Four Engined He 177 w/ Level-Headed Specs

[Zipper730]

Not necessarily as the props would be smaller with 4xDB601 variants

Looking at the Avro Lancaster as a reference, the propeller diameter was 13'0", and the He 177 was 14'8", so you could take off 20", or 10" of radius. It's gotta mean something.

 

[swampyankee]

I've read a bit about the He 177: It's one of those completely absurd designs that leave you scratching your head. Many unorthodox designs have been proposed, but most are weeded out before they ever fly, others are heavily modified into something practical, some manage to make unorthodox work, and some manage to be morphed into practical designs during flight test.

The problems with the He 177 revolved around the issue of high speed and the ability to execute moderate-angle diving-attacks that were eventually increased to 60-degree diving-attacks. I'm not sure the definition of moderate-angle diving-attacks (but if I were to make an educated guess, I would speculate more than 30 and less than 60-degrees, reasonably speaking), but it seems a substantial design requirement as it was (the aircraft would require stressing for higher airspeeds, and higher g-loads), and jacking it up further ended up requiring increases in weight, and attempts to offset it by using the DB606, and doubled landing-gear requirement just made the airplane more overly complicated (Though the DB606 wasn't an intrinsically bad engine, it did run hotter than the DB601 because of the central exhaust nozzles). The increases in weight also further complicated the engine installation, with it being recessed further back inside the wings where its position had fuel & oil lines, and electrical harnesses in close proximity, and I'm not sure it had a firewall.

It's pretty obvious with 20/20 hindsight (or the properly calibrated crystal-ball) that the Lotfernohr 7 bombsight would come online in 1941, and the dive bombing requirement would be lifted in 1942: The fact is that the Do 17/215 & Ju 88 were predominantly level-bombers that could do dive-bombing if need be. The moderate-angle dive requirement prior to late 1937 was probably already overkill, but it might have been easier to achieve weight and range requirements.

It seems 4 x DB601 would be the best choice because it'd have a smaller propeller diameter, and that'd simplify the landing-gear. I remember hearing that drag difference would be all of 3%, and that could be covered by some aerodynamic refinement.

I'll disagree with one of your first sentences: The He177 was really a very conventional aircraft, in structure and configuration. Its failures were, in my opinion, largely due to two issues: poor nacelle design, which was largely, albeit not completely, Heinkel's fault (DB should share blame here; they evidently did not give Heinkel the sort of support or information that they should have during the design phase, possibly because they had not yet gotten in), and excessive structure weight (that's mostly on the RLM). Heinkel also seemed to have some constructional problems with the prototype, where a wing failed at significantly below its design load; this is not a specification problem; it's problem within Heinkel's design office.

Even leaving out the dive bombing requirement (which was foolish for so large an aircraft), Heinkel did not do a great job on this design.

 

[Zipper730]

I'll disagree with one of your first sentences: The He177 was really a very conventional aircraft, in structure and configuration.

I edited the initial post, which I think better explained things.

I'm still surprised they had so much trouble with determining cooling requirements, because the He 119 flew before the He 177, and they would have known/learned various things that they could have designed their way around (provided they were allowed to).

The flap system seemed to be a serious problem and, I'm guessing it was to keep the takeoff run low.

 

[Shortround6]

A big problem with surface cooling, or rather several interconnected problems, was it's high maintenance and combat vulnerability. You have a lot of square footage of cooling area. A lot of potential for leaks in normal (noncombat) operations. Bullet and shrapnel holes just make things worse. Hard landings may require more work and testing before the plane is ready for operations.
A few Schneider cup racers used surface cooling. It did offer some advantages for race or record setting planes. I don't believe any combat plane or commercial plane used it.

 

[Zipper730]

A big problem with surface cooling, or rather several interconnected problems, was it's high maintenance and combat vulnerability. You have a lot of square footage of cooling area. A lot of potential for leaks in normal (noncombat) operations. Bullet and shrapnel holes just make things worse. Hard landings may require more work and testing before the plane is ready for operations.

I was reading a book Die Deutsche Luftrüstung: 1933-1945 (Band 2). On pages 184-191, there's an entry on the He 100.

They did mention investigation being required as to how it would withstand battle-damage: I'm not sure what tests were done, nor does it say in the book, but they apparently felt most of the problems were fixed: That said, they did run into problems (the surface area was smaller than expected, which required an extendable belly-radiator) with the surface-cooling system, and the oil-cooler proved to be a problem (since allowing the oil to boil was unacceptable, and the DB-601's dumped a significant amount of heat into the oil), requiring a small radiator to be added.

With a new aircraft, it would appear that they would have been able to better design a cooling surface of adequate area to dispel the engine heat adequately, but it wouldn't work an engine the size of the DB606. 4 x DB601's with surface evaporative cooling might actually have been safer than the DB606's used on the early designs, lacking firewalls and all.

 

[swampyankee]

I was reading a book Die Deutsche Luftrüstung: 1933-1945 (Band 2). On pages 184-191, there's an entry on the He 100.

They did mention investigation being required as to how it would withstand battle-damage: I'm not sure what tests were done, nor does it say in the book, but they apparently felt most of the problems were fixed: That said, they did run into problems (the surface area was smaller than expected, which required an extendable belly-radiator) with the surface-cooling system, and the oil-cooler proved to be a problem (since allowing the oil to boil was unacceptable, and the DB-601's dumped a significant amount of heat into the oil), requiring a small radiator to be added.

With a new aircraft, it would appear that they would have been able to better design a cooling surface of adequate area to dispel the engine heat adequately, but it wouldn't work an engine the size of the DB606. 4 x DB601's with surface evaporative cooling might actually have been safer than the DB606's used on the early designs, lacking firewalls and all.

The basic problem with surface cooling is the basic fact that there is a lot of surface area that can't be protected from battle damage; even a rifle-caliber machine gun would mission-kill a bomber with a surface-cooling system. A four-engined aircraft would have the same problem, except that losing one engine's cooling system would cause a 25% loss of power, vs a 50% loss.

Of course, a four-engined aircraft can use slightly smaller engines to at least maintain altitude with one engine out; if the aircraft needs 3000 shp to maintain altitude, a twin needs a total of 6000 shp, but a four-engined aircraft only needs 4000 shp, easing the cooling problem as there is less surface area. If the aircraft has the same total installed power, it would need close to the same net surface area for its cooling system.

For some fun with heat transfer, try Convection Wizard. Using that calculator (I'll check some numbers later with my heat transfer books), at 20 m/s (part way through the takeoff run), a surface radiator can dissipate about 6 kW per square meter. An spark-ignition engine dumps about 25% of its heat to its coolant (http://web.mit.edu/2.61/www/Lecture notes/Lec. 18 Heat transf.pdf), so each 2000 hp (1500 kw) engine dumps about 375 kW into its coolant, so it needs about 65 square meters of surface area for takeoff. At cruise, the heat transfer rate will be much higher (about 25 kW/m^2), so the surface area is less, but still pretty large. Aircraft engines operate best within a fairly narrow temperature range, so both over-cooling and under-cooling adversely affect performance.

 

[Zipper730]

The basic problem with surface cooling is the basic fact that there is a lot of surface area that can't be protected from battle damage; even a rifle-caliber machine gun would mission-kill a bomber with a surface-cooling system. A four-engined aircraft would have the same problem, except that losing one engine's cooling system would cause a 25% loss of power, vs a 50% loss.

As stupid as this will sound, a mission kill is the same thing as "a kill" correct?

Regardless, I did some search and, found something that might not be the most accurate source, but it does seem to indicate problems with the surface-cooling system posing a vulnerability to the aircraft.

Of course, a four-engined aircraft can use slightly smaller engines to at least maintain altitude with one engine out; if the aircraft needs 3000 shp to maintain altitude, a twin needs a total of 6000 shp, but a four-engined aircraft only needs 4000 shp, easing the cooling problem as there is less surface area. If the aircraft has the same total installed power, it would need close to the same net surface area for its cooling system.

From what I remember with the DB606 was 2 x DB601A or 2 x DB601E. That seems to produce produce something like 4 x 1350 PS, the DB-610 produced 2950, which if a single engine, would be 1475 hp.

Looking at the figures you provided with 25% of the heat dumped into the coolant, I get the following

2 x 2700 or 4 x 1350 hp = 4026.8 kW​

4026.8 kW / 4 = 1006.7 kW​

1 x DB606 = 503.3 kW​

1 x DB601 = 125.8 kW​

With 6 kW able to be dissipated in 1 square meter, I get the following

167.8 m^2 total​

83.9 m^2 per 1 x DB606​

41.9 m^2 for 1 x DB601​

That actually exceeds the He 177's wing area. That said, I do remember the He 100 had an extendable radiator to handle heat distribution at low speeds. Regardless, with approximately 25 kW being dissipated in 1 square meter of wing area, I get

40.3 m^2 total​

20.1 m^2 for 1 x DB606​

10.1 m^2 for 1 x DB601​

One thing I'd notice right away is that you'd run into problems if a more powerful engine came along. That became an issue with the P-38, though for a different reason (intercoolers).

I'm curious about cooling drag's effects on flight at high speed at the time: I've been told it increases airplane drag as much as 25% at low-speed and, depending on radiator design, goes down as you go faster. The P-51 and De Havilland Mosquito seem great examples where cooling drag was reduced to near nonexistence.

 

[Zipper730]

This sounds a little bit silly, but I remember that the design had a remote controlled turret up top, which was then augmented by a manned turret behind it: I'm curious why they couldn't just put one remote-controlled turret behind the other; then use a battle-head arrangement more like the Do 217/317 with the sighting station at the back of the battle-head?

 

[swampyankee]

As stupid as this will sound, a mission kill is the same thing as "a kill" correct?

All shootdowns before a bomber gets to its target are mission kills, but so would damage enough to force a mission abort. The latter would generally be preferable for the bomber's air force and air crew as the aircraft may be safely landed and returned to service and none of the air crew get killed.

I'm curious about cooling drag's effects on flight at high speed at the time: I've been told it increases airplane drag as much as 25% at low-speed and, depending on radiator design, goes down as you go faster. The P-51 and De Havilland Mosquito seem great examples where cooling drag was reduced to near nonexistence.

I suspect that most engineers will design the cooling system for optimal performance at high speed, and then check to make sure that it's acceptable at other conditions.

 

[RW Mk. III]

I have to say I don't think the evap cooling system was ever going to be implemented. It could work quite clearly, Schneider cup racers had it functioning. It was never a question of "can we get it sorted?," They could, given the time, resources and necessary modifications to the aircraft as a whole.

But it was chasing a dragon, it would never not be an albatross around the neck of a combat aircraft. I'll elaborate my thoughts:

It simply does not provide enough of a savings in drag to justify itself. The tradeoffs made to go faster are too great for the speed gained.

here are the significant drawbacks as I see it:

1. Economics. A regular old radiator is a contained unit, it can be manufactured by a third party at a third location. Radiators don't necessarily even need to be aircraft-specific. They can be off-the-shelf so to speak even. A good radiator is an economy of materials too, it can be designed to have the required volume and cooling area, and then plumbed into the airframe with a minimum of piping. Consider the vast amount of plumbing needed in an evap cooling aircraft. Pipes, one-way valves, check valves, pressure switches etc etc. The quality of the cooling areas themselves must be much higher too, according to the thermal conductivity properties and pressures needed. It's all very economically intense for how much benefit?

2. Attrition. As mentioned above. The thing is a glass doll. A sufficiently complex system of shut-off and pressure valves with bypass plumbing will allow punctured evap chambers to be isolated. But how many can it afford to lose? How frequently can this thing expect to avoid shrapnel? Shrapnel hits that would be virtually superficial on a more conventional aircraft are now interfering with flight-critical systems. Not to mention stresses from hard landings! The sheer resilience you'd need to a. Design into the system and b. Maintain in the field. The thing just can't afford to leak (much) unless you want to cart hundreds of kilos of spare coolant around and has an awful lot of surface area and seams/joints when you consider the evap chambers and all the accordant piping. I cannot see these things having a very good survivability rate even without damage from enemy action.

Yes the added speed helps evade (but not totally avoid) enemy fighters. But the resultant aircraft is a veritable maintenance basketcase even without holes being punched in it. Anyway that's my take on it.

 

[Shortround6]

Italian Macchi- Castoldi MC 72 Schneider cup racer.

The copper colored areas are the surface cooling, granted this plane has two large V-12s placed end to end but you need a lot of surface cooling area for large engines.
Please remember that the Schneider cup races were pretty much at sea level so the air was dense, you didn't as many cubic feet per second flowing over the surfaces as a high altitude plane would.

 

[Zipper730]

All shootdowns before a bomber gets to its target are mission kills, but so would damage enough to force a mission abort. The latter would generally be preferable for the bomber's air force and air crew as the aircraft may be safely landed and returned to service and none of the air crew get killed.

So, anything that either destroys or prevents the plane from dropping bombs?

I suspect that most engineers will design the cooling system for optimal performance at high speed, and then check to make sure that it's acceptable at other conditions.

I was kind of hoping for some kind of rule of thumb... oh well.

The copper colored areas are the surface cooling, granted this plane has two large V-12s placed end to end but you need a lot of surface cooling area for large engines.
Please remember that the Schneider cup races were pretty much at sea level so the air was dense, you didn't as many cubic feet per second flowing over the surfaces as a high altitude plane would.

That is a lot of surface area for the cooling system -- nearly the whole wing and pylon, as well as a sizable chunk of the floats.

While I've been told surface-evaporative cooling eliminates cooling drag: I'm curious if whether it just produces cooling-drag levels equivalent to a plane without a cooler, or actually produces less drag than a plane without a cooler (i.e. it'd add energy to the airflow)?

 

[swampyankee]

So, anything that either destroys or prevents the plane from dropping bombs?
I was kind of hoping for some kind of rule of thumb... oh well.

I didn't consider that the equipment would be specialized and would likely be produced only by Heinkel itself. That would greatly increase the amount of work required by Heinkel, and reduce the number of aircraft that could be cranked out.

As for attrition -- yeah it wouldn't take much to disable the aircraft -- any hit to the engine block or leading edge would basically incapacitate the aircraft. Clearly, the design would require a proper radiator: With the Germans having much favoritism to the power-egg approach, how much latitude did the designer have to design innovative radiator configrurations?

That is a lot of surface area for the cooling system -- nearly the whole wing and pylon, as well as a sizable chunk of the floats.

While I've been told surface-evaporative cooling eliminates cooling drag: I'm curious if whether it just produces cooling-drag levels equivalent to a plane without a cooler, or actually produces less drag than a plane without a cooler (i.e. it'd add energy to the airflow)?

Warming the airflow may tend to destabilize the boundary layer and increase skin friction drag; cooling the surface tends to stabilize the boundary layer and increase regions of laminar flow.

 

[wuzak]

I suspect that most engineers will design the cooling system for optimal performance at high speed, and then check to make sure that it's acceptable at other conditions.

I believe that the cooling system is usually designed for normal operation. ie cruise condition/normal power.

It is part of the reason why high performance modes, like WEP, are time limited - there is insufficient cooling to maintain the engine temperature at that power level.

EDIT: Also, a cooling system designed for WEP would be much larger and draggier than one designed for cruise.

 

[Shortround6]

Part of this depends on when the plane was designed and what the anticipated power levels were.
The P-38, P-39, P-40 and Allison P-51s could not have been designed to cool at WEP levels because WEP didn't exist when they were designed and wouldn't exist for several years.
However they were designed for full Military power, not cruise.
The worst condition for cooling is climbing. A plane with a best climb speed of 160mph has 1/2 the air per minute going through the cooling system/s as a plane flying level at 320mph. So in a long climb you have high power and low air flow.

 

[Zipper730]

Warming the airflow may tend to destabilize the boundary layer and increase skin friction drag; cooling the surface tends to stabilize the boundary layer and increase regions of laminar flow.

I should have thought of that: I'm curious if the slight turbulence is better than a very good radiator like the P-51/Mosquito?

I believe that the cooling system is usually designed for normal operation. ie cruise condition/normal power.

I figured the limit on WEP was due to the loads on the engine.

The worst condition for cooling is climbing. A plane with a best climb speed of 160mph has 1/2 the air per minute going through the cooling system/s as a plane flying level at 320mph. So in a long climb you have high power and low air flow.

The climb speeds are slower on props due to propeller efficiency right?

 

[Shortround6]

The climb speeds are slower on props due to propeller efficiency right?

No, the best climb speed is either at the point of lowest drag or just slightly above.

At the point of least/minimum drag you have the most surplus power to devote to lifting the airplane (climbing). Sometimes they wanted another 10-20,mph to ensure stability or good airflow over the control surfaces.

 

[Zipper730]

I have to say I don't think the evap cooling system was ever going to be implemented. It could work quite clearly, Schneider cup racers had it functioning. It was never a question of "can we get it sorted?," They could, given the time, resources and necessary modifications to the aircraft as a whole.

I didn't consider that the equipment would be specialized and would likely be produced only by Heinkel itself. That would greatly increase the amount of work required by Heinkel, and reduce the number of aircraft that could be cranked out.

As for attrition -- yeah it wouldn't take much to disable the aircraft -- any hit to the engine block or leading edge would basically incapacitate the aircraft. Clearly, the design would require a proper radiator.

No, the best climb speed is either at the point of lowest drag or just slightly above. ... At the point of least/minimum drag you have the most surplus power to devote to lifting the airplane (climbing).

Then why do jets have a higher climb-speed? I thought that was due to jets having better efficiency at higher speed?

 

[Zipper730]

Being that I just wanted to see what the He 177 would look like with four DB601, I sort of edited a series of images together to produce the effect. I still think it'd look better with some form of battle-head that the Ju-88 and Do-217/317 had. That said, I am doing a little rudimentary work on that.

This image obviously featured the He-177 design which was courtesy of RaspingLeech, but there were bits and pieces used from other designs: The propeller was lifted off the Avro Lincoln, and courtesy of Darth Panda & Sheepster; the nacelle was based on the Me-109F, and courtesy of Nighthunter & Radome; the nacelle paneling was based on the He 100 and, however subtle: It came from the work Nighthunter.

 

[Zipper730]

The Germans seemed to have a favoritism for modular power-egg arrangements, and a tendency to see bombers use both inline and radial (probably why you'd see so many annular radiators), was there any allowances for some kind of creative radiator arrangement more like something we'd see in the USAAF and RAF?

I'm curious because the Germans didn't seem to have good radiator in their bombers...

S Shortround6 , swampyankee , W wuzak