Engine technology free-for-all

[J.A.W.]

Throttle 'pressure drop' considerations may be contingent on carburetion/fuel-mix bleed/venturi concerns..
For fighter engines requiring rapid response to throttle inputs, large/slow turbo spool-up 'lag' was an issue..

[trinidefender]

But velocity will be lower because volume flow is lower because specific volume is lower because temperature is lower.
P&W knew there would be "some" thrust. If it was only half I wouldn't categorise it as "competitive" (the term used by trini)

why would the temperature be lower ?
the mean exhaust pressure above the turbine is far higher than the the mean exhaust pressure in the MS engine

and why should velocity be the dominant factor ?
for a given energy the thrust will be greater with lower velocity and higher pressure ( via momentum/Froude ? efficiency)

unrelated .....
the Corliss throttle (re Merlin era) was chosen via needing less force from the operator and 'had a smaller pressure drop'
I have in the past asked 'experts' whether or not the necessary property of a throttle is to drop pressure

Here is a graph comparing pressure and temperature shown along a turboshaft engine. Look what happens to the exhaust gasses as they expand through the turbines. I have a better diagram and explanation in another book however I leant it out to somebody so I'll have to get it back before I can take a picture of it.

As far as I have always been aware the exhaust drops in temperature and pressure going through the turbines. Hence where the energy to drive the shaft comes from.

[J.A.W.]

Turbo control & Corsair supercharger schematics, note the bulky intercooling requirements of air cooling..

http://www.enginehistory.org/Installati ... ontrol.jpg
http://www.enginehistory.org/Installations/F4U.gif

& it ought to be self evident that exhaust gases captured & directed through long ducts to perform 'tread-mill'
compressor duties will then have less energy to impart - as jet thrust to an airstream - than ejector types..

[wuzak]

While it is true that an engine with turbonormalising (how turbos were used during WW2 - maintaining sea level pressure to the engine proper) will have higher cylinder pressures at altitudes above a mechanically supercharged engines it is also true that the turbo requires more of the exhaust energy to power its compressor.

After the critical altitude of the turbocharger the power tends to fall off rapidly, though.

[trinidefender]

Throttle 'pressure drop' considerations may be contingent on carburetion/fuel-mix bleed/venturi concerns..
For fighter engines requiring rapid response to throttle inputs, large/slow turbo spool-up 'lag' was an issue..

This isn't actually totally true as many many fighter pilots, especially of planes like the spitfire with its automatic boost control, put the plane to full power and then left it there for most of the manoeuvres in dogfighting. Generally they were only throttled back if dive speeds got to high or engine over heating or doing certain manoeuvres.

Turbo lag wasn't nearly as critical as it is in automotive applications.

[J.A.W.]

A couple of points..
A, Spitfire pilots were certainly advised to run at higher power settings - when flying in combat zones..
..where fast enemy fighters were likely to be encountered, even if at the cost of higher fuel use..

B, The turbo'd R-2800 P-47 could pump through 275 gal/hr when 'spooled up'..
& running at anywhere near those settings was an operational problem, esp' on long range escort missions..
.. the P-47 ended up as a virtual flying gas-tank, with 'wet wings'..
& the job of juggling myriad engine management systems in combat was another burden on the pilot..
..impairing combat flying effectiveness..
..& such fiddle-faddle fettling really was the proper province of a dedicated flight engineer.. ..or automation..

Interestingly, the German radar/interception services could pick out the aircraft types by cruising characteristics
& determine countermeasure responses accordingly..
Compare the max continuous cruise speed capabilities of P-47 & P-51 as shown below..

http://www.wwiiaircraftperformance.org/ ... asheet.jpg
http://www.wwiiaircraftperformance.org/ ... IV-ads.jpg

[gruntguru]

But velocity will be lower because volume flow is lower because specific volume is lower because temperature is lower.
P&W knew there would be "some" thrust. If it was only half I wouldn't categorise it as "competitive" (the term used by trini)

why would the temperature be lower ?
the mean exhaust pressure above the turbine is far higher than the the mean exhaust pressure in the MS engine

and why should velocity be the dominant factor ?
for a given energy the thrust will be greater with lower velocity and higher pressure ( via momentum/Froude ? efficiency)

unrelated .....
the Corliss throttle (re Merlin era) was chosen via needing less force from the operator and 'had a smaller pressure drop'
I have in the past asked 'experts' whether or not the necessary property of a throttle is to drop pressure

Temperature is lower because energy is removed by the turbine - that's thermodynamics. The mean exhaust TEMP above the turbine is NOT far higher than the the mean exhaust TEMP in the MS engine. It may be a little higher but after expansion in the turbine the final temperature is much lower. For temperatures in a high PR turbo-compund cycle see page 11 of http://ntrs.nasa.gov/archive/nasa/casi. ... 001160.pdf

The energy is not "given". The energy in the turbocharged case is lower than the MS case.

The purpose of the throttle valve is to reduce density of the charge. The volumetric flow into an engine is constant at a given rpm. Changing the charge density changes the mass flow.

[Tommy Cookers]

IMO the turbo potentially has competitive exhaust thrust ......
because the mean exhaust pressure downstream of the turbine is not lower than in the mechanically supercharged engine
because the mean exhaust pressure above the turbine will be higher or much higher than in the MS engine

and because P&W thought so, they did the VDT

in support of the above, consider that with (extreme) altitude the turbo exit nozzle was restricted by manual adjustment
to regulate turbo rpm and, critically, to prevent turbo overspeeding

[Tommy Cookers]

.... For fighter engines requiring rapid response to throttle inputs, large/slow turbo spool-up 'lag' was an issue..

This isn't actually totally true as many many fighter pilots, especially of planes like the spitfire with its automatic boost control, put the plane to full power and then left it there for most of the manoeuvres in dogfighting.

........Generally they were only throttled back if dive speeds got to high or engine over heating or doing certain manoeuvres.

A former co-worker (who flew P-47s in WW2) told me that there was a placard saying .....
'if control difficulties occur at high ias remove hands and feet from the controls and open throttle fully'
this was for compressibility issues making dives uncontainable - to prevent over-stressing or runaway rpm from prop pitch problems
he tried this situation and so by accident entered his airfield circuit at about 500 mph

low or negative g can affect pitch control, very bad in 2 blade props (eg if the blades can go to different pitches)
also quick movements of (the pilot) pitch control can be bad, they can produce instant engine detonation even in light aircraft
in flight throttling up or down does not change the engines rpm, the pitch control keeps rpm constant
ime the throttle is mostly left alone when away from the airfield

[trinidefender]

.... For fighter engines requiring rapid response to throttle inputs, large/slow turbo spool-up 'lag' was an issue..

This isn't actually totally true as many many fighter pilots, especially of planes like the spitfire with its automatic boost control, put the plane to full power and then left it there for most of the manoeuvres in dogfighting.

........Generally they were only throttled back if dive speeds got to high or engine over heating or doing certain manoeuvres.

A former co-worker (who flew P-47s in WW2) told me that there was a placard saying .....
'if control difficulties occur at high ias remove hands and feet from the controls and open throttle fully'
this was for compressibility issues making dives uncontainable - to prevent over-stressing or runaway rpm from prop pitch problems
he tried this situation and so by accident entered his airfield circuit at about 500 mph

low or negative g can affect pitch control, very bad in 2 blade props (eg if the blades can go to different pitches)
also quick movements of (the pilot) pitch control can be bad, they can produce instant engine detonation even in light aircraft
in flight throttling up or down does not change the engines rpm, the pitch control keeps rpm constant
ime the throttle is mostly left alone when away from the airfield

That was an aircraft specific problem though. Most of those fighters were designed so that adding a bunch of throttle added in some pitch up moment. Therefore it makes me think that the placard was there so when the pilot throttles up it slowly pulls the plane out of the dive. Not much else you can do if you are going fast enough to hit compressibility problems.

All the same very nice information. Does your friend have any other stories that may be able to help us on how the turbocharged PW-2800 performed such as lag issues or anything?

A slight correction though. Throttle and prop control really depends on the conditions. For takeoff and climb out, the prop would be set at a fine (low AoA/high rpm) setting allowing the engine to go to full rpm and the throttle setting usually at max continuous power. For cruise the prop lever is advanced to a coarse (higher AoA/lower rpm) setting. Throttle was then set for desired cruise speed. This reduced fuel consumption. When entering a combat area, to my knowledge, the prop was then put back to a fine pitch and set to allow combat power rpm. The throttle is then advanced to combat power or 5 minute combat limit power if needed.

As far as I'm aware, all the turbocharged WWII aero engines controlled their turbo rpm through the use of a conventional wastegate. Is your suggestion that turbo rpm could be governed by the use of a variable exhaust outlet? If so wouldn't this cause quite large back pressure issues?

[wuzak]

As far as I'm aware, all the turbocharged WWII aero engines controlled their turbo rpm through the use of a conventional wastegate. Is your suggestion that turbo rpm could be governed by the use of a variable exhaust outlet? If so wouldn't this cause quite large back pressure issues?

I think he is speaking of Pratt & Whitney's VDT engines. The turbo speed was controlled by a nozzle, which controls the mass air flow through the system.

It wokred on teh test benchm but in flight needed constant adjustment by the flight engineer, because there was no control system at the time tot could deal wth it.

[trinidefender]

As far as I'm aware, all the turbocharged WWII aero engines controlled their turbo rpm through the use of a conventional wastegate. Is your suggestion that turbo rpm could be governed by the use of a variable exhaust outlet? If so wouldn't this cause quite large back pressure issues?

I think he is speaking of Pratt & Whitney's VDT engines. The turbo speed was controlled by a nozzle, which controls the mass air flow through the system.

It wokred on teh test benchm but in flight needed constant adjustment by the flight engineer, because there was no control system at the time tot could deal wth it.

I was more referring to in production aero engines. I should do more reading about those VDT engines. Don't know to much about them.

Question then. Did the nozzles on the VDT engine cause back pressure problems at all? Also with the associated extra back pressure what was the power loss to the crankshaft? What about things like valve timing and exhaust scavenging and the like? There would be a massive difference in exhaust back pressure between sea level when the nozzle would be fully closed to stop the turbo spinning to fast and high up when the nozzle opens up to reduce back pressure to allow the turbo to run higher pressure ratios to maintain manifold air pressure equivalent to sea level. Can you give me more information?

[Tommy Cookers]

@ both
pilot training films show that P-47 turbo rpm had to be limited by pilot action in response to a graduated flashing light
this at high&rising altitude, compressor load was falling but exhaust pressure:atmospheric pressure differential increasing

the pilot mentioned is now deceased from natural causes, anyway the war was a sensitive topic
he survived flying a Spit 14 with reversed aileron cables (then fittings were made foolproof)
Spit 14s were limited to 3 lb boost takeoff as torque overloaded 1 wheel and tyres burst (so he tried a 0 boost takeoff - 'once !')
Sq Ldr Martindale had in 1943 a prop rpm runaway at around Mach .92, this maybe was the cause of the placarding of RAF P-47s

OT topic - many planes eg Merlin Mustang, most Spits had more prop pitch at low flight speeds than useful for thrust
simply to absorb the power and contain the rpm when the pilot opened up (these airframes were intended for about 1100hp)
this was a large part of the infamous torque problem in this situation
airframes intended for around 2000 hp eg Corsair and P-47 had bigger props
there was some NACA report on this aspect of performance

fine pitch is by even today's definition for takeoff and landing use, so whatever the combat pitch was called it was not called fine
downwind checks are iirc ..... B-U-M-P-F-F ..... where P means means pitch selection 'fine'
fighters preferred to avoid low rpm cruise, being scared of plug fouling

I had quite a lot of Pitts etc time and a little Harvard time, you really do leave the throttle alone most of the time
there's a manifold pressure gauge, this is useful for setting the (uncalibrated) pitch control
but the prop has much more braking (when throttled back) effect than a fixed pitch prop
this is useful in many aircraft (the Boeng 247 designed around this) but an underpublicised danger in light, draggy aircraft
throttle movement from closed to rather open and back when landing can cause engine stall from overpitch (I did it in a Decathlon)

at Kissimee they had 2 TF-51s available (with a minder) to anyone, presumably they still have them

[trinidefender]

@ both
pilot training films show that P-47 turbo rpm had to be limited by pilot action in response to a graduated flashing light
this at high&rising altitude, compressor load was falling but exhaust pressure:atmospheric pressure differential increasing
100000rpm+ ?

the pilot mentioned is now deceased from natural causes, and the war was a sensitive topic
he survived flying a Spit 14 with reversed aileron cables (then fittings were made foolproof)
Spit 14s were limited to 3 lb boost takeoff as torque overloaded 1 wheel and tyres burst (so he tried a 0 boost takeoff - 'once !')

OT topic - many planes eg Merlin Mustang, most Spits had more prop pitch at low flight speeds than useful for thrust
simply to absorb the power and contain the rpm when the pilot opened up (these airframes were intended for about 1100hp)
this was a large part of the infamous torque problem in this situation
airframes intended for around 2000 hp eg Corsair and P-47 had bigger props

fine pitch is by even today's definition for takeoff and landing use, so whatever the combat pitch was called it was not called fine
downwind checks are iirc ..... B-U-M-P-F-F ..... where P means means pitch selection 'fine'
fighters preferred to avoid low rpm cruise, being scared of plug fouling

I had quite a lot of Pitts etc time and a little Harvard time, you really do leave the throttle alone most of the time
there's a manifold pressure gauge, this is useful for setting the (uncalibrated) pitch control
but the prop has much more braking (when throttled back) effect than a fixed pitch prop
this is useful in many aircraft (the Boeng 247 designed around this) but an underpublicised danger in light, draggy aircraft
throttle movement from closed to rather open and back when landing can cause engine stall from overpitch (I did it in a Decathlon)

at Kissimee they had 2 TF-51s available (with a minder) to anyone, presumably they still have them

Well the higher you got the faster the turbocharger would have to spin to maintain boost pressure and of course it isn't tied to the engine so nothing slowing it down. Especially with the greater pressure differential at the turbine from the low pressure outside at high altitude. As far as I am aware, turbo over speed was a constant concern for many pilots running at high power settings at high altitude.

From what I am aware the fighters didn't exactly utilise what you or I would consider low RPM cruise but it would still be somewhat lower than full power. Think for the merlins it was 2650rpm for high speed cruise power settings and 3000 rpm was used for takeoff/climb and combat power settings.

Considering climb and combat used the same rpm and the props were CS (constant speed propellers change propeller blade pitch automatically to maintain a particular rpm for any confused readers) units, it does make sense that the same setting would be used for both. If not then they would be close together on the propeller lever.

Well the Spitfire MK 14 was a griffon spit. Those things put out something like 2200 hp at sea level on a frame that originally had a little over 1000 hp. Surprising how well and how developed the spitfire got considering how far back it's roots went. The Spitfire MK 22 and MK 24 had stronger main wheels and tyres to stop the bursting issue. Another factor is at the griffon engine rotated in the opposite direction as the merlin so it caught out many experienced merlin spit pilots when they were operating on muscle memory and put in the wrong rudder direction. Have heard stories that on some takeoffs full rudder wasn't even enough to keep the airframe straight on takeoff.

Think I may have flew into Kissimmee a few years ago.

[J.A.W.]

If of interest, the 'Pilots Notes' - handbooks for operating these machines - are readily available on line..

The P-47 document contains turbo control specifics which include warnings about certain characteristics,
such as surging & overheating on take-off & turbo stall 'lag' at altitude.. ..this machine sure kept its pilot busy..

& predictably negative unintended consequence, the classic Spitfire prop-rotation reversal from Merlin to Griffon..

The British decided to standardise rotation, hence Centaurus, Griffon & Sabre all turned the same way..
However, the Merlin predated this standard & turned opposite-wise, ( & many tens of thousands more were flown),
so the newer, but fewer built mills were the odd ones out, with numerous needless Spitfire take-off dramas..
The Spit 14 P-Ns advises against being too timid with the application of boost for take-off - due to the risk
of high TEL level fuel fouling the plugs..

Bigger aircraft such the Sabre powered Tempest, even spinning the largest prop (14ft) of any WW2 single engined fighter..
..could be given what ever take-off boost level the pilot chose.. ..Sabre 7 was rated at 3,500hp for take-off @+20lbs/ADI..

Curiously both Merlin & Allison V12s could be operated with alternative prop rotation, for use in 'handed'
twin-engine powered types, the Allison was designed to offer alternative reverse recip' core rotation,
& R-R did a reduction gear mod to achieve the effect..