RPM vs manifold pressure

Honest, reliable, iconic American fighter
new reply
User avatar
Medtner
A2A Mechanic
Posts: 1350
Joined: 30 Sep 2013, 10:10
Location: Arendal, Norway
Contact:

RPM vs manifold pressure

Post by Medtner »

I've had 2-3 hours in the P-40 today, and what a lovely aircraft! It has been a long time - the 182 has had me thoroughy occupied.
The flight was partially inspired by a new P-40 pilot here on the forum and also the fact that there is a T6 in the works - I needed to get reaquainted with these warbirds. The sound of the Allison engine never stops fascinating me.

However I'm noticing that whenever I lower the RPM the MP will also lower. Isn't this supposed to be the other way around?
Erik Haugan Aasland,

Arendal, Norway
(Homebase: Kristiansand Lufthavn, Kjevik (ENCN)

All the Accusim-planes are in my hangar, but they aren't sitting long enough for their engines to cool much before next flight!

User avatar
DHenriques_
A2A Chief Pilot
Posts: 5711
Joined: 27 Mar 2009, 08:31
Location: East Coast United States

Re: RPM vs manifold pressure

Post by DHenriques_ »

Medtner wrote:I've had 2-3 hours in the P-40 today, and what a lovely aircraft! It has been a long time - the 182 has had me thoroughy occupied.
The flight was partially inspired by a new P-40 pilot here on the forum and also the fact that there is a T6 in the works - I needed to get reaquainted with these warbirds. The sound of the Allison engine never stops fascinating me.

However I'm noticing that whenever I lower the RPM the MP will also lower. Isn't this supposed to be the other way around?
Lowering RPM literally means the pistons are sucking in less air, thus less vacuum past the MP probe. Should result in a rise in MP.
DH

User avatar
Medtner
A2A Mechanic
Posts: 1350
Joined: 30 Sep 2013, 10:10
Location: Arendal, Norway
Contact:

Re: RPM vs manifold pressure

Post by Medtner »

DHenriquesA2A wrote:
Medtner wrote:I've had 2-3 hours in the P-40 today, and what a lovely aircraft! It has been a long time - the 182 has had me thoroughy occupied.
The flight was partially inspired by a new P-40 pilot here on the forum and also the fact that there is a T6 in the works - I needed to get reaquainted with these warbirds. The sound of the Allison engine never stops fascinating me.

However I'm noticing that whenever I lower the RPM the MP will also lower. Isn't this supposed to be the other way around?
Lowering RPM literally means the pistons are sucking in less air, thus less vacuum past the MP probe. Should result in a rise in MP.
DH
This is what I thought - and this is correctly modelled in the 182 (and in the other warbirds?). That means that there is a bug there somehow?
Erik Haugan Aasland,

Arendal, Norway
(Homebase: Kristiansand Lufthavn, Kjevik (ENCN)

All the Accusim-planes are in my hangar, but they aren't sitting long enough for their engines to cool much before next flight!

User avatar
DHenriques_
A2A Chief Pilot
Posts: 5711
Joined: 27 Mar 2009, 08:31
Location: East Coast United States

Re: RPM vs manifold pressure

Post by DHenriques_ »

Medtner wrote:
DHenriquesA2A wrote:
Medtner wrote:I've had 2-3 hours in the P-40 today, and what a lovely aircraft! It has been a long time - the 182 has had me thoroughy occupied.
The flight was partially inspired by a new P-40 pilot here on the forum and also the fact that there is a T6 in the works - I needed to get reaquainted with these warbirds. The sound of the Allison engine never stops fascinating me.

However I'm noticing that whenever I lower the RPM the MP will also lower. Isn't this supposed to be the other way around?
Lowering RPM literally means the pistons are sucking in less air, thus less vacuum past the MP probe. Should result in a rise in MP.
DH
This is what I thought - and this is correctly modelled in the 182 (and in the other warbirds?). That means that there is a bug there somehow?
Could be. I'll pass it on to Scott for possible change in an update. Thanks. Good catch ! Our forum community in action :-))
DH

User avatar
Medtner
A2A Mechanic
Posts: 1350
Joined: 30 Sep 2013, 10:10
Location: Arendal, Norway
Contact:

Re: RPM vs manifold pressure

Post by Medtner »

DHenriquesA2A wrote: Could be. I'll pass it on to Scott for possible change in an update. Thanks. Good catch ! Our forum community in action :-))
DH
Thanks to you too! :-)

(btw I can't find words on how much it tickles me that the T6 is in development now! New blood to the warbird-family! :-D)
Erik Haugan Aasland,

Arendal, Norway
(Homebase: Kristiansand Lufthavn, Kjevik (ENCN)

All the Accusim-planes are in my hangar, but they aren't sitting long enough for their engines to cool much before next flight!

User avatar
Jacques
Senior Master Sergeant
Posts: 2376
Joined: 26 Jun 2011, 17:54
Location: West Coast, USA

RPM vs manifold pressure

Post by Jacques »

Hi Dudley and Medtner,

I've noticed this behavior in the P-47 as well, and just confirmed it during a flight. I reduce MP to 32" then brought the propeller back to 2150 RPM which resulted in a drop of MP to around 30" if not just a tick below. Could you forward this on as well if it is considered abnormal?

JP

User avatar
DHenriques_
A2A Chief Pilot
Posts: 5711
Joined: 27 Mar 2009, 08:31
Location: East Coast United States

Re: RPM vs manifold pressure

Post by DHenriques_ »

Jacques wrote:Hi Dudley and Medtner,

I've noticed this behavior in the P-47 as well, and just confirmed it during a flight. I reduce MP to 32" then brought the propeller back to 2150 RPM which resulted in a drop of MP to around 30" if not just a tick below. Could you forward this on as well if it is considered abnormal?

JP
No problem.

AviationAtWar
Technical Sergeant
Posts: 899
Joined: 30 Nov 2014, 19:07
Location: US
Contact:

Re: RPM vs manifold pressure

Post by AviationAtWar »

Mustang is the same way. I thought manifold pressure was dropping because the engine is turning the supercharger slower at a lower RPM?

User avatar
Erlk0enig
A2A Mechanic
Posts: 692
Joined: 06 Jan 2010, 07:21
Location: Germany

Re: RPM vs manifold pressure

Post by Erlk0enig »

AviationAtWar wrote:Mustang is the same way. I thought manifold pressure was dropping because the engine is turning the supercharger slower at a lower RPM?
Correct. Increasing the rpm of a radial compressor directly increases the pressure ratio and therefore the mp (if no mp regulation device is used). For the higher mass flows however at the extreme right choking border of the compressor map, there won't be any rise possible in pressure ratio any more.
Best Regards,
Ralf

Image
R9 7900 X, 64 GB DDR5, RTX4090, HP Reverb G2

User avatar
gulredrel
Master Sergeant
Posts: 1275
Joined: 12 Jun 2011, 02:11
Location: Germany

Re: RPM vs manifold pressure

Post by gulredrel »

With the supercharged engines you can only see the MP rise at near idle RPMs and low MP. I think, this is correct in the P-40.
"Give me a ping, Vasili. One ping only, please."

User avatar
Medtner
A2A Mechanic
Posts: 1350
Joined: 30 Sep 2013, 10:10
Location: Arendal, Norway
Contact:

Re: RPM vs manifold pressure

Post by Medtner »

I can't argue against better knowledge - I don't know that much about this, but based on what little I know and what I have seen it still seems weird.

Have a look at the following videos, with Kermit Weeks doing runups and propeller-exercises. With some careful attention one clearly sees him setting MP to the current air pressure: 30 inches in both cases, and that it rises to 33-34 ish when he cycles the prop.

AT6:

https://youtu.be/gAHNnHBBJT8?t=755

P-51
https://youtu.be/J1F_UJaaP1A?t=334

Edit:

https://www.youtube.com/watch?v=eOXxUApaaWo
In this part with the P-51 one can see that the MP stays put or (maybe - not completely clear) even rises ever so slightly when Kermit lowers the RPM after take off.

Listen to the wonderful engine, by the way! Goosebump-material! :-D
Erik Haugan Aasland,

Arendal, Norway
(Homebase: Kristiansand Lufthavn, Kjevik (ENCN)

All the Accusim-planes are in my hangar, but they aren't sitting long enough for their engines to cool much before next flight!

User avatar
gulredrel
Master Sergeant
Posts: 1275
Joined: 12 Jun 2011, 02:11
Location: Germany

Re: RPM vs manifold pressure

Post by gulredrel »

P-51 has a manifold pressure regulator, so you set your desired MP. If anything changes (e.g. engine RPM), but the engine can still produce your desired MP, the regulator will compensate for the changes in MP which are caused by changing RPM.
On the other engines, you directly control the throttle plate.

Interesting here: http://www.avweb.com/news/pelican/Pelic ... 081-1.html There are some others, describing turbo are supercharger behaviour I htink.
"Give me a ping, Vasili. One ping only, please."

new reply

Return to “P-40 Warhawk / Tomahawk”

Who is online

Users browsing this forum: No registered users and 9 guests