantage after all?

Can anyone tell what are the average piston speeds on these engines?

A P-38 ran a 6.5:1 mechanical compression ratio the extra 5"-8" boost made that the equivalent of 9.5/10.5:1 increasing the efficency ie fuel economy.

10:1 CR is a good common average for me so great! 6.5:1 is impossibly sluggish, as is any less than 8:1.

@ alt a 10:1 mechanical CR would be more like, I dunno, 5:1?

Whereas a boosted motor can start @ 8:1 @ 4" boost, then switch to 8" @ high alt?

That way the the performance stays more-or-less exactly the same? (Same FCR of 10:1 on all examples - not accurate, just a random figure ).

This is not an auto application the numbers for an Allison
Bore - 5.5" Stroke - 6.0"
Compression - 6:1
Max Piston speed - 3,200ft/min
Max rpm - 3,000-3,200

Also a the limiting speed on the props was determined by the tip speed going supersonic, for a 12" prop thats about 1,600rpm.

As long as the rpm is fast enough to to not overload the crank bearings the boost is fine. Compression ratio is a relative thing boost increases it in proportin to the ammount that 6:1 at 60" is the equivalint of 18:1 (1 atmosphere normal + 2 boosted).

Boost does help push the piston down but it has added more content to compress so its harder for the piston to get to the top of the stroke.

As for sluggish, remember an aircraft is under boost all the time and the engine acceleration can't be too high because of the prop. A prop also acts like a flywheel in both weight and initeria, A 2800 has been known to spin the prop shaft right off if the throttle is slamed on to fast!

Increasing mechanical ratio when you have a controllable pressure source available is kind of risky because as the mech ratio goes up so does detonation, boost in easier to control and still get max power where you need it.

Would more boost at high alt cause any of these problems?

Compressing air that's twice as thin twice would make it the same?

- Sorry, don't know how to speak properly on air quality - half as thick?

Altitude really doesn't change anything regarding detonation but will eventualy the reduced air pressure/density will be to much and boost will start dropping. High altitude also reduces cooling capacity because there is less air to transfer heat to.

Thats the beauty of a turbo up to its max speed it will go faster to gather enough to produce boost.

I suppose headwinds and high-pressure etc would be a knightmare, also the cunning tactic of slipstreaming would not be so cunning in an aeroplane, so I hear? - unless you can fly without lift?

Not really a big deal but the turbulance can make it harder to control. Most aces liked to get to 100yards before firing!

Vortices can be an issue anywhere but as a rule they increase with weight and speed.

True, but it's usually bad design that starts it, looking at it I suppose the early Mustangs (Razorback) would suffer most, bubble canopies and Me109/Spitfire shapes shouldn't.

I dunno if weight would affect them?

I do know that the placement of the radiator on the P-51 was in part to fill the area thad vortices from the wing/fusaage juncture would be eliminated to reduce drag.

That makes sense, but wouldn't it reduce lift slightly?

Also the heat from the rad created forward thrust IIRC?

I wonder that the placement of the P38's rads would be good or this, but would take in the warm air of the engine's and turbo's?

Anything in the air that is faster than that air creates a vortice, which is a nice way of saying disturbance. Weight as exibited by the lift required to carry it will intensify and vorticeies. Razor backs are less inclined to create drag - the P-51 lost 3-5 mph with the bubble canopy.

I don't think the rad placement on the P-51 affected lift.

Thrust? The thrust is created by the addition of heat expanding the air, that expansion difference is the thrust. The P-51 had a largeplenum (open space) that slowed the air and allowed the maximum transfer of heat. A little some sources say 100lbs some up to 600hp. When I look at the numbers I think the 100hp number is bretty close. The real help was the best cooling system with the least drag of just about any WWII fighter.

The P-38 had the same thrust producing sys the P-51 cooling system had but less plenum inside ment less thrust out the back but the P-38 almost never overheated so it was fine.

Also is torque (lb/ft) of any importance to a prop-plane?

Thats what spins the prop! Tourque is the actual power output mesurement - hp is the time in which it does it.

wmaxt