Use of boost during ground attack role.

[Gunfreak]

Is this needed? 

Are there any advantages of not using it(less engine stress?)

[Mud]

Check out this video.  If I remember correctly, using boost below 7k feet is rather pointless or even disadvantageous.

 

[Aernov]

You decide if it is needed or not. The purpose of turbosupercharger (boost) is to allow you to get MP levels unreachable with geared supercharger (throttle) alone. Usually you can ground attack without the boost since at low altitudes your geared supercharger will give enough MP (up to 52'' at sufficient speed). If you want to do a high speed attack at 52'' MP - use full throttle and, if that's not enough - boost. If you want even higher speed to pass AAA quickly - get 64'' MP with throttle+boost+water. If you give full throttle before engaging boost, the only disadvantage is that you are required to remember that settings above 42'' and 2550 RPM are "time-limited" and can damage the engine with prolonged use and inadequate cooling. 7000 ft restriction is only actual for grade-91 fuel, DCS gives us 100.

[grafspee]

If you want high speed ground strike, you just need to start this from higher alt. 52 inch or 64 inch during dive makes little difference, as well you can use 42/2550.

[MadKreator]

I always disconnect boost and use it only at higher altitudes when required to keep MP nominal. I’m sure there are many reasons/ situations where one could/ should use it, but I find no real advantage at lower altitudes. I’ve also read that when using it  at low altitudes that it draws more power than it can produce… although I’m not sure if that performance metric is really modeled in DCS.. I can’t really tell a difference one way or the other

Edited April 12, 2024 by MadKreator

[grafspee]

22 hours ago, MadKreator said:

I always disconnect boost and use it only at higher altitudes when required to keep MP nominal. I’m sure there are many reasons/ situations where one could/ should use it, but I find no real advantage at lower altitudes. I’ve also read that when using it  at low altitudes that it draws more power than it can produce… although I’m not sure if that performance metric is really modeled in DCS.. I can’t really tell a difference one way or the other

 

If you want obtain 64 inch you have to use boost lever even at SL.

Power lose is up to 300hp but only between throttle/boost lever interconected and throttle/boost lever splited technique. Using boost lever at low altitude always adds power. Turbocharging is very or maybe the most efficient forced induction.

[Cool-Hand]

Use it if you need the manifold pressure. It definitely nets more airspeed or altitude if you need to zoom climb, that way the MP wont fall off a few thousand feet up with just the supercharger.

[Bozon]

Normally I just link the boost to the throttle and forget about it. I don’t fly high enough where firewalling the throttle and adjust power with the boost is useful.

At altitudes relevant for me (up to 20k) max power is always achieved by firewalling both throttle and boost, without over-boosting. At low altitudes boost is not needed at all and even eats some power - however, this just means that for the same actual power I need a little higher manifold pressure, so I burn a little more fuel. This slightly sub-optimal operation tradeoff is worth the no-hassle of a linked boost, at least for me with a throttle setup that is not the most supportive of 3 separate boost / throttle / RPM axes. YMMV.

[Mr_sukebe]

I was told that using the turbo at low altitude increases parasitic power loss and is not recommended.

I’ve never tried to verify it, but I do fly the P47 with that assumption and always take off and land with the turbo boost disengaged

[Cool-Hand]

I've read that as well but it seems to be a bit of an internet myth. If you leave them linked at low altitude you won't get the benefit of full supercharger power and be using the Turbo add to the partial power to get to your manifold pressure desired setting. In that case there is a power loss being experienced by running the turbo and only partial supercharger since if you run both at max you can overboost the engine. Linking them both, you are only getting partial power from both if you aren't taking off or something.

If you don't link them and max out the supercharger on the engine and then still are shy of the desired manifold pressure, you can add the Turbo to make up for extra power. Its the same principle as going higher and needing the Turbo to make up for supercharger loss of power with altitude being only a single stage supercharger. The Turbo is there to make up for the lack of variable speed supercharger and is also why it gets faster the higher it goes with no drop offs in performance until it's max altitude or maximum turbo speed is encountered.

Worst case test it in DCS for yourself, only takes a few minutes.

Edited April 22, 2024 by Cool-Hand

[grafspee]

@Mr_sukebe, @Cool-Hand Exactly as you said. The worst spot to link throttle with boost lever is low alt range. At low alt supercharger alone is capable to provide decent boost. boost.

With throttle alone you can get 46inch for take off, but even for full power take off procedure from manuals is to move boost lever to take off stop and advance throttle fully, this boost lever take off stop should provide that 52inch for take off unfortunately our DPS P-47 does not have boost lever take off stop.

Manuals clearly says that at certain conditions up to 300bhp can be lost due to throttle/boost interconnection, i always use split throttle/boost, at higher alt i manage boost only with boost lever.  

Edited April 22, 2024 by grafspee

[Art-J]

2 hours ago, Cool-Hand said:

Worst case test it in DCS for yourself, only takes a few minutes.

Hear hear!

In DCS power loss from supercharger throttling seems to be either not simulated all that deeply, or, more likely, one has to look really hard for a rare and ugly combination of input parameters to make it anywhere close to that "up to 300 BHP" and start noticing some sort of a really meaningful difference in performance.

Below are screens taken in Caucasus free flight mission. Test goal - get the engine up to about 46" MAP at about 1500 ft using a) full supercharger and no turbo, and b) lots of turbo and only a little supercharger (so little in fact, that landing light warning came on, note the positions of levers). I'm sure the engine shouldn't like nasty supercharger throttling in the latter case, but from airspeed point of view there was no difference whatsoever (well, maybe +/- 2 kts 'cause it's difficult to keep the -47 flying level and steady).

Repeated the test at 8000 ft, aiming at 52" MAP this time, with various combinations of both levers, added interlinked combo, all with same results - no speed difference beyond margin of error.

All and all, to me It doesn't seem to be worth it - fiddling with levers separately I mean. I'm not seeing noticeable performance penalty (if any) for flying interlinked all the time. OK, there is one "inconvenience" penalty rather than performance one - turbo lag with resulting MAP getting bipolar disorder at low altitudes . For this reason I sometimes do like flying with split levers, as the throttle alone has more linear and predictable output. Nicer to manage and can be supplemented with boost later I admit. More often, however, I just interlink them and call it a day. The plane will fly equally fast anyway. Granted, level speed is not everything and I suppose there might be some other "drawback" simulation running in the background, but unless one flies Reflected's campaigns or some really long missions, it's probably not a big factor.

[Gunfreak]

1 hour ago, Art-J said:

Hear hear!

In DCS power loss from supercharger throttling seems to be either not simulated all that deeply, or, more likely, one has to look really hard for a rare and ugly combination of input parameters to make it anywhere close to that "up to 300 BHP" and start noticing some sort of a really meaningful difference in performance.

Below are screens taken in Caucasus free flight mission. Test goal - get the engine up to about 46" MAP at about 1500 ft using a) full supercharger and no turbo, and b) lots of turbo and only a little supercharger (so little in fact, that landing light warning came on, note the positions of levers). I'm sure the engine shouldn't like nasty supercharger throttling in the latter case, but from airspeed point of view there was no difference whatsoever (well, maybe +/- 2 kts 'cause it's difficult to keep the -47 flying level and steady).

Repeated the test at 8000 ft, aiming at 52" MAP this time, with various combinations of both levers, added interlinked combo, all with same results - no speed difference beyond margin of error.

All and all, to me It doesn't seem to be worth it - fiddling with levers separately I mean. I'm not seeing noticeable performance penalty (if any) for flying interlinked all the time. OK, there is one "inconvenience" penalty rather than performance one - turbo lag with resulting MAP getting bipolar disorder at low altitudes . For this reason I sometimes do like flying with split levers, as the throttle alone has more linear and predictable output. Nicer to manage and can be supplemented with boost later I admit. More often, however, I just interlink them and call it a day. The plane will fly equally fast anyway. Granted, level speed is not everything and I suppose there might be some other "drawback" simulation running in the background, but unless one flies Reflected's campaigns or some really long missions, it's probably not a big factor.

Would be interesting to read a book by a real plot in what they actually did. None of the P47 war memories I've read go into such detail. 

On the other hand it seems real pilots mostly just floored it until the combat was over. And did stuff to the engine that would kill it in seconds in DCS.

 

[Nealius]

3 hours ago, Art-J said:

turbo lag with resulting MAP getting bipolar disorder at low altitudes

This is the part that drives me nuts on takeoffs and low altitude flying in the Jug. The turbo lever axis is so non-linear and there's such a lag between adding turbo and seeing the turbo needle move that I'm constantly in an overshoot/undershoot cycle trying to get desired MP.

[grafspee]

I can add to discussion that even if i move throttle and boost lever at take off roll turbo does not spool up at all, it star catching up very late just before take off.

 

12 hours ago, Gunfreak said:

Would be interesting to read a book by a real plot in what they actually did. None of the P47 war memories I've read go into such detail. 

On the other hand it seems real pilots mostly just floored it until the combat was over. And did stuff to the engine that would kill it in seconds in DCS.

 

Iirc early version of p-47 didn't have throttle/boost lever interconetion at all so we can safely assume that pilots at firsts use throttle and boost lever separately.

Yeah this is the fact that DCS model engines way too sensitive, i could list couple famous examples but probably all of you know what i have in mind.

[grafspee]

I assume that worst case scenario is when air temp is very high and this additional temp increase in turbocharger goes above carb air temp limit. It is specifically noted in manual to take off w/o turbocharger when temps are high outside.  It is indeed not stated anywhere in manual at what condition this up to 300bhp loss appears. This knowledge probably was passed via flight instructors. But my guess is that highest power decline will appear when carb air temp pass upper redline.

[Brigg]

I would say it is to do with carb air temperature as in the installation test flight manual for the -59 engine it mentions loss of bhp when temperature goes above normal operating temp 

[Brigg]

On 4/23/2024 at 11:17 AM, grafspee said:

I can add to discussion that even if i move throttle and boost lever at take off roll turbo does not spool up at all, it star catching up very late just before take off.

 

Iirc early version of p-47 didn't have throttle/boost lever interconetion at all so we can safely assume that pilots at firsts use throttle and boost lever separately.

Yeah this is the fact that DCS model engines way too sensitive, i could list couple famous examples but probably all of you know what i have in mind.

Yeah that's correct early models didn't have interconnected controls. Pilots were killing their engines often by retarding the throttle before the boost so the USAAF told Republic to come up with a solution as they had had enough of the size of the cheques they were writing them