correct as-is P-51D speed bug

[amazingme]

  I believe there's an issue with the maximum speed that P51 can achieve @ different rpm/boost settings. I could achieve 585kph (~364mph) using 67"/2600rpm then the speed dropped to normal levels after WEP was engaged.. 565kph (~350).

  Video and track attached.

 

server-20210718-032230.zip

[grafspee]

62"/ 2600rpm faster then 67"/3000 rpm. And engine looks like run just fine at 2600rpm and 62".

I remember yo-yo was saying something about prop tip going to close to speed of sound at 3000rpm and this kills prop's thrust at high speeds.

I'm not sure if engine can handle this power settings in RL.

Edited July 18, 2021 by grafspee

[KenobiOrder]

The actual aircraft was tested to higher speeds in every real life test. The top speed on the deck of the Mustang on a standard day was 375mph. In no real world test of the Mustang was it ever slower at higher RPMs at lower RPMs. This is an indisputable empirical fact, and anyone stating otherwise is in the wrong.

Edited July 18, 2021 by KenobiOrder

[nighthawk2174]

I mean with a prop if you put in more power you should get more work out of the engine and get a higher top speed.  As you go faster on the rpm yes the prop efficiency will fall but this doesn't just cause you to go slower your ability to push faster just falls off rapidly.  You don't just loose thrust because your efficiency falls.  And as said above the charts show the top speed is 375 and if the mustang in game doesn't meet this then its modeled incorrectly its that simple.

Edited July 18, 2021 by nighthawk2174

[grafspee]

2 hours ago, KenobiOrder said:

The actual aircraft was tested to higher speeds in every real life test. The top speed on the deck of the Mustang on a standard day was 375mph. In no real world test of the Mustang was it ever slower at higher RPMs at lower RPMs. This is an indisputable empirical fact, and anyone stating otherwise is in the wrong.

 

I have never seen speed test reports showing speed for different rpm and same boost

for example 61" and 3000rpm and 61" 2700rpm

I don't see 375mph S.L top speed.

http://www.wwiiaircraftperformance.org/mustang/p-51d-na-46-130.html

When testing remember to set STD atmosphere 1013.25 hPa and 15C

My tests, similar results, lowering rpm allow to gain couple kts higher speed.

Top speed 67" 3000rpm 365MPH and top speed 67" 2600 rpm 374MPH

Apparently Power difference between 3000 and 2600 rpm is smaller then gains on prop thrust at lower rpm, this results with higher top speed.

If any one wonder why my prop stopped, yes i hit the water

 

 

 

P-51 speed test.trk

Edited July 18, 2021 by grafspee

[KenobiOrder]

11 hours ago, grafspee said:

I have never seen speed test reports showing speed for different rpm and same boost

for example 61" and 3000rpm and 61" 2700rpm

I don't see 375mph S.L top speed.

http://www.wwiiaircraftperformance.org/mustang/p-51d-na-46-130.html

When testing remember to set STD atmosphere 1013.25 hPa and 15C

My tests, similar results, lowering rpm allow to gain couple kts higher speed.

Top speed 67" 3000rpm 365MPH and top speed 67" 2600 rpm 374MPH

Apparently Power difference between 3000 and 2600 rpm is smaller then gains on prop thrust at lower rpm, this results with higher top speed.

If any one wonder why my prop stopped, yes i hit the water

 

 

 

P-51 speed test.trk 1.13 MB · 1 download

 

The speed chart you quoted was estimated. That is not a flight test. It is also faster than the DCS P-51.

 

DCS is clearly just wrong, which is pretty par for the course with the WW2 modules. Half the planes dont even model compression effects, the thermo-model for engines is bonkers, the P-47s prop overspeeds in a manner directly contradictory to the manual....the list goes on and has for years. If the Mustang could have gone faster IRL at a lower RPM, than the engineers would have specified that lower RPM as the best RPM for performance instead of 3000 RPM.

[Magic Zach]

13 hours ago, KenobiOrder said:

The actual aircraft was tested to higher speeds in every real life test. The top speed on the deck of the Mustang on a standard day was 375mph. In no real world test of the Mustang was it ever slower at higher RPMs at lower RPMs. This is an indisputable empirical fact, and anyone stating otherwise is in the wrong.

 

How do you know?  67", 2650RPMs was never tested in flight, as far as I can find.

It's not that wild of a thing to happen either, if you think about it.  When the plane is at high speed with full RPM, the governor dictates that the engine limit itself to 3000RPM.  But this also means that as the airspeed increases the propeller isn't biting as much air as it could.  When you lower the prop pitch control, the governor needs to lower the engine RPM.  And ofc to do this, it increases blade angle to apply more resistance on the engine.  However at the same time, the increased blade pitch allows each blade to generate more lift (or thrust rather, applied horizontally like they are) than at a shallower angle at 3000RPM.
The reasons this probably wasn't done or mentioned in tests or manuals etc etc is that it's probably absolute hell on the engine.  I'd imagine if this was done in reality, it'd require a complete strip-down upon landing.

Actually I found a similar question under a different forum (ED overlords please don't delete this, it's relevant) that has its own answers as well
Why does increasing rpm with same mp reduce airspeed? - The A2A Simulations Community

Edited July 18, 2021 by Magic Zach

[grafspee]

I have only one concern, can engine handle 67" at 2600 rpm ? 

If the pitch of the blade was a concern then they would change prop's reduction gear ratio to set lower prop rpm, so engine could stay at 3000rpm.

Major factor is the tip of the blade speed, at high rpm and high speed it loses thrust.

 

 

Edited July 18, 2021 by grafspee

[=475FG= Dawger]

6 hours ago, Magic Zach said:

How do you know?  67", 2650RPMs was never tested in flight, as far as I can find.

It's not that wild of a thing to happen either, if you think about it.  When the plane is at high speed with full RPM, the governor dictates that the engine limit itself to 3000RPM.  But this also means that as the airspeed increases the propeller isn't biting as much air as it could.  When you lower the prop pitch control, the governor needs to lower the engine RPM.  And ofc to do this, it increases blade angle to apply more resistance on the engine.  However at the same time, the increased blade pitch allows each blade to generate more lift (or thrust rather, applied horizontally like they are) than at a shallower angle at 3000RPM.
The reasons this probably wasn't done or mentioned in tests or manuals etc etc is that it's probably absolute hell on the engine.  I'd imagine if this was done in reality, it'd require a complete strip-down upon landing.

Actually I found a similar question under a different forum (ED overlords please don't delete this, it's relevant) that has its own answers as well
Why does increasing rpm with same mp reduce airspeed? - The A2A Simulations Community

 

You are on the right track here. 67 inches at 2700 RPM would make you faster until the cylinder heads blew off. 

[grafspee]

So engine rpm range from 2700-3000 is only to allow engine to operates at high MP IRL, so peak power settle around 2600-2700 rpm.

2700rpm advantage over 3000rpm - lower charger temp, aggressive spark timing those 2 add power, higher prop efficiency = higher top speed.

Edited July 19, 2021 by grafspee

[grafspee]

15 hours ago, Magic Zach said:

 When the plane is at high speed with full RPM, the governor dictates that the engine limit itself to 3000RPM.  But this also means that as the airspeed increases the propeller isn't biting as much air as it could.  When you lower the prop pitch control, the governor needs to lower the engine RPM.  And ofc to do this, it increases blade angle to apply more resistance on the engine.  However at the same time, the increased blade pitch allows each blade to generate more lift (or thrust rather, applied horizontally like they are) than at a shallower angle at 3000RPM.

This is wrong way of thinking.

Engine don't limits it self to 3000rpm if prop would detach engine rpm would go crazy high, it is always governor which limits engine rpm.

3000 rpm or 2700 rpm always sits in optimal pitch. As plane accelerate pitch increases, so it "bites as much as it can " within optimal blade AoA.

Question is, how much power engine develops at 2700rpm.

Power chart at constant 46" boost from 2700 to 3000 rpm would solve the mistery 

What about blade velocity, isn't decreasing blade velocity also decrease generated lift ??

Prop's blade similar to wing, you get lift from AoA and speed, extreme AoA levels provide more lift but at great drag cost same in the prop.

 

Edited July 19, 2021 by grafspee

[KenobiOrder]

Higher RPM means more power. A P-51 and 61 inches of boost and 3000 RPM is slower than the P-51 at 67 inches of boost and 3000 RPM, and that is slower than a plane at 75 inches of boost and 3000 RPM.

 

We have speed charts for Mustangs at 3000 RPM and 75inches of boost. Clearly it is false that the propeller could not absorb more power since the plane was faster and climbed significantly better at the same RPM but more manifold pressure. Later P-51 H's would produce the same RPM but have even higher manifold pressures of something like 90 inches or whatever.

 

As an empirical matter of fact it simply not the case the the propeller could not adsorb/output more thrust at 3000rpm.

 

Edited July 20, 2021 by KenobiOrder

[Yo-Yo]

On 7/18/2021 at 4:24 AM, amazingme said:

  I believe there's an issue with the maximum speed that P51 can achieve @ different rpm/boost settings. I could achieve 585kph (~364mph) using 67"/2600rpm then the speed dropped to normal levels after WEP was engaged.. 565kph (~350).

  Video and track attached.

 

server-20210718-032230.zip 4.62 MB · 3 downloads

Didn't you think you just killed the engine before the second part of your test?  My test shows 367 mph at 61/(2650...3000) and 373 mph at 67/3000

 

The report that was used

[grafspee]

367mph at 61/ 2650...3000 rpm, so you got the same top speed in this rpm range? how do i read that?

On my end P-51 clearly start accelerating when i start reducing rpm.

[Yo-Yo]

The speed depends on radiator scoop position as well, so the max speed depends even at the pre-history of your flight? because the scoop position is not a determoned fuction of coolant temperature. It will stop in different positions as the coolant temperature is in limits. I. e. if your limits are T1 and T2, the scoop can stop finally at any position where heat balance is obtained for the temperature between T1 and T2. The scoop position as well as engine power and compressed air temperature affect charge temperature. But there is exhaust thrust that decreased with rpm decreasing. A lot of factors, indeed. By the way, the blade tip Mach is about 0.92, so the losses starts to rise very fast in this region.
For example, starting at ISA SL at 570 gives you this kind of excessive specific power log.
Three charts are a bit overlapped time history of 3000-2650-3000 profile.
So, reducing rpm you can add few mph, but this overboost condition can be dangerous. And the effect itself lays within 1%, so less than real world measuring ability.
 

[grafspee]

@Yo-Yo thanks.

[amazingme]

On 7/28/2021 at 11:41 PM, Yo-Yo said:

Didn't you think you just killed the engine before the second part of your test?  My test shows 367 mph at 61/(2650...3000) and 373 mph at 67/3000

 

The report that was used

  Thanks a lot for the reply! The engine seemed fine, no issues at all.. I noticed that people would abuse this 'strategy' online, otherwise I wouldn't have made this post. It doesn't break the engine unfortunately and it's more noticeable if you dive a little bit before. The exhaust thrust may be too much or there's something off, it shouldn't be too noticeable.. just as you've stated above..