twin boom designs

[Mace2004]

Sometimes aircraft design decisions have reasons that can be pretty obscure.  Some of the early designs put in two booms to make room for the prop in a pusher configuration and it's pretty obvious that this allows them to mount the prop on the aircraft centerline as opposed to above it. The flying boxcar (C117?) used the aft end of the main fuselage as a large cargo door.

I don't have any specific knowledge about why the P38 uses that configuration as it has significant limitations including greater mass away from the aircraft centerline and reduced visibility however, in this case, it's probably a simple matter of "packaging".  What's the best way to mount two high power engines on a fighter?  Dornier put one in the nose and one in the tail which was probably far more efficient from a performance perspective than placing them in the wings but it had other issues including reduced efficency and cooling of the aft prop/engine.

All airplanes have to make room for the engines, fuel, pilot and weapons and need to do it in the lightest weight and lowest drag package that can handle the loads.  Also, cost and complexity weigh in heavily (no pun intended).  My guess is the Johnson looked at the two engines mounted in the wings and, based on the performance requirements for the plane, decided that if he's got to have two large engine nacels hanging out there he may as well use them and eliminate the redundant conventional fuselage. This probably gave him the combination of weight and performance he needed.

Some would say this is also a beautiful design...others might ask "why build a fighter with a hole already in it?"

[Krusty]

They wanted x performance. They could not get it with 1 engine. They needed 2 engines. Pure speculation says that putting the engines on booms would reduce drag vs. in under-wing engine pods. Plus the gear, oil coolers, and turbochargers are right there, in close proximity to the engine they serve, instead of being routed all the way around the fuselage and central tail unit (greater simplicity).

[Furball]

Originally posted by Oldman731
Thanks.  Every time I look at one of those contraptions, it reminds me of what a coward I truly am.

- oldman

Have you ever seen close up, the construction of a First World War aircraft and the materials used?

I don't think you are a coward... those men were just incredibly brave!

[BaldEagl]

Funny what a little Internet search can turn up (from 2 sources):

* The Lockheed P-38 was designed in response to a 1937 US Army Air Corps (USAAC) specification designated "Project X608" for a fast high-altitude twin-engine interceptor, capable of 580 KPH at an altitude of 6,100 meters (360 MPH at 20,000 feet). Five companies -- Consolidated, Curtiss, Douglas, Lockheed, and Vultee submitted proposals.
The Lockheed team was under the direction of Clarence "Kelly" Johnson, who would eventually design a string of famous aircraft up to the SR-71 Blackbird Mach 3 spy plane. Johnson's initial concepts for the new fighter covered a range of configurations, but the Lockheed team finally decided on a scheme with twin booms to accommodate the engines, and with the pilot and armament in a central nacelle. The aircraft was designated the "Lockheed Model 22". The engines were to be supercharged 12-cylinder, vee-inline, water-cooled Allison V-1710 engines.
When Johnson selected the Allison, it had not been rated at even 746 kW (1,000 HP), but it was really the only large inline engine available in the US at the time. The propellers would rotate in opposite directions to eliminate the effect of torque. The General Electric B-1 superchargers were positioned in the booms, behind the engines. Armament was to consist of four machine guns in the nose of the nacelle, clustered around a cannon. The design featured tricycle landing gear, making the aircraft one of the first with such a feature.
The Lockheed submission won the competition on 23 June 1938 and was awarded a contract to build a single "XP-38" prototype. The XP-38 was rolled out in December 1938 and first flew on 27 January 1939, with USAAC Lieutenant Benjamin Kelsey at the controls, who by no coincidence had helped drive Project X608 in the first place. Kelsey found he couldn't retract the flaps and thought of bailing out, but he managed to get back down on the ground safely. The problem was quickly fixed and later flights went much better, demonstrating the impressive performance of the type.
------------------------------------------------------------------------------------------------------------
The increasing tension in Europe, and the combined build up of the German and Japanese military forces, caused the United States to look at its own military condition. The fact was that the United States was lagging far behind other countries in military strategy and equipment. In order to address this problem, the military decided to initiate a design contest for a next generation interceptor. This aircraft would far exceed any other fighter currently in production in performance and armament.
The result of this desire for a new fighter was Specification X-608. In Specification X-608, the aircraft was required to maintain a top speed of 360 mph, whereas previously designed aircraft were only required to maintain a top speed of 300 mph. This aircraft would also be required to operate at altitudes of 20,000 and contain firepower three or four times higher than any other existing fighter aircraft. Essentially, this aircraft would be able to perform any mission any other fighter would be capable of, but not limited to any specific one.
At the time of this specification, the state of the aircraft in the United States military was lagging far behind any other country in Europe. The typical aircraft in service was limited to roughly 500 lbs. of armament. Usually this consisted of 2 machine guns and the ammunition. This new design would be the first of what would be considered modern aircraft. Vultee, Curtis, and Lockheed all submitted designs for this contest.
Hal Hibbard and Kelly Johnson headed the Lockheed team. Overall, there were only 50 engineers working on the new prototype. At this time, there were no indications of immediate war, and most of the designers and engineers were working on other designs, which were non-military related. The Lockheed team knew they would face stiff competition, but they believed their many innovations would set their design apart from the other entries. Hibbard and Johnson decided to build their prototype, the Model 22, around the new Allison V-1710 engine. This engine was capable of running at 1000 hp for over 150 hours. They immediately realized that a twin-engine fighter would be required to fulfill the X-608 requirements. Also new General Electric turbosupercharger development would enhance the high altitude performance of their new design. Johnson admired the Spitfire wing design, but realized that if he used a similar design, the Model 22 would not possess adequate long-range capabilities. In fact, early Spitfires contained 85 gallons of internal fuel, whereas the P-38 would eventually contain 400 once in operational status. The new Model 22 would be a heavy aircraft, and its wing characteristics were not considered acceptable for a fighter. The Lockheed design team felt that with the new engine development, turbosuperchargers, and their innovative design elements, it would indeed be an effective fighter. The design would also allow for a 23-mm Madsen cannon and four .50 caliber Colt MG-53 with 4,000 rounds of ammunition. Kelly Johnson was able to back up their designs with calculations, which would allow for accurate predictions about the aircraft performance. In June 1937, Lockheed was awarded the contract for the building of 1 prototype. If tests proved promising, Lockheed was informed they could expect an order for fifty others.

[Soulyss]

From what I can gather the basic explaination for the P38 design was that the specifications and demands placed by the army @ the time of it's design required 2 engines since there was not a single engine in production that they tought would fit the bill.  They came up with a design that doubled the available power while minimizing the weight gain.

[AquaShrimp]

P-38s had booms to house the gargantuan turbo chargers that the horrid allison engines so needed.  

When it was suggested that the P-38 switch to the elegant and powerful Merlin engines, the booms would have then housed additional fuel.  But the engine switch was never completed.

[Captain Virgil Hilts]

Originally posted by AquaShrimp
P-38s had booms to house the gargantuan turbo chargers that the horrid allison engines so needed.  

When it was suggested that the P-38 switch to the elegant and powerful Merlin engines, the booms would have then housed additional fuel.  But the engine switch was never completed.

Any idea how wrong you are? Completely is the word that fits best.

Merlins would screw it up, Lockheed did a study, and found speed and climb would be reduced, along with range.

There's nothing wrong with the Allison engine either. Hell, thirty years after the war, they were putting Allison rods in the Merlins so that Merlins could run enough boost to win at Reno and not blow up.

Turbocharged Allisons had more power at high altitude than the Merlin could ever hope to have.

And had the War Production Board allowed Allisons to be fitted with two speed two stage superchargers, even Allisons without turbochargers would have had equal or superior high altitude performance.

Oh, and none of the GE turbochargers were gargantuan. They're not really that big at all. And yes, I've seen them up close and personal. I was building Allison engines over 20 years ago.

[AquaShrimp]

Turbocharged allison engines never reached parity with supercharged merlin engines.  Speed and range reduced by switching to merlin engines?  Extremely unlikely.

Lockheed ran a study comparing a Merlin XX powered Lightning with a standard V-1710 powered variant. The reported speed difference was over 25 mph, favoring the Merlin powered airplane. Climb performance was similar to the Allison powered machine.

Yet another study in 1944 compared V-1710s producing 1725 bhp and "advanced" Merlins using "special" fuel and producing 2000 bhp (no altitude specified). The Merlin powered version could supposedly attain 468 mph at 30,000 feet, which was considerably better than the Allison powered version.

The results of this study show a P-38 with almost identical speed when compared to the P-51D (around 440 mph at 24,000 feet), and slightly better climb. The key to the comparison is the value of the drag coefficient used, and the weight estimate. If the basic, zero-lift drag coefficient of the early P-38s (E-H models) is used, the V-1650-7 powered Lightning is slightly faster than the Mustang. Using P-38J drag data, the Mustang is slightly faster. In either case the P-38 climbs better.

[Spikes]

Originally posted by Bor3d
P-82 Twin Mustang

I don't know if this counts...

I have a close up of a P-82. Dayton Ohio AF Musem. It is hard to see that it is a P-82 because it is so close. I took a pic of it to show Betty it.

[Murdr]

From a previous discussion...

Originally posted by Badboy
I think this extract from LOCKHEED HORIZONS Issue Twenty Three provides the answers to your questions, and the source of the sketch posted previously... Note that the image posted previously appears to have been cropped to omit the initials of the author.  

"A number of configurations were conceived by the Lockheed design team to meet the single-place twin-engine requirements of Project X608, Figure 5. (note the initials):

The most conventional arrangement placed each of the engines in wing-mounted nacelles, concept 1. Layouts indicated that the internal placement of the many elements of the propulsion, gear, and armament systems could not be arranged in a manner that used the airplane internal volume to best advantage.

Placement of the two engines in the fuselage along with the pilot led to three other configurations, concepts 2, 3 and 6. Two of these designs used gear boxes and drive shafts so that the propellers were mounted on the wing, either in a tractor or pusher propeller arrangement. Complexities of gearing and shafting and anticipated problems of engine cooling eliminated these ideas. Concept 6 provided for a tractor propeller in the forward fuselage combined with a pusher propeller at the rear of the fuselage. Complementing this arrangement were two tail booms spanned at the rear by the horizontal tail. Problems with rear-engine cooling and the propeller strike hazards related to pilot escape gave reason to discard this design.

The twin-boom arrangement had attractive advantages, so two other alternatives, concepts 4 and 5, were laid out for study. The design which placed the pilot in a fuselage pod along the airplane centerline was found to be the most preferred of these two designs. Concept 4 became the final, most logical choice mainly because the boom arrangement nicely accommodated all the elements of each powerplant, allowed room for landing gear stowage, and at the same time provided proper aircraft balance. Behind the counter-rotating Curtiss 3-bladed propellers and the V-1710 Allison engines there was room for the inlet scoops and the General Electric turbosuperchargers and cooling scoops. The main landing gear wheels could be retracted be hind the superchargers, just in front of the Prestone radiators. In addition, there was space for air induction and cooling ducts, as well as room for other aircraft systems components.

In this arrangement, the propeller slip stream would provide ram air for cooling, and increased air flow over the two rudders so as to enhance direction control. The hot Prestone coolant was removed from the cockpit area. All the internal fuel was contained within the wing. Maximum efficiency in the use of airplane volume was achieved and favorable airplane fineness ratio and low frontal area offered minimized parasite drag."

Hope that helps...

Badboy

Also, here are some more detailed design concept drawings.

[joeblogs]

The problem is that from the early 1930s the aircorps told allison the engine would be fitted with a turbosupercharger, so they never devoted time to developing two speed or two stage gear driven superchargers. But turbo development came on slowly and thus the P40 and the P39 suffered.

BTW the turbosupercharger itself wasn't that big, it's all the piping and the intercooler that takes up the space. Same for the P47 installation.

-Blogs

Originally posted by AquaShrimp
P-38s had booms to house the gargantuan turbo chargers that the horrid allison engines so needed.  

When it was suggested that the P-38 switch to the elegant and powerful Merlin engines, the booms would have then housed additional fuel.  But the engine switch was never completed.

[Angus]

The last one,(6) - with puller and pusher looks very interesting.
No one is almost a Mossie

[Captain Virgil Hilts]

Originally posted by AquaShrimp
Turbocharged allison engines never reached parity with supercharged merlin engines.  Speed and range reduced by switching to merlin engines?  Extremely unlikely.

Wrong answer. By 29000 feet the Merlin is down to nearly 1000HP. At 29000 feet, the -30 Allison still has nearly 1700HP in WEP.

According to the Lockheed study, the Merlin installation was 1000 poundsheavier, the range and speed was reduced, as well as climb. Ask Warren Bodie, he wrote the book on the Lightning, and was a Lockheed engineer. Lockheed actually wanted to try the Merlin in the P-38, but it was never done. No P-38 was ever built with Merlin engines, according to the Lockheed documents. Any studies were conjecture at best.

At 470MPH, the P-38 will compress above 26000 feet, even in level flight. And a Merlin won't make enough HP to get close to that figure, even if you put Hamilton Standard props on it. Two Allisons will make about 3400HP at 26000 feet. Two Merlins, even of the type from a P-51D, won't make more than 2000HP. If you didn't have 2000HP, with Curtiss Electric props, the damned thing probably wouldn't even hit 400MPH. There's no way in Hell a pair of Merlins could make a P-38 hit 468MPH at 30000 feet, and even if they did, the damned thing would be in a full compression dive.

The -30 Allison, running at Lockheed and Allison spec boost and RPM for the P-38L-5-Lo, will power the plane to AT LEAST 442MPH. According to the Lockheed test logs. The USAAF didn't accept the Lockheed and Allison ratings, that's why the P-38L is not faster than the P-38J in most references. The P-38L actually IS faster, run at the factory ratings.

The P-38K setup, with a special set of F-15 Allisons (better than the -30), and Hamilton Standard props, topped 450 easily, and would probably hit 460. It would also climb at 4800 FPM. The big drawback to the P-38 was not the Allisons, but rather the crappy Curtiss Electric props. By the way, the reason Allison didn't make any more of the engines they made for the P-38K is that the USAAF didn't even accept the ratings for the -30, so they knew they wouldn't accept the hotter F-15 engines from the P-38K. The War Production Board wouldn't even allow Lockheed to stop each line one at a time for two weeks per line to make the P-38K, despite demonstrating in 1943 that the P-38K had superior performance in range, speed, and climb to ANY U.S. fighter of the day. The P-38K was a proven aircraft, and in early 1943 was superior to the P-51D in almost every measure, even though the P-51D wasn't even produced until nearly a year later.

And by the way, the USAAF removed the turbocharger from the P-39. Had they not done that, the P-39 could have been one of the finest fighters of the war. After the war, one of the fastest racing planes around was a P-39 with a turbocharged P-38 Allison.

The Merlin has a lot of good points, but it was not really superior to the turbocharged Allison, other than simplicity.

[Angus]

Which Merlin?
And regarding Simplicity and such, why were there never any Allison driven single engined fighters with such output at altitude?
Compare a Merlin powered P51 vs Allison? Or the P40?

[Captain Virgil Hilts]

Originally posted by Angus
Which Merlin?
And regarding Simplicity and such, why were there never any Allison driven single engined fighters with such output at altitude?
Compare a Merlin powered P51 vs Allison? Or the P40?

Even the Merlins tuned for high altitude performance couldn't match the turbocharged Allisons for power above about 28000 feet. The problem is that the higher the altitude, the faster you have to turn a supercharger to create a given pressure. But the faster you turn a supercharger, the more power it takes to turn it. At some point, it takes more power to turn the supercharger than can be gained from the pressure generated. Also, above a certain speed, a supercharger will cavitate, and actually produce less pressure. And finally, the faster the supercharger runs, the more it heats the air, making the air less dense, even though it has high pressure.

Now the same principles do apply to a turbocharger, but to a far lesser degree. Both the P-47 and P-38 used the same basic GE turbocharger, and both were capable of overspeeding the turbocharger at high altitudes.

The P-38 was the only Allison equipped fighter with a turbocharger. The P-39 originally had a turbocharger. However, in the interest of streamlining for drag reduction, among other reasons, the USAAF/USAAC had Bell remove it. The P-39 COULD have had 1725 to 1750HP at 29000 feet, but they took away the turbocharger before the plane went into production. Just imagine the small, lightweight P-39, equipped with the -30 series 1725HP Allison, four 50's in the wings and a 20MM cannon shooting through the hub of a Hamilton Standard paddle prop.

There are any number of reasons that the Allisons were never equipped with a two speed crank driven supercharger, although none of them are particularly good reasons. Mostly the reasons given have to do with simplicity, expediency, expense, and related excuses. The Allison could have been equipped with a two speed supercharger, it just wasn't. Same thing applies to props. Curtiss could have been tasked with building hydrostatic paddle props, instead of the crappy electric thin blade props, but they weren't. The Curtiss prop most commonly fitted to Allison equipped planes was inefficient, and unreliable. Pilots who flew planes equipped with Curtiss props continually complained of burned out generators and blown fuses or popped circuit breakers.

Although Bodie spoke highly of the idea of a Merlin powered P-38 in his book, he also said the conversion was never attempted, and no one knew if it would actually work as well as was thought. Bodie, at least when I was swapping emails with him, seemed to think the F-15 powered P-38K with Hamilton Standard props was better than any Merlin conversion could have hoped to be. And remember, the P-38K had the early 3 blade prop, not the later four blade. You would only need to look into the dramatic improvement seen in the P-47 when Republic swapped from the Curtiss Electric prop it shared with the P-38 to the Hamilton Standard paddle prop to see how much the P-38 would have gained with the same prop, even without the F-15 Allisons.