Ram Effect

[funkedup]

I just realized that if you take an AH plane above rated altitude, manifold pressure will increase with airspeed.  Neato.  

[R4M]

oh.....  

I was thinking you were about to talk about me and my wonderful effect in this game  

BTW, Manifold gauges in LW planes still don't change with altitude. -----------> Had to hijack yet another thread! MUHAHAHAHAHAHAHAH  

[Tac]

I thought the alt monkey allied pimps shot them down before they got to altitude  

[funkedup]

Yeah I only tested this on the P-51B.  I'll check some others.

I'm also doing some testing vs. real world fuel consumption charts.  So far HTC engine model is looking pretty impressive.  

[Glasses]

Funked when you get the results could you post some Manifold /RPM settings for Endurance in the tested aircraft?

Since I asked even Pyro last time and he was "too drunk" to say anything about Vy ,Vx or fuel endurance.  

[funkedup]

P51-D, Sea Level Aces High (10x Fuel Multiplier) Manual
RPM MP Gallons Time Speed GPH MPG Speed GPH MPG
3000 61" 85 205 355 149 2.4 N/A 182 N/A
2700 46" 85 317 325 97 3.4 315 86 3.7
2500 43" 85 390 305 78 3.9 300 74 4.1
2150 40" 85 499 285 61 4.6 280 61 4.6
1800 36" 85 605 255 51 5.0 255 51 5.1
1600 30" 85 783 200 39 5.1 220 39 5.6

[ 12-12-2001: Message edited by: funkedup ]

[Seeker]

What does this mean and why is it interesting?

<areoengineeringly challenged.....>

[funkedup]

It's interesting because it shows you which engine settings to use for maximum range, and it shows that at least on the P-51D, HTC have a quite accurate model of power and fuel consumption with respect to manifold pressure (throttle) and RPM.

The chart is a comparison of real life P-51D fuel consumption and AH P-51D fuel consumption, at sea level, full internal fuel load.

1st column is RPM, 2nd column is Manifold Pressure in inches Hg.  These settings are right out of the manual for the airplane.

3rd column is how much fuel I burned in my test (I burned off the fuselage tank), 4th column is the time to burn off the fuel (in seconds), 5th column is my TAS in mph, 6th column isis gallons per hour, adjusted for the 10x fuel multiplier, 7th column is miles per gallon, also adjusted.

8th, 9th, 10th columns are TAS, gallons per hour, and miles per gallon from the manual for the airplane.  

As you can see there is some variation between AH and the manual, but not a heck of a lot.  Looks like a very nice job by HTC of matching historical performance in this area.

[ 12-12-2001: Message edited by: funkedup ]

[funkedup]

It just occured to me that Seeker might have been asking about the ram comment.

The air intake on the Mustang (under the prop spinner) was designed (like a lot of other WWII planes) to use the speed of the plane to "ram" air into the engine.  This increases manifold pressure and gives more power.

Above rated altitude, the supercharger can no longer provide the maximum allowable manifold pressure (61").  So the ram effect really comes in handy to increase power.  And in AH you can see the manifold pressure increasing as you speed up above rated altitude, which indicates that AH is calculating the manifold pressure increase due to ram effect, which says something for the engine model.

[Seeker]

Thanks Funked.

One more thing though, do super chargers have waste gates, or blow off valves? How do you avoid over pressurizing the system in a high speed dive?

[funkedup]

Merlins in the Mustangs in AH (V-1650-3 and V-1650-7) had an automatic manifold pressure regulator on the carburetor which automatically varied (opened and closed) the throttle butterfly to maintain constant manifold pressure as set by the pilot's "throttle" lever.  So if too much air was being rammed in a dive the regulator would gradually close the butterfly until the manifold pressure was at the desired level.

I think some of the turbosupercharged planes might have had a wastegate.  I'll look into it.

[Seeker]

That's exactly what I meant, Funked. All the turbo engines I've ever seen have some kind of waste gate to avoid over boost, and it suddenly struck me that I can't remember a reference to a similar facility in a super charged engine, which begs the question: why not?

[funkedup]

In airplanes it's a really bad thing for range and endurance to dump unburned fuel.  And except for injected engines like BMW 801 or DB or Ash-82FN I think they generally added fuel upstream from the supercharger(s).

[ 12-12-2001: Message edited by: funkedup ]

[Seeker]

You're obviously right with regard to the importance of economy on those engines, but even F1 turbo cars have waste gates, and their fuel is measuerd pretty much to the metre.

Odd, isn't it?

[Staga]

One quite big difference between turbo-chargers and mechanical driven blowers is superchargers' rpm (and thus airflow thru charger) is dependable about engines rpm (well there's two-stage/two-speed and variable speed like in Daimler-Benz's engines 'cause engines need to adjust to different heights/air pressures) when pressure turbo-charger gaves depends how much exhaust gas engine produces.
"Pop-Off valves" and WasteGates usually works when you cut the throttle in turbo-charged engines when turbo-charger is stil giving full pressure thus dramatically increasing pressure in inlet manifold.

btw co-worker use to drive a Volvo 142 which was faster in 1/4 mile than Porsche 911... That car was a real "sleeper" with Garrett turbo and water-injection