Turbosupercharger Use: 7000ft? 12000 ft? 3500 ft?

[Istari6]

I'm learning the P-47 by reading through the User Manual and Chuck's Guide.  I've also been doing a lot of external reading, then trying out the DCS Thunderbolt.

One area of confusion is proper use of the turbosupercharger.  The guides talk in different places about not needing the turbo until 7000 ft at minimum, or even 12,000 ft.  Some guides suggest interlocking the Turbocharger for takeoff, others say leave Boost off and only take off with throttle alone.

Yet when I take off using MP only and Boost off, I get 45" MP maximum, which drops pretty rapidly as I climb.  By 3500 ft, I'm dropping below the 42.5" for Climb, even with full throttle.  Feels like I need to start bringing in the Turbo much earlier than guides are suggesting.

My testing is on the Normandy 2.0 map from Chailey AFB, outside temperature 10C, which I'd expect to actually improve engine performance over typical ISA of 15C.

What am I doing wrong?

[razo+r]

It depends on many factors how much MP you'll get at a certain throttle and boost setting in the P-47. Factors like airspeed, altitude, temperature, pressure, RPM etc. all play a role here. On good, cold days you will have to move the boost lever later than on hot days, as the ambient density will be higher. It's kind of pointless to say at which altitude you will have to use the turbo without knowing all these variables... 

But don't worry about these numbers in the guides. Use the turbo at any time to reach the desired MP value.

About interlocking, you can do that, however you will lose some performance. Interlocking can be a good idea if you're still new to it and learning the quirks, but you should eventually stick to moving the levers induvidually.

As for using turbo, when powering up, adjust RPM first, then throttle, and once throttle is full open, use the additional boost lever, and then eventually the mixture lever if needed. For reducing power, it's almost the other way, first boost lever, if that one is all the way back use throttle and then adjust RPM.

[grafspee]

Those guides does not provide full picture, they pull some quote from manuał w/o background.

P-47's engine is equipped with single stage single speed supercharger and on top of that you have turbocharger.

45" is max MP achievable w/o turbocharger at SL, if you want more MP you have to use turbocharger (Boost lever).

If you take off w/o turbocharger at 45" then during climb MP will drop because ambient pressure drops with alt.

Full picture is that P-47 unlike P-51 for training purposes was operated with 91 octane fuel which lowers max allowable MP, for take off 40" and for climb 35"  with those limits use of turbocharger below 7k ft wasn't necessary. 

Edited May 6, 2023 by grafspee

[MD44]

After watching this vid, it really helped me understand the workings of the throttle and boost.  The other vids in the series here are also very useful.

 

[Istari6]

8 hours ago, razo+r said:

It depends on many factors how much MP you'll get at a certain throttle and boost setting in the P-47. Factors like airspeed, altitude, temperature, pressure, RPM etc. all play a role here. On good, cold days you will have to move the boost lever later than on hot days, as the ambient density will be higher. It's kind of pointless to say at which altitude you will have to use the turbo without knowing all these variables... 

I'm familiar with the principles of air density changing with temperature, and thus improving or decreasing internal combustion engine performance.  That's why I was surprised that on a day that's colder than ISA (10C rather than 15C), I seemed to need the turbosupercharger well below 7000 ft.  The other factors are close to standard - Chailey AFB is at 95' above SL, so not a factor.  RPM was 2550 and MP was set to 42.5" for initial climb, but I found myself going to full throttle just to hold that 42.5" and falling behind at 3500 ft.  

6 hours ago, grafspee said:

Those guides does not provide full picture, they pull some quote from manuał w/o background.

P-47's engine is equipped with single stage single speed supercharger and on top of that you have turbocharger.

45" is max MP achievable w/o turbocharger at SL, if you want more MP you have to use turbocharger (Boost lever).

If you take off w/o turbocharger at 45" then during climb MP will drop because ambient pressure drops with alt.

Full picture is that P-47 unlike P-51 for training purposes was operated with 91 octane fuel which lowers max allowable MP, for take off 40" and for climb 35"  with those limits use of turbocharger below 7k ft wasn't necessary. 

Aha!  This seems to answer it.  If I'm following, the recommendations about turbo not being required before 7000 ft (in typical conditions) is because you could maintain at least 35" to that altitude before you topped out the throttle.  I'll try that and see if DCS behaves similarly.  45" at SL at max throttle without turbo is exactly what I'm getting in DCS.  

I'm assuming the single stage supercharger harnessed to the Double Wasp is operating even at SL?  There's no power surge during the climb as it kicks in?

Edited May 7, 2023 by Istari6

[Art-J]

3 hours ago, Istari6 said:

I'm assuming the single stage supercharger harnessed to the Double Wasp is operating even at SL?  There's no power surge during the climb as it kicks in?

You wouldn't get above ambient ~30" at SL if it wasn't . Yes, that is correct assumption.

Annoying and uncommanded power changes are a thing though when operating with levers interconnected. This is where lack of MAP regulator in DCS Jug makes life a little harder when turbo can't make up its mind if it wants to boost already or not. You're gonna spend some time chasing the gauge needle with the levers.

[Istari6]

56 minutes ago, Art-J said:

You wouldn't get above ambient ~30" at SL if it wasn't . Yes, that is correct assumption.

Annoying and uncommanded power changes are a thing though when operating with levers interconnected. This is where lack of MAP regulator in DCS Jug makes life a little harder when turbo can't make up its mind if it wants to boost already or not. You're gonna spend some time chasing the gauge needle with the levers.

Right, that makes sense!  Should have realized that myself.  Good point.  Yes, I've noticed the "turbo lag" and I'm getting a better feel for how to compensate.  Fortunately, the Double Wasp seems pretty resilient to momentary surges without breaking.  

[grafspee]

4 hours ago, Istari6 said:

I'm assuming the single stage supercharger harnessed to the Double Wasp is operating even at SL?  There's no power surge during the climb as it kicks in?

 

Single stage single speed supercharger is permanently connected to engine crankshaft and there is no power surge or kick in behavior because those are related to dual speed superchargers like in P-51 or Spitfire where at certain alt high speed kicks in.

[Istari6]

11 hours ago, grafspee said:

45" is max MP achievable w/o turbocharger at SL, if you want more MP you have to use turbocharger (Boost lever).

If you take off w/o turbocharger at 45" then during climb MP will drop because ambient pressure drops with alt.

Full picture is that P-47 unlike P-51 for training purposes was operated with 91 octane fuel which lowers max allowable MP, for take off 40" and for climb 35"  with those limits use of turbocharger below 7k ft wasn't necessary. 

Ran the test, and happened exactly as you predicted.  Kept climbing without turbo, and crossed 40" MP at 4500 ft, 35" MP at 7800 ft.  This is with a slightly colder day than ISA (10C), so makes sense I could get higher than 7000 ft before needing turbo, given that 35" was the old recommended MP for climbing under 91 octane.  

1 minute ago, grafspee said:

Single stage single speed supercharger is permanently connected to engine crankshaft and there is no power surge or kick in behavior because those are related to dual speed superchargers like in P-51 or Spitfire where at certain alt high speed kicks in.

Thanks!

Edited May 7, 2023 by Istari6

[grafspee]

18 minutes ago, Istari6 said:

Ran the test, and happened exactly as you predicted.  Kept climbing without turbo, and crossed 40" MP at 4500 ft, 35" MP at 7800 ft.  This is with a slightly colder day than ISA (10C), so makes sense I could get higher than 7000 ft before needing turbo, given that 35" was the old recommended MP for climbing under 91 octane.  

Thanks!

 

Other factor is RAM air, this thing adds boost depends on airspeed, faster you flying more additional boost you get.

So in manuals those values are given for static or with out RAM air.

Thing that you could get 35" at 7800ft is affected by ram air, if you level out and let plane accelerate you will notice that MP will go up.

When i said that w/o turbo you can get 45" MP it only match for static or w/o Ram air, if you close boost lever and enter dive you will get even +50" w/o turbo.

So critical alt for supercharger is affected by speed.

Edited May 7, 2023 by grafspee

[Nealius]

Could be poor practice but I don't bother with thinking about what altitude to use or not use boost. If I'm not getting the MP number I want, I add bost. If I'm getting too much I back off the boost. Though there are times I need to add boost then reduce throttle to get the MP number I want which feels a bit finicky and weird.

[Yo-Yo]

I think, there is a  simple rule of thumb: add turbocharger as a last mean when rpm are set, throttle at max and you have lack of MP.
And as you reduce power (MP), turbo must be retarded first. It is very unwise to let turbo overcompress the air and then  throttle it down.

[grafspee]

@Nealius Just like @Yo-Yo siad.

[Nealius]

@Yo-Yo @grafspee While climbing at 42" 2550rpm, throttle max, the boost lever is too sensitive to make fine adjustments of MP. For example, if my MP drops to 39" I move my boost lever 1mm and the MP goes all the way to 45" or more. If I reduce boost lever 1mm I go back to 39." Therefore I must add boost to get 45" or more then reduce throttle to bring it down to 42". If I should not be reducing throttle in that situation, then how do I maintain a specific MP when the boost lever won't allow adjustments in 1" increments?

[grafspee]

I use left throttle lever in warthog hotas to control turbo and i am able to control boost via turbo with neurosurgeon precision.

[Nealius]

44 minutes ago, grafspee said:

I use left throttle lever in warthog hotas to control turbo and i am able to control boost via turbo with neurosurgeon precision.

As do I but I do not have neurosurgeon precision with it. 0 deadzone, 0 curves, 100% saturation on both axes. Did you do any axis tuning?

I should mention this lack of precision is always when climbing around 4,000-10,000ft. As I get higher, particularly up in the 20,000+ range my boost lever precision seems normal.

Edited May 23, 2023 by Nealius

[Yo-Yo]

2 hours ago, Nealius said:

As do I but I do not have neurosurgeon precision with it. 0 deadzone, 0 curves, 100% saturation on both axes. Did you do any axis tuning?

I should mention this lack of precision is always when climbing around 4,000-10,000ft. As I get higher, particularly up in the 20,000+ range my boost lever precision seems normal.

 

It's not criminal to use throttle for fine adjustment.

[peachmonkey]

4 hours ago, Nealius said:

As do I but I do not have neurosurgeon precision with it. 0 deadzone, 0 curves, 100% saturation on both axes. Did you do any axis tuning?

I should mention this lack of precision is always when climbing around 4,000-10,000ft. As I get higher, particularly up in the 20,000+ range my boost lever precision seems normal.

 

see if you can re-calibrate the boost lever axis ?

Visually, looking at the boost gauge, when in 4k-10k range the boost gate (?) isn't very effective/efficient, i.e. the boost lever mus be pushed way too forward for it to start affecting the MP. And higher you go the more you gotta back off the lever to avoid overboosting. Maybe the boost lever in your HOTAS is missing some finer calibration at the forward position?

I've flown P47 with X55, X56, Warthog, and now Virpil Throttles, and I have no problem with the fine control of boost at any altitude. There are no curves/saturation on my boost axis either, it uses the default settings.

Edited May 23, 2023 by peachmonkey

[grafspee]

Difficulty in controling boost via turbo comes from how turbo works.

You need to build up enough exhaust pressure for turbine wheel so compressor wheel get enough rpm to build up boost, but once it happens MAP rises and exhaust gases pressure rises too this affect turbine wheel to spin even faster and so, you can call it a chain reaction.

So to spin up turbo you need to move boost lever far but once it catch boost you have to retard boost lever immediately.

Second thing is that DCS P-47 uses automatic turbo regulator which maintain  exhaust back pressure within design limits, so this explains why MAP changes in climb or descent.

Edited May 23, 2023 by grafspee

[Art-J]

I believe Nealius is talking about climbing through 5k-6k altitude where turbo, which used to sleep till that point, wakes up, realizes there's a 42" job to do and starts boosting in a very sensitive, non-linear manner. That being said, I also use left Warthog throttle lever to control it (no curves or deadzones) and can confirm that with correct working hardware it should be relatively easy to keep MAP within +/- 1 of that 42. Output should be sensitive, but not spiking.

[Nealius]

Art-J basically understands what I'm getting at, and now it makes some sense given the mechanical explanation by grafspee and peachmonkey. Though if it's not criminal for me to adjust with throttle while also boosting (keeping boost lever physically aft of the throttle lever) then I may just keep that method for now. 

[SloppyDog]

On 5/24/2023 at 5:12 AM, Nealius said:

Art-J basically understands what I'm getting at, and now it makes some sense given the mechanical explanation by grafspee and peachmonkey. Though if it's not criminal for me to adjust with throttle while also boosting (keeping boost lever physically aft of the throttle lever) then I may just keep that method for now. 

That's the right way to do it, boost always equal or less than throttle.

Our old friend Greg explains that in fine detail here:

 

 

[grafspee]

Remember that interconnected throttle and boost lever may reduce as much as 300 hp output power.

[Caldera]

Hey All,

I have read all of the above.  Great discussion!

I was myself wondering why my MP was so low at lower altitudes, but I seemed to remember information saying not to use the turbo boost until >12k feet.

 

My comments:

40mmHg of boost in reality is not allot on an MP absolute pressure gauge.  30mmHg is about sea level pressure and about 15 psia or 0 psi.  So 40mmHg MP is about 20 psia or 5 psi.  And 60mmHg would be only about 30 psia, 15 psi or roughly two atmospheres.  In effect, two atmospheres essentially doubles the displacement of the engine.

As an example, my turbo diesel was able to run at 70-80 psi boost (with two in series turbos).  That is about 95 psia.  A diesel engine does not have a throttle plate.

The carburetor throttle plate acts as a restriction on an automobile gas engine.  I do not know how it works exactly on the R-2800, but to maintain mixture control I assume that it has one.  So anytime the throttle plate is less than full open you would be making boost for less effect on engine performance.  Basically making boost for the cost of adding HP load (supercharger) on the engine.  The supercharger uses allot of HP. 

With the throttle plate wide open, then all of the HP used to power the supercharger is converted to engine power more efficiently.  Saying all that, the supercharger gearing is probably set up with this and a given engine RPM in mind to work the best.

From what I can gather the turbo (supercharger) on the P-47 was huge.   First off, with allot of rotating mass it would not spool instantly.  This "turbo lag" is what I see when I close the waste gate and direct engine exhaust gas to the turbo vice dumping it to atmosphere.  Second off, for efficiency, the turbine casing for the turbo would be large.  The reason is that a more restricted turbine casing would provide faster spool up, but also create more back pressure on the engine.  Any back pressure on an engine reduces efficiency.  And the sizing of the turbine itself must match the HP load of the compressor needed to supply the desired boost.  The turbo compressor uses allot of HP.

Using a turbo, in general, is like running an engine in a pressurized box.  Where the atmospheric pressure in the box is set equal to the back pressure on the engine.  So for an example, say the engine back pressure was 20 psia the engine would be less efficient than if it were running at sea level or about 15 psia.  The turbine causing back pressure on the engine does this very thing.  The net effect is positive HP gain, due to the positive effect of intake pressure increased by boost.  This effect can be significant.

As an example, my turbo diesel could produce 70-80 psi of boost, but with 80-90 psi of back pressure.

Running a supercharger and a turbo supercharger (P-47) in series has its advantages and disadvantages.  The biggest disadvantage is if the throttle plate is not wide open.  This acts as a restriction for the entire system.  The supercharger boost is restricted by the throttle plate.  The turbo boost is restricted by the supercharger.  Total boost increasing as the sum of the two.  There could be situations of making allot of boost that couldn't be used.  The supercharger consuming HP powered directly by the engine and the turbo by its increased effect of back pressure on the engine.  Essentially wasting much of the HP needed to created that boost.  Making allot of boost requires allot of HP.

As an example, for my turbo diesel (I once calculated) that to produce ~600 HP the turbo itself was using ~300 HP or about 50%.  For jet engines with higher compression ratios this number can be higher or about 65%.  

HP is not produced for free, fuel must be burned.  Check your gas gauge...  YMMV

 

I suppose more information than anyone really wanted.  If you did read I hoped you did enjoy, and be sure to give feed back.

Caldera

 

[grafspee]

It is worth to remember that P-47 engine or any ww2 fighter engine operates at constant max MP regardless of altitude with small exemptions.

Throttle is main way to control power of spark ignition engine + device which can measure air flow mass and introduce right amount fuel.

Supercharger is driven from crankshaft directly, so way to control it is to reduce inlet pressure by throttle to not exceed max allow pressure.

If we take 2 engines R-2800 from P-47 and one on them we would strip supercharger of it.

And test both of them at 30 inch of MAP, test would tell us that engine w/o supercharger would produce a lot more power, why ?

First thing is that supercharger need power to comperes air, second thing is that while compressing air, temp is rising so not only supercharger eats Hp by it self but also it rises inlet air temp which reduce power even more. Hotter air = less air density.

Turbocharger uses exhaust gases to propel compressor, but in this case most of the power comes from exhaust inertia and exhaust pressure called backpressure, backpressure impact engine's power at much lower rate then supercharger driven from crankshaft.

Turbocharger needs power same as supercharger to compress air but most of it comes from exhaust gases inertia which impact engine power output in limited range.

Very high exhaust back pressure can force exhaust valves to freely open and poison the engine with exhaust gases  but this is very unlikely to happen in those engines which are very big and run low boost.

Any use turbocharger w/o throttle being wide open, evoke increased air inlet temp and increased back pressure which both reduce power output of the engine, which can reach 300hp.