Allison engines vs Merlins for racing

[Samuel]

Re: Allison engines vs Merlins for racing

Some of the Allison's used for the Hydroplanes had an axillary stage supercharger. Would this be a viable option?

Supercharger: The engine supercharger (second stage supercharger) is contained in the accessory housing and is driven from the flexible inner member of the hydraulic vibration damper. The impeller is 10.25" diameter with 15 vanes and includes a separate rotating reverse-curved inducer guide vane inlet guide, the relationship with the impeller being maintained by the common splined shaft. The diffuser is cast integrally with the supercharger cover, which also contains the inlet to which the injector throttle body mounts. The impeller is overhung, with the shaft supported by two floating lead-bronze steel backed bearings placed on both sides of the supercharger drive gear. The bearing are pressure lubricated with engine oil.

The auxiliary stage is contained in a separate assembly coupled to the engine accessory section. It is intended to provide a critical altitude of 25,000 ft by delivering air to the engine supercharger at pressures close to sea-level when at critical altitude. This requires large volume of low density air to be handled by the auxiliary supercharger -- because of the density of the air at 25,000 ft the supercharger must move 2.23 times more air volume than the engine supercharger, and compress the air to about 2.7 times to deliver sea-level conditions. The auxiliary stage drive was obtained by a power-takeoff from the starter gear which connects to a driveshaft incorporating a universal joint, the driveshaft being contained in a tube coupling the engine accessory section with the remote auxiliary stage. The driveshaft hydraulic torque converter connected to the step-up gears contained in the auxiliary supercharger housing. The torque-converter provides variable speed for the supercharger by varying the amount of oil and therefore the coupling of the torque converter. The speed controlled by a boost-regulating system, permitting infinitely variable control of the speed of the auxiliary stage which was used to control manifold pressure so that power could is controlled while the throttle remains wide-open. The auxiliary supercharger consumes 490 hp from the crankshaft at the 2250 hp WER rating.

http://www.thunderboats.org/history/history0323.html

[Arctic Cat]

Re: Allison engines vs Merlins for racing

Jack Sandberg experimented with a pair of Garret turbochargers on the Allison in Tipsy Miss in 1974, but they never ran in competition.

A few years back I was working with Frank Taylor after he had bought Dago Red from the Costo's. He was open to all sorts of suggestions, including experimenting with a turbocharged Allison--to the point where I pointed him in the direction of Ed Cooper Jr., who built and ran possibly the strongest turbo Allisons ever run in the Unlimited Hydroplanes. I know Frank did a lot of feasibility discussion with Ed about getting a few for Dago.

At the same time I was talking to Kerch about his thoughts on running a turbo Allison in a Mustang since he had worked on J.R.'s P-63. He said that it certainly had possibilities, but thought that he figured you'd have to run at least three turbochargers PER BANK in order to get comparable horsepower of an Allison rod Merlin. And in the voice that only Kerch can muster he said "Where would you put it????"

The technology of turbos have come a long way since 1974! I will say that you couldn't just "slap a pair on" it would have to be a part of a whole engine management system. Electronic fuel injection, electronically controlled ignition etc.

[440_Magnum]

Re: Allison engines vs Merlins for racing

As noted on another thread, Bruce Lockwood wrote an excellent article in AirClassics about the "Tube" Merlin; and he also mentions the amount of thrust produced by the exhaust of that type of engine setup. So, my question is: would not a turbo-ed set up loose much of this exhaust thrust?

I'm an electrical guy, not a thermo or aero specialist. But from the little thermo and fluids I've had, I would think that an internalized exhaust system (no drag from 12 stacks hanging in the breeze) with a lower-velocity exit that is faired in and dumps the exhaust flow at the right place on the airframe to reduce afterbody drag (see earlier comments about jets) could potentially be a better overall package than the stacks, even if stacks produce more thrust. Getting more torque to the prop and reducing drag could conceivably more than offset the lost thrust. Just a SWAG....

Now, where are you going to PUT that piping in an airframe that already has the cylinder heads so closely cowled that the stacks poke out the sides of the cowl...???? Different question.

Nothing can be viewed in isolation. An airplane (any machine, really) is simultaneously optimizing many equations in many variables.

[L.E.D.]

Re: Allison engines vs Merlins for racing

I would think that the exhaust thrust is a bigger win as it is. Given that the exhaust pressure is constant, the power delivered to the airplane by the exhaust stacks increases with speed, without requiring the prop to spin faster.

The prop is already close to the limit of the crankshaft power it can absorb, so any extra made available by removing the supercharger would go to waste.

[440_Magnum]

Re: Allison engines vs Merlins for racing

I would think that the exhaust thrust is a bigger win as it is. Given that the exhaust pressure is constant, the power delivered to the airplane by the exhaust stacks increases with speed, without requiring the prop to spin faster.

The prop is already close to the limit of the crankshaft power it can absorb, so any extra made available by removing the supercharger would go to waste.

Perhaps so, but my gut still says that there's a lot of stack drag, especially since each individual stack only produces one impulse of thrust for approximately 1/2 turn of the crank out of every two rotations of the crank, and its just hanging in the breeze making drag all the rest of the time. Roughly speaking (an exhaust pulse isn't exactly 1/2 turn of the crank...) 25% of the time its thrusting, 75% of the time its dragging. If the stacks could be smoothly blended into a single pipe with a single exit from the airframe on each side, and all done without adding excessive back-pressure, would the drag/thrust ratio from the exhaust system be better? Even comparing non-turbocharged with stacks to non-turbocharged with collectors?

[Curt_B]

Re: Allison engines vs Merlins for racing

Perhaps so, but my gut still says that there's a lot of stack drag, especially since each individual stack only produces one impulse of thrust for approximately 1/2 turn of the crank out of every two rotations of the crank, and its just hanging in the breeze making drag all the rest of the time. Roughly speaking (an exhaust pulse isn't exactly 1/2 turn of the crank...) 25% of the time its thrusting, 75% of the time its dragging. If the stacks could be smoothly blended into a single pipe with a single exit from the airframe on each side, and all done without adding excessive back-pressure, would the drag/thrust ratio from the exhaust system be better? Even comparing non-turbocharged with stacks to non-turbocharged with collectors?

Externally, the stacks could be cleaned up by incorporating a little trailing edge to the last stack. Since they are all lined up, the air only sees one stack per side as external drag essentially. Now when the valve's closed, the pressure in the hole, or base pressure, would determine internal drag of each stack. How rapidly that pressure changes between open and closed valve would need to be determined, and whether the closed position has time to reach base pressure levels before the next open. Don't know if they make pressure transducers that can operate that rapidly, and would think you'd need multiple transducers in a stack to see if the pressure is even throughout.

Or you could just weld it all up, and fly. Lol. Would think some stacks would be out of tune though.

[grampi]

Re: Allison engines vs Merlins for racing

Larger Diameter = greater prop tip speeds = not good

It couldn't be geared down so as to turn less RPMs?

[L.E.D.]

Re: Allison engines vs Merlins for racing

It couldn't be geared down so as to turn less RPMs?

Most of the fast Mustangs are geared differently than stock - here's a really detailed discussion of the nose case on a Merlin:

http://www.enginehistory.org/GfG/gearing.htm

[Sparrow]

Re: Allison engines vs Merlins for racing

The "stock gear ratio's for Packard engine's are .479 and .420. The "stock" gear ratio's for Rolls is .471 and .420. There have been custom gear ratios made at .361 and .383. The .383 ratio lasted almost one lap for Tiger at the Denver Air Race. .383 gears work fine at high RPM and low MAP, they didn't like 3450/120", failed the pinion gear. We had a gear set made at another ratio that was designed for 6000HP which is a 30 % safety margin. Ran several races with them, no issues.

[440_Magnum]

Re: Allison engines vs Merlins for racing

We had a gear set made at another ratio that was designed for 6000HP which is a 30 % safety margin. Ran several races with them, no issues.

You KNOW I think that's the coolest thing anyone's done, right? . Hope you get the opportunity to put that setup out there again, I really think its right for way more than a handful of reasons. And I'm probably ignorant of a lot of other reasons...

[440_Magnum]

Re: Allison engines vs Merlins for racing

Since they are all lined up, the air only sees one stack per side as external drag essentially. .

Lots of good stuff in your post, but I want to talk about this one. They're "all lined up," I agree. But they all point slightly down, which leads me to think that whoever designed them that way thinks the local airflow is slightly downward across the bank of stacks. So maybe the airflow "sees" more than just one?' And to complicate things, looking at the heat bluing and soot stains on some pictures (these dang museums keep their birds so clean its hard to find a good sooty picture these days!!), it actually looks like the local airflow sweeps UPWARD aft of the cylinder banks....

I'm not sure that what's being used could really be improved on enough to matter, but it sure SEEMS like something weird is going on with down-canted stacks and an up-swept heat mark on the fuselage.

[Curt_B]

Re: Allison engines vs Merlins for racing

Lots of good stuff in your post, but I want to talk about this one. They're "all lined up," I agree. But they all point slightly down, which leads me to think that whoever designed them that way thinks the local airflow is slightly downward across the bank of stacks. So maybe the airflow "sees" more than just one?' And to complicate things, looking at the heat bluing and soot stains on some pictures (these dang museums keep their birds so clean its hard to find a good sooty picture these days!!), it actually looks like the local airflow sweeps UPWARD aft of the cylinder banks....

I'm not sure that what's being used could really be improved on enough to matter, but it sure SEEMS like something weird is going on with down-canted stacks and an up-swept heat mark on the fuselage.

Lol, it's all logic, and packaging, and operational considerations. The local airflow is upward because the lift of the wing produces streamline patterns that far forward. They're canted down for clearance I'd imagine, and for operational considerations like preventing roasting the plexiglass. What they wouldn't have realized is that the downward angle alleviates trim drag a little too. Some things just all line up, intentionally or unintentionally, to give a cool airplane.

Drag is produced by pressure integrations on a surface. The exhaust is air, as opposed to a surface, so a pressure integration on the exhaust/jet stream wouldn't exist as drag on the airframe. The OD of the last stack should be a little larger, with a nice trailing edge to capture the turbulent flow coming off the forward stacks for streamlining.

[mayday51]

Re: Allison engines vs Merlins for racing

Curt,
Would you mind doing a quick pic/illustration (don't fret quaity,just convey idea) my weak mind just can't quite assemble it all.

Thnx !

[Sparrow]

Re: Allison engines vs Merlins for racing

Packard spent a lot of time on R&D with those exhaust stacks. If I recall, they provide 200 HP worth of thrust. Sparrow

[John]

Re: Allison engines vs Merlins for racing

Packard spent a lot of time on R&D with those exhaust stacks. If I recall, they provide 200 HP worth of thrust. Sparrow

That generation did an incredible job(back in the day) with the design elements of the WW2 fighters, it never ceases to amaze me.