The A2A Simulations Community

"Come share your passion for flight"
It is currently Thu Nov 23, 2017 9:46 am

All times are UTC - 5 hours




Post new topic Reply to topic  [ 12 posts ] 
Author Message
 Post subject: RPM vs manifold pressure
PostPosted: Mon May 18, 2015 6:39 pm 
Offline
Technical Sergeant
User avatar

Joined: Mon Sep 30, 2013 10:10 am
Posts: 827
Location: Arendal, Norway
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!


Top
 Profile  
 
PostPosted: Mon May 18, 2015 7:21 pm 
Offline
A2A Chief Pilot
User avatar

Joined: Fri Mar 27, 2009 8:31 am
Posts: 4274
Location: East Coast United States
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


Top
 Profile  
 
PostPosted: Mon May 18, 2015 7:25 pm 
Offline
Technical Sergeant
User avatar

Joined: Mon Sep 30, 2013 10:10 am
Posts: 827
Location: Arendal, Norway
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!


Top
 Profile  
 
PostPosted: Mon May 18, 2015 7:27 pm 
Offline
A2A Chief Pilot
User avatar

Joined: Fri Mar 27, 2009 8:31 am
Posts: 4274
Location: East Coast United States
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


Top
 Profile  
 
PostPosted: Mon May 18, 2015 7:30 pm 
Offline
Technical Sergeant
User avatar

Joined: Mon Sep 30, 2013 10:10 am
Posts: 827
Location: Arendal, Norway
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!


Top
 Profile  
 
 Post subject: RPM vs manifold pressure
PostPosted: Mon May 18, 2015 8:21 pm 
Offline
Senior Master Sergeant
User avatar

Joined: Sun Jun 26, 2011 5:54 pm
Posts: 1624
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


Top
 Profile  
 
PostPosted: Mon May 18, 2015 8:25 pm 
Offline
A2A Chief Pilot
User avatar

Joined: Fri Mar 27, 2009 8:31 am
Posts: 4274
Location: East Coast United States
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.


Top
 Profile  
 
PostPosted: Mon May 18, 2015 9:01 pm 
Offline
Staff Sergeant
User avatar

Joined: Sun Nov 30, 2014 7:07 pm
Posts: 390
Location: US
Mustang is the same way. I thought manifold pressure was dropping because the engine is turning the supercharger slower at a lower RPM?

_________________
Image

AviationAtWar on Youtube


Top
 Profile  
 
PostPosted: Mon May 18, 2015 11:42 pm 
Offline
A2A Mechanic
User avatar

Joined: Wed Jan 06, 2010 7:21 am
Posts: 569
Location: Germany
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


Top
 Profile  
 
PostPosted: Tue May 19, 2015 1:55 am 
Offline
Technical Sergeant
User avatar

Joined: Sun Jun 12, 2011 2:11 am
Posts: 836
Location: Germany
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."


Top
 Profile  
 
PostPosted: Tue May 19, 2015 4:56 pm 
Offline
Technical Sergeant
User avatar

Joined: Mon Sep 30, 2013 10:10 am
Posts: 827
Location: Arendal, Norway
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!


Top
 Profile  
 
PostPosted: Wed May 20, 2015 12:39 am 
Offline
Technical Sergeant
User avatar

Joined: Sun Jun 12, 2011 2:11 am
Posts: 836
Location: Germany
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/Pelicans-Perch-15-Manifold-Pressure-Sucks-182081-1.html There are some others, describing turbo are supercharger behaviour I htink.

_________________
"Give me a ping, Vasili. One ping only, please."


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 12 posts ] 

All times are UTC - 5 hours


Who is online

Users browsing this forum: No registered users and 4 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group