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#4628964 - 01/09/18 07:48 AM Grumman F6F Hellcat
supton Offline


Registered: 11/09/08
Posts: 11976
Loc: NH
Stumbled across a training film last night, and I have to confess a bit of ignorance, as a complete non-pilot. When they say 30 inches of manifold pressure, is that essentially no boost at idle, at ground level? (around the 3 minute mark). Later on takeoff manifold pressure was 45-50 inches. I see that 2 inches of mercury is 1psi, but I'm not sure if they subtract off the 15psi at sea level. How do I correspond that into boost that an automotive engine would be?



Also, at 7:35 rudder control is set to 1.5 degree right, instead of neutral. Why is that? [This I suspect will be beyond my understanding.] I suspect that the prop impacts a rotational force on the plane, but I don't see how rudder would be used to counteract that.

Lastly, I like how they use gallons of fuel yet temps are in Celsius. Odd mix of units back then.


Edited by wwillson (01/09/18 09:00 AM)
Edit Reason: subject clarification

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#4628986 - 01/09/18 08:27 AM Re: Grumman F6F Hellcat [Re: supton]
Astro14 Offline


Registered: 10/10/10
Posts: 8540
Loc: Virginia Beach
So, manifold pressure is just that: the absolute air pressure in the manifold.

Now, most conventional gasoline engines, particularly those with carburetors like this one, run at vacuum all the time. If you hook up a vacuum gauge to your car, it would be at positive vacuum all the time, except when at wide-open throttle, and even then with some flow restrictions, it might be at a bit under sea level pressure. So, 30" MAP is just a bit above wide open throttle without boost. IF you were to have a MAP gauge on your car, you might never see 30" because of those restrictions, atmospheric conditions (pressure, temp, etc.) and design. At idle, you might see 5-10" MAP and up to the high 20s when you really get on it...

So, 30" MAP is a lot of air moving through the engine.

Boost takes it from 30" up to 45". Combat power (short use, hard on the engine) might be as high as 50". At Reno, you might see some old warbirds pushed up to 60" of pressure.

You put a bit of right rudder trim in for takeoff to counter the P-factor. At normal cruise power/airspeed, the rudder will be neutral, but on takeoff, the direction of engine rotation and prop angle (upward blade vs. downward blade) pull the airplane to the left. P-factor causes a bit of pitch up, too, but in this airplane, you'll be countering that with the stick for rotation anyway...

And no, the F-14 had none of this...but I've got a few old warbird flight manuals that I've read just for fun...including the F-4U Corsair.


Edited by Astro14 (01/09/18 08:30 AM)
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#4629020 - 01/09/18 09:07 AM Re: Grumman F6F Hellcat [Re: supton]
Linctex Offline


Registered: 12/31/16
Posts: 6148
Loc: Waco, TX
If you are sitting on the ground with the engine stopped and the barometric pressure that day is 29.99 in HG, you will see 30" on the manifold pressure gauge.

Start the engine, at idle you will see ~14 in HG or so (depends)

Full throttle will be about 28" (carb restriction)

Anything above that is "boost"
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#4629030 - 01/09/18 09:17 AM Re: Grumman F6F Hellcat [Re: supton]
supton Offline


Registered: 11/09/08
Posts: 11976
Loc: NH
Still trying to wrap my head around it... I think I know where I'm getting confused, psi versus inches of mercury. Engine vacuum is in inches, while boost can be in inches or psi.
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#4629065 - 01/09/18 09:59 AM Re: Grumman F6F Hellcat [Re: supton]
FowVay Offline


Registered: 06/02/02
Posts: 2679
Loc: Southeastern USA
Standard pressure and temperature yields pressures as follows:
14.7 PSI at sea level
29.92" Hg at sea level

When a normally aspirated engine is running the manifold pressure drops due to the engine drawing in the air and the butterfly valve causing the restriction. If you go wide open throttle you will instantly see the manifold pressure jump UP due to the removal of the restriction (butterfly valve is now open). The manifold instantly assumes atmospheric pressure.

On a boosted engine you will get manifold pressures greater than atmospheric due to mechanical input. Atmospheric pressure is lower the higher that you go in altitude so boosting the manifold pressure helps to overcome air starvation at high altitudes.
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#4629074 - 01/09/18 10:07 AM Re: Grumman F6F Hellcat [Re: supton]
Yah-Tah-Hey Offline


Registered: 05/26/14
Posts: 3409
Loc: Columbus,Nebraska
I remember when I was working on my P rating from the FAA that an instructor was talking about how much boost the Pratt & Whitney 4360 radial engine could produce. Believe he said the engine had the capability of blowing the jugs off at take off if the flight engineer didn't have things set up correctly.Also remember what a pita it was trying to set up the timing with all those magnetos and cylinders. Shouldn't have been *%$#@^& around with recips anyway as the airlines had changed over to gas turbines years ago.

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#4629091 - 01/09/18 10:25 AM Re: Grumman F6F Hellcat [Re: supton]
Win Offline


Registered: 02/05/03
Posts: 4448
Loc: Arkansas
Originally Posted By: supton
... rudder control is set to 1.5 degree right, instead of neutral. Why is that? .... I suspect that the prop impacts a rotational force on the plane,


P factor - It's a big deal on light piston twins - it makes one engine "critical". Some light twins had counter rotating props to try to mitigate this. The Twin Commanche comes to mind. https://en.wikipedia.org/wiki/Critical_engine

That Hellcat probably has so much P factor that you need trim help to manage it.
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#4629102 - 01/09/18 10:33 AM Re: Grumman F6F Hellcat [Re: supton]
OneEyeJack Offline


Registered: 09/14/10
Posts: 7479
Loc: S California
Did you know that the skin of the F6F was layed out to use the source aluminum panels with as few cuts as possible? The sheets of aluminum were supplied to size without requiring a trim cut and the trim was handled once at the end of the section being covered. It was not unusual to finish a section and not have to trim the last panel at all. Regardless of how well the F6F flew, it was a masterpiece of mass production. It went together on the production like a preplanned kit, which it was. The F6F was one of the first fighters that started life with exactly the power and thrust it was designed to have. It did not receive an underpowered unreliable powerplant to start its production life like almost every other fighter that the US produced. This I idea of getting a good, powerful and reliable engine was a real game changer for planes like the F6F and F4U and other propeller planes to follow. When the jets came out it was back to underpowered, unreliable powerplants once again.

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#4629134 - 01/09/18 10:55 AM Re: Grumman F6F Hellcat [Re: FowVay]
supton Offline


Registered: 11/09/08
Posts: 11976
Loc: NH
Originally Posted By: FowVay
Standard pressure and temperature yields pressures as follows:
14.7 PSI at sea level
29.92" Hg at sea level

When a normally aspirated engine is running the manifold pressure drops due to the engine drawing in the air and the butterfly valve causing the restriction. If you go wide open throttle you will instantly see the manifold pressure jump UP due to the removal of the restriction (butterfly valve is now open). The manifold instantly assumes atmospheric pressure.

On a boosted engine you will get manifold pressures greater than atmospheric due to mechanical input. Atmospheric pressure is lower the higher that you go in altitude so boosting the manifold pressure helps to overcome air starvation at high altitudes.


Right, I recall engines developing 15-20 inches of negative pressure at idle, and zero at WOT (give or take); and then boosted engines developing 5-15psi of boost at full tilt (max boost pressure as regulated by the design). 15psi of boost is 2x the airflow through the engine and therefore 2x the power output of an NA engine (give or take).

It's the units and absolute vs gauge is throwing me.
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#4629173 - 01/09/18 11:30 AM Re: Grumman F6F Hellcat [Re: Win]
440Magnum Offline


Registered: 02/01/09
Posts: 8754
Loc: Texas
Originally Posted By: Win
Originally Posted By: supton
... rudder control is set to 1.5 degree right, instead of neutral. Why is that? .... I suspect that the prop impacts a rotational force on the plane,


P factor - It's a big deal on light piston twins - it makes one engine "critical". Some light twins had counter rotating props to try to mitigate this. The Twin Commanche comes to mind. https://en.wikipedia.org/wiki/Critical_engine

That Hellcat probably has so much P factor that you need trim help to manage it.


P-factor is a huge factor on most WWII era fighters. The P-51 Mustang can be flipped on its back if too much power is applied at low airspeed, when the rudder and ailerons don't have enough authority to counteract the gyroscopic forces from the prop. There's a placard in the cockpit stating the maximum manifold pressure allowed below a critical airspeed. Go-arounds are the riskiest time because the reflex is to just throw all the power available at it... but if the speed is too low, Very Bad Things (tm) happen.

I think the Bearcat is similar, not sure about the bigger ones like Thunderbolts and Hellcats.
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#4629189 - 01/09/18 11:45 AM Re: Grumman F6F Hellcat [Re: supton]
ArrestMeRedZ Offline


Registered: 11/11/08
Posts: 933
Loc: Las Vegas
Originally Posted By: supton

Lastly, I like how they use gallons of fuel yet temps are in Celsius. Odd mix of units back then.


WWII US fighters used gallons, modern US jet fighters use thousands of pounds of fuel. A fuel reading of 7.6 over 10.4 would indicate 7600 pounds in the fuselage tanks with 10400 pounds the total internal fuel. External tanks usually aren't calibrated so you only have an idea of their fill status by time in flight.

Temps in Celsius make more sense for a lot of reasons. One is the standard rule of thumb that outside air temps drop 2 degrees C for every thousand foot gain in elevation. Works great for Celsius, not so easy for Fahrenheit. Not sure what the Russians use with altitude in meters.

Modern aircraft use nautical miles for distance measurement, and nautical miles per hour (knots) for speed. A nautical mile is approximately 6080 feet, unless it's a radar nautical mile which is 6000 feet. Is that confusing enough?

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#4629224 - 01/09/18 12:18 PM Re: Grumman F6F Hellcat [Re: Win]
cjcride Offline


Registered: 11/06/09
Posts: 1815
Loc: Ontario Canada
Originally Posted By: Win
Originally Posted By: supton
... rudder control is set to 1.5 degree right, instead of neutral. Why is that? .... I suspect that the prop impacts a rotational force on the plane,


P factor - It's a big deal on light piston twins - it makes one engine "critical". Some light twins had counter rotating props to try to mitigate this. The Twin Commanche comes to mind. https://en.wikipedia.org/wiki/Critical_engine

That Hellcat probably has so much P factor that you need trim help to manage it.

The Cessna Skymaster appeared to be the safest prop twin to have 1 engine fail.

I read the noise level was quite high so sales never really took off.


Edited by cjcride (01/09/18 12:31 PM)

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#4629439 - 01/09/18 04:03 PM Re: Grumman F6F Hellcat [Re: supton]
bubbatime Offline


Registered: 03/18/08
Posts: 5627
Loc: South Florida
That was a powerful airplane for the time. I remember reading that it was so powerful that things could really go bad for the pilot if they over did it. Like torque steer would rotate the plane right into the drink if you left the deck with full throttle.
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#4630405 - 01/10/18 12:02 PM Re: Grumman F6F Hellcat [Re: cjcride]
billt460 Offline


Registered: 03/30/15
Posts: 4411
Loc: Glendale, Arizona
Originally Posted By: cjcride
The Cessna Skymaster appeared to be the safest prop twin to have 1 engine fail. I read the noise level was quite high so sales never really took off.


I don't believe you could get your multi engine rating in that airplane, because of it's centerline thrust. Flying it did not teach you how to handle asymmetrical thrust from having an engine out on one side.

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#4630748 - 01/10/18 05:55 PM Re: Grumman F6F Hellcat [Re: billt460]
Astro14 Offline


Registered: 10/10/10
Posts: 8540
Loc: Virginia Beach
Originally Posted By: billt460
Originally Posted By: cjcride
The Cessna Skymaster appeared to be the safest prop twin to have 1 engine fail. I read the noise level was quite high so sales never really took off.


I don't believe you could get your multi engine rating in that airplane, because of it's centerline thrust. Flying it did not teach you how to handle asymmetrical thrust from having an engine out on one side.


That's true, and in fact, there are other airplanes in which you can get a multi-engine rating but the words "limited to center thrust" are included as a restriction on your rating.
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