Engine quastion about He-177

[rousseau]

I've known that two engines was crowded cramed in one nacelle on He-177 per se. So cause a serious problem about how to cooling the engine. I think both of air-cooling and liquid-cooling was used on He-177's engine nacelles but still hard to make them works well. Therefor, I was wondering, why Nazi didn't set engins as fore-and aft? Such layout of engine has had already used on Farman 222 before He-177 was born. Both problems of head-on area of engine and cooling engine would be solved.
Here invite your thoughts

 

[Parmigiano]

I'm not a super-expert, but I think that the fore and aft layout would probably had simplified the gearing of the 'powertrain' (possibly at the expense of a less optimized aerodynamics of the wing/nacelle) but I don't see how it could have (alone) improved the cooling, especially for the water cooled DB.

 

[DerAdlerIstGelandet]

The whole reason behind the coupled engines was to allow the aircraft to be perform Dive Bombing duties.

 

[Njaco]

Adler, I just noticed your comment under your siggy and I'm cracking up!!!

Adler is right, it was to streamline the plane for dive-bombing purposes. After all the problems Ernst went on his own and tried a conventional four engine design.

 

[ccheese]

fly boy said: "isn't that the first jet bomber? becasue i have flown one in a flight sim before and i know how it handles.

Good one, Adler..... I love it...

Charles

 

[Kurfürst]

The He 177s DB 610s problem was not cooling related but lubrication related. It was a problem early DB 605s suffered from (the DB 610 were really just two 605s coupled together), also in other airraft like Bf 110, 109..

It was solved in the automn of 1943 by adding an oil de-aerator.

 

[Njaco]

Kurfurst wasn't there also a problem with leaking fuel or maybe oil that collected in the cowlings and caught fire from the heat at times? maybe another plane.

 

[rousseau]

The whole reason behind the coupled engines was to allow the aircraft to be perform Dive Bombing duties.

I think a tandem setting engine could get a smaller head on area for diving but no reduce the amount of engines.

 

[Graeme]

I think a tandem setting engine could get a smaller head on area for diving but no reduce the amount of engines.

Would such an arrangement destroy the aerodynamics of what was meant to be a high performance multi-role bomber?

I've also read (somewhere) that a pusher configuration is less efficient than a tractor. Maybe someone more knowledgeable in this area can confirm/deny this.

 

[HoHun]

Hi Graeme,

>I've also read (somewhere) that a pusher configuration is less efficient than a tractor.

Actually, the pusher tends to be more efficient as the tractor accelerates the airflow over the fuselage/nacelle/wing, and the faster airflow means more drag.

However, the same faster airflow can be an advantage when it comes to take-off and landing as it means more lift, and depending on the layout also better controllability by provding a slipstream to keep the tail aerodynamically effective.

With regard to WW2 aircraft, the Dornier Do 335 had different single-engine characteristics depending on it flew on the tractor front engine or on the rear pusher engine

According to Eric Brown, it was faster on the rear engine.

Regards,

Henning (HoHun)

 

[SoD Stitch]

I've also read (somewhere) that a pusher configuration is less efficient than a tractor. Maybe someone more knowledgeable in this area can confirm/deny this.

Actually, as HoHun said, it's the other way around; there is no parasite drag on a pusher propeller, but there is on tractor propeller. I know that the Do 335 was faster with the front (tractor) propeller feathered, flying on the rear (pusher) propeller, than the other way around. Also, our Cessna 337 Skysmasher was faster on just the rear engine than it was on just the front engine.

 

[Graeme]

Actually, the pusher tends to be more efficient as the tractor

Actually, as HoHun said, it's the other way around

So much for my memory. Thanks HoHun/SoD Stitch.

Why then don't we see more 'pushers' (with regards to props) in the sky?

 

[HoHun]

Hi Graeme,

>Why then don't we see more 'pushers' (with regards to props) in the sky?

The standard single-engine tractor layout naturally allows good propeller ground clearance in take-off and landing attitude. If you go for a pusher, you introduce the risk of propeller ground strikes when the angle-of-attack is increased too far.

To keep the landing angle within limits, it's sensible to add particularly effective flaps not necessary on a similar tractor aircraft. If you plan on achieving the same low take-off and landing speeds as with the tractor aircraft, you have to make the flaps yet more effective, introducing even more cost, complexity and weight to the airframe. (Dornier's early patents on a Do 335-style push-pull aircraft actually included a variable incidence wing for that purpose - talk about complexity!)

You also need a stiffer fuselage if you're going to use a pusher propeller in the tail (unless you go for a canard ... Rutan VariEze - Wikipedia, the free encyclopedia ). Willy Messerschmitt's Me 109 layout - with engine, wings, main gear and fuselage all joined through a single structural member, the firewall - illustrates the advantage of the traditional tractor layout. For a pusher layout, you'd need something like an Airacobra with the extension shaft going aft to a pusher propeller behind the tail ... and if you have seen drawings of the structure of the P-39's fuselage structure, you have probably noticed that quite a bit of metal went into that.

For multi-engined aircraft, it's probably a bit easier to design for pusher engines as ground clearance is a minor issue. However, for multi-engined aircraft, take-off and landing distances tend to be important, and the benefit of the accelerated airflow from the propellers increasing lift is a point designers like to employ to improve the low-speed characteristics of their aircraft. The B-29 for example used tractor engines, while the B-36 built for the same role used pushers. The B-29 used a relatively small high-aspect ratio wing of high wingloading, while the B-36 used a relatively larger, lower aspect-ratio wing which I guess was not quite as advantageous for long range as the B-29's wing.

In short, it comes down to the propeller configuration being a the result of a compromise inspired by the intended purpose of the specific type

Regards,

Henning (HoHun)

 

[Graeme]

Very informative answer HoHun! Thanks.

One last query. The ducted propeller. So many promising designs. Generally 'pusher' in layout with the propeller protected. Yet to my knowledge no large scale manufacture ever occurred for any of them. I would have thought that the General Aviation market would have embraced the idea. Any thoughts as to why success evaded them?

 

[rousseau]

So a dim conclusion here may be is that designor of He-177 was never aware of tandem setting engine is actually useful layout for their trouble?

 

[DerAdlerIstGelandet]

So a dim conclusion here may be is that designor of He-177 was never aware of tandem setting engine is actually useful layout for their trouble?

Not for this designs intended purpos as HoHun has pointed out.

 

[antoni]

During 1936 the Luftwaffe issued details of its requirements for the new "Bomber A". From the start it was a demanding specification, and called for an aircraft with a maximum speed of 335 m.p.h., able to carry 4,400lb of bombs to a target 1,000 miles away or 2,200lb to a target 1,800 miles away. Moreover, it had to be strong enough to make medium-angle dive attacks, descending at 40-50°.
This formidable specification demanded an aircraft able to out perform, by a considerable margin, any bomber and outrun any fighter then in service. To meet the challenge the Heinkel design team displayed great ingenuity in their P.1041 design study for a well-proportioned aircraft of all metal construction, with an exceptionally clean airframe. Early on, however, its advanced features presented serious problems.
Power for the new bomber would be from a pair of Daimler Benz DB606 coupled engines, Each DB606 comprised two DB601 liquid-cooled 12-cylinder inverted-vee inline motors, mounted side-by-side and driving a single four-bladed propeller through a connecting gear train. Each combined engine gave an output of 2,600 h,p, A clutch arrangement allowed either engine in a pair to be shut down in flight, so the aircraft could cruise on two engines to extend its endurance. Installing the engines in two nacelles produced an aerodynamically cleaner arrangement than placing the four engines in individual nacelles. Moreover, concentrating the weight of the propulsion system well inboard gave the bomber increased manoeuvrability for the medium-angle dive-attack role.
In a further move to reduce the drag of the engine installation, the designers sought to dispense with the drag-producing radiators usually employed with liquid-cooled engines. For an attempt on the world air speed record title Heinkel company had pioneered an evaporative cooling system.The water coolant was pressurised, allowing the temperature to rise to 1,100C before steam formed. The coolant was then ducted away and depressurised. at which point steam formed. The water was separated off and returned to the engine, while the steam was piped to condensers in the wing leading edges cooled by the airflow, There the steam condensed and the coolant was pumped back into the engines.
The system worked well enough during a record-setting flight lasting about 15min, and in March 1939 a much-modified Heinkel He 100 fighter raised the world air speed record to 463 m.p.h.
The cooling of a D6606 running for several hours on end, was a different matter It quickly became clear that dissipation of the heat generated by the D8606s was far beyond the ability of the evaporative cooling system. The designers reverted to conventional radiators, increasing drag.

In November 1938 tile Helnkel company received an order for six prototypes. At the same time the type received its official designation - Heinkel He 177. On November 19, 1939, the He 177V1 prototype made its maiden flight. The chief of the Rechlin flight test centre, Dipl-Ing Franke, took the bomber into the air but almost immediately the engines started to overheat and he returned to the airfield after just 12min airborne. Franke reported favourably on the bomber's handling, apart from the engine overheating, although he also complained of the inadequate effect of the tail surfaces, undue
vibration from the propeller shafts and a degree of aileron flutter. To cure the flutter the prototype was fitted with a new fin and tailplane of 20 per cent greater area.
As pilots explored the bomber's performance it became clear that its maximum speed was disappointingly low; a mere 286 m.p,h. Moreover, they found that the He 177's range was little more than 3,100 miles, a quarter less than specified.
A few weeks later He 177V2, the second prototype, joined the test programme. This aircraft was generally similar to the V1 but retained the original, smaller tail surfaces. During an early test dive the V2 developed severe control flutter and broke up in mid-air, killing the pilot Consequently the \/3, V4 and V5 prototypes, then nearing completion, were all given the enlarged tail surfaces. Each measure taken to improve handling added more weight. and each successive prototype was somewhat heavier than its predecessor.
Diving trials resumed using tile V4 prototype, but during one of these the aircraft suffered a malfunction of the propeller-pitch mechanism, failed to recover from its dive and crashed into the Baltic.
The last aircraft of the prototype batch, the V5, was used to test the bomber's armament. To save weight this aircraft had its 13mm machineguns replaced by 7-9mm weapons. Early in 1941, during a simulated low-level attack, the V5 suffered a double engine fire, flew into the ground and exploded.
Thus, during the early test programme three of the first five He 177 prototypes crashed, taking their crews to their deaths.

As the test programme continued there remained the difficulty of meeting the requirement for the bomber to make precision attacks in dives of up to 60°. Pilots found that to get the He 177 into a diving attitude they had first to reduce speed, then very slowly ease forward on the control column to get the bomber's nose to drop, Almost as soon as the bomber was established in its dive it was time to initiate a relatively gentle pull-out Even with this gentle handling the He 177 was easily overstressed and would pop rivets, It was far too large for steep dive attacks,

However, the new Lotfe tachometric bombsight, working on a similar principal to the American Norden bombsight entered Luftwaffe service, It produced accuracies in horizontal bombing attacks similar to those obtained in diving attacks, so the He 177's dive-bombing role was quietly dropped, and after the initial pre-production batch the dive brakes were omitted.

In an effort to get the new bomber into service the sixth prototype was sent to Bordeaux-Merignac in France for initial operational trials during the summer 1941. But these revealed several shortcomings and the aircraft was returned to Germany.
The Luftwaffe began receiving the pre-production A-O series, totalling 35 aircraft in November 1941. These aircraft explored the performance envelope and technical features,

It became clear that the problem of engines overheating and sometimes catching fire had more than one cause. Some cases were blamed on the common exhaust manifold which carried the exhaust gases away from the two inner cylinder blocks, and became excessively hot in flight. Moreover, if spilt oil collected in the bottom of the cowling it could ignite. Sometimes the fuel injectors delivered too much fuel if the pilot throttled back too quickly. If the fuel connectors were insufficiently tight, or were defective, excess fuel might spill on to a hot part of the engine.
Owing to the bid to save weight the He 177 lacked firewalls at the rears of the nacelles. Thus even a small fire could quickly run out of control and reach a wing fuel tank.
It was found that some highly stressed engine parts had poor lubrication. In extreme cases this led to connecting-rod bearings seizing-up and smashing through the crankcase. Lubricating oil spilling on to a hot exhaust pipe could also cause a fire. Daimler-Benz launched a major effort to diagnose and cure the DB606's failings, but the problem of engines overheating and catching fire would run for a lot longer.

 

[barkhorn45]

and then there was the junkers jumo 222 which i believe{i'm currently sitting in my truck at a truck stop in waco tx}were two jumo 213's coupled together and used in a few Bomber B programe aircraft designs.I don't believe it suffered from the same problems as the db 610.Also from what i've read the low oil pressure problem of the db 605 was never completely solved which perplexed the engineers at the time.

 

[Aurum]

It seems more easier and troublefree could be installation of pair but mechanically not combined engines. They could work independently on two coaxial propellers. By the way diameter of propellers then be sufficiently reduced.

 

[Gibbage]

The Allison V-3420 had no such problems.

It was never used in any production aircraft, but many many prototype aircraft.

A varient of the B-29, the B-39, was tried with the V-3420's in case the R-3350's didnt work out.

And the XB-19, served for 6 years on V-3420's as a cargo aircraft.