A-36 Apache

[Iskandar Taib]

What I'm getting at is that you can't turn off the second blower stage. What you CAN do is change the gears so that the two stages run faster (at high altitude) or slower (at low altitude). The blower absorbs a lot of power that can otherwise go to the prop. At low altitudes, you use the low gear because, if you don't, 1) you get more boost than you need, and 2) the amount of added power you generate doesn't make up for the amount you lose turning the blower. The reason you have two stages is that at high altitudes you need them to get the needed boost. But you have to gear them lower at lower altitudes.

So it would make sense, if the second blower stage didn't really give you an advantage close to the ground, to delete it if you're not flying at high altitudes. But in fact, it does. Note that the two stage Merlins gave more output than the single stage ones even at low altitudes. Part of this had to do with the intercooler, part of it that the two stage Merlins were built stronger to take the added boost. So the Merlin should have had higher output than the Allison did regardless of the altitude - just the fact that it needed a bigger radiator testifies to that. But even with less power the Allison Mustang was as fast or faster up to 10,000 feet. Why? I suspect the inlet scoop area had something to do with it. The Meredith effect helps, but it can't cancel out all of the radiator intake drag, and intake drag would be severest at low altitude.

 

[Anonymous]

From what I understand, you never get as much thrust out of the Meredith Effect as it takes to overcome the drag of the radiator inlet. You can recover quite a bit, but it is never zero sum or better. In any case, there is only so much thrust that can be generated out of the heat produced by the engine. Make the inlet larger than is optimal, and you get more drag. The inlet size really does matter - the bigger it is, the bigger the drag. You make it just as big as it needs to be to cool the engine, no bigger. If the size didn't matter, then you could make the inlet as big as you wanted. This also applies to radials, by the way.

See:

http://www.supercoolprops.com/ARTICLES/gwhite.htm
http://www.supercoolprops.com/ARTICLES/ducted_cooling.htm

I don't think the Allison was making more power than the two stage Merlin, even on the deck. If they did, people would be dumping the two stage supercharger for racing! Instead, they finesse the inlet, and spray liquid on the radiator core. Incidentally, Supercool is wrong about the Spit IX. It did use the Meredith effect (see Quill's book), though the radiator design wasn't as efficient as on the Mustang, and there were other sources of drag. The Mustang manages to hide most of the bulk of the radiator core inside the fuselage - it takes up a large amount of space below and behind the pilot, while the scoop and inlet represent a comparatively small "bump" in the fuselage profile.

I have to say I'm dubious of the articles above (especially the first one). It claims the meredith effect was tweaked in the Cal Tech wind tunnels. The effect cannot really be evaluated in a static test of that nature. The engine must be running and the cooling system operational for the effect to occur. Certainly the shapes were studied in the wind tunnel, but if you research the P-51 development you will see these guys did most of their work utilizing mathematics rather than physical tests. Physical tests were conducted to confirm or refute the mathematical conclusions, but the process was not one of trial and error as implied.

Also, he seems to take Lee Attwoods story about the P-51 at face value. If you study the P-51, you will see that there was a scism of sorts between Atwood, who was an excutive on the P-51 team, and Edgar Schmued and Ed Horkey, who were the actual designers. Attwood tried every trick in the book to claim the P-51 design features were his ideas, but the truth seems to indicate otherwise. For instance, he did not make claim of having brought the info about the "Meredith Effect" to the team from the British researcher F.W. Merdith done in 1935 until after Edgar Schmueds death in (I think) 1986. But this is refuted by the fact that the heat pump design of the P-51 cooling system had been on the table at NAA for years within Schumed's design team, long before Atwood returned from his sales trip to England.

It is kind of telling that Atwood waited until after Schmued's death to start making his claims as "father of the P-51". Ed Horkey has disputed Atwoods claims from the moment he made them. I tend to believe Schmued was the force behind the P-51's design, not Atwood. Atwood's baby was the B-25 - a plane sorely lacking in any kind of innovation. Schmued's babies, the P-51 and the F-86 were loaded with innovations, and Schmued was the one whose team was designing hypothetical fighters back when they were tasked to design the AT-6 trainer when Atwood got the B-25 project. Atwood had no such reputation of zeal to design a fighter - he was more of... a businessman/salesman.

You can get a glimps into the whole debate by reading the following articles:

http://www.airspacemag.com/asm/mag/supp/jj99/Mustang.html

http://www.airspacemag.com/ASM/Mag/Index/1996/AS/wmtm.html

-------------------------------

I certainly agree there is an optimal size for the scoop. My understanding is that the radiator thrust system overcame 90-100% or more of the cooling system drag, depending on altitude (pressure), ambient temperature, and how hot the particular plane was setup to run. Sometimes they used a Prestone formulation that allowed temps up to 350 degrees F, but these were highly caustic and hard to handle for ground crews, and usually lower temp solutions were utilized.

From the Spitfire thread:

-----------------------------------

First off lets look at the Bf109 scoop/cooling design:

As you can see the radiators are indeed quite small. The "boundary layer diverter" mechanism was a fix for a problem discovered on the E models. The radiator is mounted to the bottom of the scoops and the boundary layer is allowed to flow through a space between the top side of the radiator and the wing. This helps to avoid injestion of the turbulent boundary layer which makes the radiators more efficient than they would be if there were larger but no space was provided for the boundary layer. Because the boundary layer has to make a significant turn upward to follow the diverter, this is only partially effective and at high speeds as the boundary layer gets thicker and has more mass and as pressure builds in the scoop, the boundary layer still lifts off the bottom of the wing and around the scoop entirely resulting in the "gulping" effect. The boundary layer diverter design helps to get a little more efficiency out of the small scoops of the 109 but it hardly "solves" the issue. There is no significant thrust generated for a number of reasons which I'll cover further down.

Now lets look at the P-51 radiator-thrust design:

First off, as is quite apparent, the P-51 radiator is HUGE compared to the two scoop radiators of the Bf109. Furthmore, the radiator has three to four times frontal area of both Bf109 radiators combine, which makes it inherantly more efficient for transfering heat.

Next, lets consider the boundary layer diversion method. On the P-51 the scoop is spaced more than an inch and a half away from the bottom surface of the wing (this varied a bit through different models). This means the boundary layer misses the scoop inlet entirely and encounters no obstruction that could rip it away from the scoop inlet until it is well past the inlet. The problem is completely solved.

Now lets look at how the thrust system works. First high speed cold air enters the scoop and proceeds down a widening passage which acts as an expansion chamber. The expansion chamber futher cools the air, slows its velocity, and increases the pressure (I know this is counter-intuative but its true). Then the (relatively) slow moving air passes through the radiator grilling, which is designed in the form of little ">" shapped fins stretched over the tubeing to form a sort of one-way valve. The heated air then passes into a narrowing passage which acts as a compression chamber.

When air passes through the radiator it is heated unevenly. Air molecules which make contact with the radiator fin elements are super-heated. Those that pass mid-way between the elements are much less heated. These molecules exchange heat in the compression chamber. One hot molecule and one cold molecule take up less volume than two warm molecules (assuming the total heat energy level is the same). So the air in the compression chamber is being compressed by its momentum into the narrowing passage and at the same time it is expanding as the heat in the molecules is transfered from the hottest molecules to the cooler molecules.

Finally, the hot air is vented through the thrust nozzel, which is designed and regulated for pressure. This provides thrust. At medium-high to high speeds, the jet of air comming out the thrust nozzel is supersonic, which provides usable thrust beyond speeds where a prop is no longer able to provide much thrust. Not only that, but the stream of hot expanding air is directed right into the wake of the fuselage. This wake is where parasitic drag normally "sucks" the plane back, and is the biggest part of an airplane's drag. Just like a tracer bullet, the P-51's exhaust fills the vacuum wake and reduces drag.

The Bf109 cooling system lacks both an expansion chamber and a compression chamber. The cold boundary layer air is re-introduced into the radiator exhaust in the space behind the radiator, virtually eliminating the chances of producing much thrust from expanding air. The cooling flaps at the back of the scoops are not designed to sustain high pressures behind the radiator, nor to control the outflow to generate a supersonic thrust stream, they are there simply to regulate the radiators to prevent excess cooling, primarily in dives. And finally, the radiator exhaust does not flow into the fusealge wake to help cancel out the parasitic drag.

The 109 has no meaningful "Meredith effect" thrust!

Note: The "Meredith effect" explanation is incomplete, trying to attribute all of the advantage of the radiator design to the thrust generated. A good portion of the advantage was the projection of super-heated air into the vacuum wake of the fuselage, nullifying parasitic drag.

----------------------------

The Spitfire lacked the boundary layer diverter portion of the Bf109, so it may have generated more thrust than the Bf109, at the cost of injesting the boundary layer. To overcome the cooling inefficiency caused by boundary layer injestion, the British simply made the scoops and radiators larger and larger. But without the expansion chamber, compression chamber, and pressure nozzel, not much thrust would be developed. And because the radiators don't feed into the vacuum wake behind the fuselage, the reduction in parasitic drag is minimal.

Finally, I'd also point out that the Mossie ustilized the Meredith effect, and it did so rather effeiciently - but only at two speeds (one being maximum)where the cooling flow was just right.

Also the Japanese Zero utilized the effect, and the this was stolen and transfered to the later model Corsairs. But the effect on these planes was much less (1/3rd?) than on the P-51. I'm not really sure how it was done.

=S=

Lunatic

 

[DaveB.inVa]

The Baugher site has the Allison in the A-36 rated at 1325hp at 3000' and the P-51B rated at either 1400 or 1450 hp depending on the engine. Not that great of a difference.

Im not so sure about them being built stronger either. Read a little and youll find that the Allison was regarded as being a bit stronger than the Merlin. In fact Diego Red's connecting rods are from an Allison.

There are plenty of other variables in there as well besides the scoop, how about weight and for instance as RG mentioned the prop.

 

[Anonymous]

I don't know if the Allisons were stronger than the Merlins or not. However, I do know the Packard Merlins were stronger than their Rolls Royce cousins. They were made to stricter tolerances using better materials and the cores were cleaned up more. RR was under tremendous production pressure and lacked the materials and time that Packard enjoyed.

=S=

Lunatic

 

[Iskandar Taib]

The Baugher site has the Allison in the A-36 rated at 1325hp at 3000' and the P-51B rated at either 1400 or 1450 hp depending on the engine. Not that great of a difference.

750 to 1025 HP is fairly significant.

Im not so sure about them being built stronger either. Read a little and youll find that the Allison was regarded as being a bit stronger than the Merlin. In fact Diego Red's connecting rods are from an Allison.

Yup! But those were just the rods, and the rods were from a late model Allison. And Allisons aren't used in racing - if the basic engine was good enough, you could hop it up like they hop up the Merlins.

This makes for VERY interesting reading:

http://www.supercoolprops.com/ARTICLES/gwhitegearheads.htm
http://www.supercoolprops.com/ARTICLES/gwhite_reno.htm

There are plenty of other variables in there as well besides the scoop, how about weight and for instance as RG mentioned the prop.

Weight, yes. I don't think there was much of a difference in the rest of the fuselage between the A and B (the A had its air intake on the top, like a P-40 did, because that's how the Allison was built), don't think there was much of a difference, if any, in the wings. Not sure how the prop would have mattered - they used a four blader on the B because the Merlin's power needed to be dissipated, so using the A's prop wouldn't have made it any faster, I don't think.

 

[KraziKanuK]

Iskandar Taib,

the Allison is used for in modern racing. There is an Unlimited hydro (U-3) that uses a dual turbo charged Allison. It has even won some races against the T-55 engined boats.

 

[DaveB.inVa]

The Baugher site has the Allison in the A-36 rated at 1325hp at 3000' and the P-51B rated at either 1400 or 1450 hp depending on the engine. Not that great of a difference.

750 to 1025 HP is fairly significant.

[

Yeah 750 to 1025hp is significant... but 75 to 125 isnt. I think you've injected a couple zeros into your calculation.

 

[the lancaster kicks ass]

or just miss-calculated............

 

[DerAdlerIstGelandet]

seriously miss-calculated

 

[the lancaster kicks ass]

dude i have to do 6+5 on the calculator, to me what he did was complicated maths, and i'm in top set maths at GCSE level!!

 

[DerAdlerIstGelandet]

Im just keeping the convo going wiht what I said. I dont have anything to add to what they are saying.

 

[CharlesBronson]

North American A-36A Invader
514th FS, 27FBG, 12th AF USAAF
Tunisia, 1943

 

[the lancaster kicks ass]

very nice..........

 

[DerAdlerIstGelandet]

Yeah good drawing.

 

[stug3]

Bump for a very informative thread from the past about my favorite plane.

 

[GregP]

We built one for Tom Friedken at Chino (Steve Hinton's Fighter Rebuilders). It's at his ranch in Texas. Flies great and the dive brakes work just fine.