Griffons and Allisons

[kiwiracefan]

Re: Griffons and Allisons

Sparrow, many thanks for your post... i learn something new pretty much every day and its those in the know such as yourself who post with great info and the inside knowledge that keeps me coming back

[wyhdah]

Re: Griffons and Allisons

There is pretty good information about what Sparrow is talking about with the boats over at thunderboats.org. I know Wayne doesn't like hot linking so you'll have to go digging. If you google Hydroplanes and Griffons it should take you there.

[Exhaustgases]

Re: Griffons and Allisons

Just to add the PR100 was used on 4360's.

[MARK SHANE]

Re: Griffons and Allisons

One recurrent theme is that Merlins and Griffons are "fuel injected." That is not true in the injection form of the word. They have various fuel pump configurations, however the fuel is "injected" into the eye of the supercharger through discharge nozzles. Basically it is a 1940's version of throttle body fuel injection. The "carburetor" is an automatic metering device that has an external fuel line that feeds the fuel directly into the blower through the nozzle.


In the boats when they were running Griffons, they were using PR 58 "carburators" on them and they found that the carb was acting as a choke by limiting airflow into the engines as a sizing factor of the air inlets of the carb itself. The PR 58's went away and they put PR 100's on them and it was a whole different animal with all kinds of air for the air compressor. An engine is basically in simple terms an air compressor, which if it's starved for air it won't work up to its full potential.

The exhaust stacks, particularly on Merlins in Mustangs had a great deal of research and developement put into them. They aren't just a random size and shape. They serve a purpose and were developed to produce the greatest amount of thrust out of them as possible. A lot of the "popping" you hear when a Merlin is idling comes from the fact that cool air is being sucked back into hot exhaust valves. There are many more factors than that, however this is one cause of a Merlin barking at idle, in particular when it is hot just after landing.

Sparrow

Thank you for the info Sparrow I never realized that the exhaust also provided thrust thats cool, I was always under the impression that the prop did all the work.

[440_Magnum]

Re: Griffons and Allisons

Thank you for the info Sparrow I never realized that the exhaust also provided thrust thats cool, I was always under the impression that the prop did all the work.

Read up on the history of the Convair 240 (and later versions) "Convairliner." One of its big advances was a very deliberate use of exhaust thrust from its two P&W radials. Exhaust force is also used to aid cooling air flow on the 3350, 4360, (and now the one 2800) powered Sea Furies as well as other radial racers like Rare Bear and Czech Mate. The exhaust stacks are tucked into a cowl air exit and their blast provides both thrust and a venturii effect to suck hot air out of the back half of the cowling, thus providing a bigger pressure differential across the cooling fins on the cylinders, and in turn allowing a tighter cowl-to-spinner gap.

There's been a lot of yakking back and forth over the years about whether Merlin Mustangs use their exhaust thrust to the best possible extent. Some believe that the short stacks provide the most thrust for the least backpressure on the piston. Others argue that since each stack only spends about 1/4th of its total time producing thrust (its a 4-stroke engine, so the exhaust stroke accounts for 1/4 of the time any given cylinder is doing something), it therefore is producing drag by hanging out in the slipstream for the other 3/4 of the time.

I don't know if anyone has actually done computer modelling or in-air testing to prove it one way or the other, and I don't have a clue which might be better. Certainly more races have been won by Merlin Mustangs with conventional stacks, but then many many more have competed with conventional stacks so that doesn't prove one way or the other.

[Donald McMullin]

Re: Griffons and Allisons

Attached is summary of NACA findings regarding Jet-Thrust power from exhaust stacks. I could not attach PDF file because of size restriction.


From NACA - REPORT No. 765
EXHAUST-STACK NOZZLE AREA AND SHAPE FOR INDIVIDUAL CYLINDER EXKAUST-GAS JET-PROPULSION SYSTEM

By BENJAMIN PINKEL, L. RICHARD TURNER, FRED VOSS, and LEROY V. HUMBLE

"Large gains in net thrust can be obtained by use of jet
stacks. This thrust power is proportional to airplane speed
and at 500 miles per hour at a ratio of altitude pressure to
inlet-manifold pressure of 0.2, the jet-thrust power is 27
percent of the engine thrust horsepower."

Deep Enough

[Sparrow]

Re: Griffons and Allisons

So, think about this..... In a Mustang at altitude for instance, take into consideration the "flow" of air through the engine. What would the differential pressure be from the intake manifold and through the cylinders to outside atmosphere via the exhaust stacks. The differential is pretty great from internal "Atmospheric pressure" (from the blower) to what the external atmospheric pressure is in open air. Bet the "flow rate" of air through the air compressor (engine) gets greater the higher in altitude you go. The boost rate from the blower would also affect this.

Also, maybe I'm pickin fly $hit out of the pepper, but to say the exhaust doesn't help that much in thrust cause it's only prevalant 25% of the time? How many exhaust "puffs" are there from every cylinder when the engine (Merlin) is running 2300 RPM at cruise?

Sparrow

[Curt]

Re: Griffons and Allisons

lets check the math
should be 575 pr min x 12 =6,900 Farts Pr Minute

[Sparrow]

Re: Griffons and Allisons

That was kinda the point, that many "pulses" shouldn't be discounted as not of much use. Packard did a lot of R&D on exhaust stack shape and size to arrive at the design. It just wasn't random parts. If I recall correctly, it seems like it was 200 HP derived from exhaust thrust.

Sparrow

[rmudbugs]

Re: Griffons and Allisons

lets check the math
should be 575 pr min x 12 =6,900 Farts Pr Minute

If you ask my wife, thats slightly more than me.

[Sparrow]

Re: Griffons and Allisons

Well, I guess you'll hve that. After all, sometimes quality is better than quantity........

[440_Magnum]

Re: Griffons and Allisons

That was kinda the point, that many "pulses" shouldn't be discounted as not of much use. Packard did a lot of R&D on exhaust stack shape and size to arrive at the design. It just wasn't random parts. If I recall correctly, it seems like it was 200 HP derived from exhaust thrust.

Sparrow

Oh, I agree that the thrust is NOT negligible. I just stated both sides of the debate ;-)

And I think the question is really about whether something could be done to get a little more thrust if the stacks could be faired over, or if a smooth manifold with a single outlet would work. But it seems obvious from looking at the exhaust stains on any Mustang that the flow isn't straight back parallel to the engine in that area, so the downward angle of the stacks is important. A single "log" manifold might actually have MORE drag because of that.

Anyway, its above my pay grade....