rpm vs. man. pressure

[xtinct]

Hello,
what's the difference between rpm and manifold pressure when it comes to use throttle? When (if evre) do I need to adjust rpm in flight since my throttle only adjusts the man pressure.

Thanks for your help
xstinct

[Soda]

You can adjust the rpms with the +/- keys on the numberpad as a separate thing from the throttle (which u may have on your stick or may be using the =/- keys on the main part of the keyboard).  Generally, I don't think many people adjust rpms except under very certain conditions, gliding being one of them.  Most of the aircraft will glide far better with reduced rpms so if your engine is off and you want to glide, press and hold the - key on the numberpad until you see the rpms drop.  When you start your engine again you can press + on the numberpad, or, you can press WEP (P) for a second and that will increase your rpms back up to full.

reducing rpms should also impact fuel use, but that is a combination of rpms and throttle position.  I have some numbers I tried on the Typhoon though and the loss in speed from reducing rpms or throttle did not help extend range.  It helped extend duration of engine run-time, but not range, because you were slower.

Hope that helps,

[Patches]

Another area where reducing throttle and RPMs may come in handy is when escorting bombers to and from a long distance target...

[Kweassa]

Generally, put your throttle at 75~80%, with the RPM around just 2000.. it may be placebo, but with that settings, a whole lot of fuel is conserved, it seems to me.

[Tarmac]

Certain (few) planes on the HTC help pages have different man and RPM settings for different things.  The 190A8 is the one I can think of off the top of my head.  

I think it's 33" man 2100rpm for max range in the A8.  
http://www.hitechcreations.com/ahhelp/models/models.html

And I think it's only the original planes that have these engine settings listed.

[Munkii]

I have to agree with Kweassa on this one, I was limping home with less than 25% fuel in both my RW and my Main tank in an F4U today, I was crusing about 325 so I dropped RPM's to 2k and manifold pressure to about 40lb's and I made it almost 2 sector's, tried to taxi to the rearm pad on landing.  Then Iran outta fuel about 20 feet away.  

I don't think I would have made it had I cruised at a higher RPM and Manifold pressure.

BTW I only lost about 25 TAS when I dropped the throttle and RPM's.




edit: fixed kweassa's name

[ccvi]

if you change both manifold pressure and rpm, you can't tell whether the increase in range is from reducing manifold pressure or rpm.

[Soda]

Just for reference, I ran a number of tests like this on the Typhoon for the current release of AH and it showed that range was maximized at full throttle (or even WEP) and max rpms.  Fuel duration was shortest but the loss in speed from lesser settings lead to an overall loss of range.  Strange, it surprised me and maybe I didn't consider all the potential settings, but nothing gave me a longer range than WEP and full rpms in the Typhoon.

Data:
Typhoon
RA/LA: 13:30 (6:45 with only 1 tank burned)
tested at sea-level

rpms       Man         Speed   Run Duration   Range
3.6 (full)  9 (WEP)   370          6:45             41.64miles
3.6 (full)  8 (full)      355          6:45             39.993 miles
3.5          8 (full)      350          6:45             39.26 miles
3.6 (full)  6              327          7:15             39.558mile
3.5          7              331          7:05             39.079miles
3.0          6              303          7:50             39.512miles

You can see what happens, reduced throttle appears to extend fuel duration but the reduced speed hurts total range.  I tested the durations at several throttle settings and it seems 100% was 6:45, 75% 7:18 and 50% 11:13.... at 50% though it was difficult to keep the plane airborne.

I think some planes do better (F6F?  Maybe Levthan had some numbers on that) but the Typhoon was better to run at full throttle.  This was a pretty quick and dirty test though but I was careful to adjust the fuel burn modifier in order to get accurate numbers.  Normally I would have agreed with Kwessa et al on this, but my tests of the Typhoon didn't show what I thought they should.

(sorry about the crummy chart above... it's kinda hard to read)

[Ghosth]

Only time it really pays IMO to drop RPM is if you have a dead engine and a ways to glide to reach home.

Feather the engine, set autoclimb (will keep the plane at its best glide speed) and let it glide.

I've done a sector & a half on 10k in a la7.
B17's will also glide a sector on 10k.

[Patches]

With reference to WEP and full throttle....sure...if you have enough altitude and very little fuel, and are far from home, I agree, burn it on WEP...get as close as you can, as quickly as you can...and when out of fuel...minimize RPM's and glide as far as you can from your present altitude.

What I was aluding to in my previous post was that fighters can extend their escort time and range by watching altitude and fuel consumptions rates by timing their climbs to altitude with the bombers, and when up high...20k or so, reduce throttle and RPMs to match the bombers...so that the fighters are not always running full throttle and full RPMs which they don't need once above the bombers and in a good bounce position....not sure this makes any sense....I think I know what I want to say, however, I'm not sure I'm saying it properly.

Perhaps I am over-generalizing...dunno

[Soda]

Patches,
  The tests I ran, mind you they were only for the Typhoon, showed that duration of engine run time was improved by throttling back.  What they didn't show though was any improvement in range.  Basically, if you had to fly the maximum range then you wanted to run at full throttle (and WEP)... so if you were low on gas, WEP and run for home, don't lean it out and try to cruise back slower.  This was basically opposite to what I would have expected but that's what it seemed like at least in the Typhoon.

So, for bomber escort, throttling back would not help other than to make it easier to keep formation with the bombers.  It might extend your loiter time, which might be a concern, but if range was the issue then you better just run at full throttle.  Typically on escort you are better to keep your speed up nice and high anyway so you can react to things better.  Making a zig-zag path at full speed gives you more options to counter the enemy than just throttling up before combat.  The only time I cruise in escort at less than full throttle (not WEP mind you) is when I want my engine to cool.. then I cut the engine, reduce rpms, and glide a bit to let the engine cool off quickly.

[empty]

The aircraft modeled in the game have constant speed propellers.  The prop turns at a set RPM based on the pitch of the propeller blades.  Reducing the RPM increased the AOA (angle of attack) of the blades.  This is why reducing the RPM for gliding improves the glide ratio.  The Manifold pressure number is an indicator of how much power is being produced by the engine and will vary with altitude and turbo/supercharger speeds.  An example of that is the P51D/B.  These two aircraft have similar engines but the supercharger shift points are different.  The P51D switches to hi-boost at 13K and the P51B at 16K.  Watch the manifold pressure as you climb through these altitudes. (I may have the reversed.)

There are optimal settings for the aircraft depending on the flight mode.

For combat you should fly with you RPM at MAX.  If you are cruising and/or need to conserve fuel a reduced RPM and throttle setting will help.

I don't have specific data; however, reducing RPM 50% (generally 2000-2400 RPM depends on airplane and range of adjustment available) and reducing manifold 40-50% results in significant fuel savings.

[mia389]

Originally posted by empty
The aircraft modeled in the game have constant speed propellers.  The prop turns at a set RPM based on the pitch of the propeller blades.  Reducing the RPM increased the AOA (angle of attack) of the blades.  This is why reducing the RPM for gliding improves the glide ratio.  The Manifold pressure number is an indicator of how much power is being produced by the engine and will vary with altitude and turbo/supercharger speeds.  An example of that is the P51D/B.  These two aircraft have similar engines but the supercharger shift points are different.  The P51D switches to hi-boost at 13K and the P51B at 16K.  Watch the manifold pressure as you climb through these altitudes. (I may have the reversed.)

There are optimal settings for the aircraft depending on the flight mode.

For combat you should fly with you RPM at MAX.  If you are cruising and/or need to conserve fuel a reduced RPM and throttle setting will help.

I don't have specific data; however, reducing RPM 50% (generally 2000-2400 RPM depends on airplane and range of adjustment available) and reducing manifold 40-50% results in significant fuel savings.

I was waiting to see the words" constant speed props" Just went over all of this stuff tuesday in class. Im currently taking my PPL.
FBsmokey

[Blue Mako]

If you are running low on fuel the best way to maximise range is to climb on full throttle at best climb speed until you run out of fuel (I always keep a little bit in the tank for the approach and landing).  Shut down the engine, reduce engine RPM and leave auto climb autopilot on.  This will give you the best glide angle = furthest gliding distance.

This gives you better range than simply going level and WEPing as the higher alt gives you better fuel economy and also a higher groundspeed (note wind layer at 16k ft!) due to the fact that the TAS you acheive is higher than lower down.

[KurtVW]

I can't speak for AH, being new here... But, as a real world pilot I can tell you that basically, the RPM controls are much like the transmission in your car.

High RPM will deliver superior climb and accelleration performance, and lower numbers (within limits) will provide superior fuel economy.

Manifold pressure can be compared with your foot on the gas pedal in a car.  More power, and greater fuel consumption.

So, High RPM and High Manifold pressure is much like driving your car in first gear with the gas pedal floored... You'll get mean accelleration or hill climbing, but you are working the engine to death.

Medium settings are like highway cruising in your car.. These settings vary from one aircraft to the next.

Generally speaking, reducing RPM and manifold pressure while in straight and level cruise flight by 10% or so should manifest fuel savings.

During a dog fight, you will definitly want full RPM and power as needed.

Hope that helps!

Kurt