Heinkel He 177

[Zipper730]

An online buddy asked me if I'd ask questions about the He-177 here: I agreed since I already asked some questions about the same plane and we both have unanswered questions about it.

They both revolve around both the engines and their use

1. Engines: The overall drag twin engines is less than four, and applies for the propellers as well. Neither of us are really sure if this applies across the board from takeoff-speed, or from cruise-speed, to Vne. We both think that low drag would have been useful for accelerating in shallow dives, but neither of us have any idea how much a difference it makes in practice.

It's obvious that the engines were mounted fairly close to the spar, and this caused major problems with the aircraft: What isn't obvious is why this is done. Drag seems a good possibility, though she said it could be due to the weight of the engines.

2. Use of Engines: Were the engines pulled back to the stops or pushed up a bit in dives?

 

[Shortround6]

I have seen an estimate of a 3% reduction in drag for the twin installation used on the He 177 vs the 4 engines. Can't remember where though
Drag is drag and would apply at all speeds.
Location close to the spar is probably due to the weight, getting the weight close to the CG and keeping engine mounts short for the dive bomber pull out. Could be wrong.

Most aircraft used part throttle in a dive. It was sometimes a bit of a balancing act depending on propellers. after a certain speed in the dive the airstream is trying to drive the engines, too low a throttle setting and props act like airbrakes. Too high a throttle setting and you risk overspeeding the engine and wrecking it. Most engines would tolerate some degree of overspeed (v-12s better than radials) at part (1/4?) throttle. This also depended on the pitch range of the propeller as those planes that the pilot had manual control of the pitch were set to the coarsest pitch possible before a dive bomber style attack. This is in general as a few planes did try using the prop as a speed brake. Usually this didn't work as it placed a large strain on the prop and reduction gears in the opposite directions that they were designed for. This could be made to work but required heavier prop hubs and reduction gears (and housing/bearings) than normal.

Although not definitive I hope this helps. Perhaps someone has a pilots manual that would describe the dive procedure of the he 177.

 

[Zipper730]

I have seen an estimate of a 3% reduction in drag for the twin installation used on the He 177 vs the 4 engines.

Was this estimate made before or after it flew? If I do my math right that would bring the top speed down by about 10 mph.

Location close to the spar is probably due to the weight, getting the weight close to the CG and keeping engine mounts short for the dive bomber pull out.

Gotcha

Most aircraft used part throttle in a dive.

Did the Germans have automatic boost-control?

Although not definitive I hope this helps.

It's definitely a start

Perhaps someone has a pilots manual that would describe the dive procedure of the he 177.

I hope so!

 

[Zipper730]

Has anybody found the dive-angles the plane was originally designed for? I know after the Spanish Civil War, the dive angle would be increased to 60-degrees. Prior to that point it was at some lower value, and considered to be some form of glide-bombing.

 

[Zipper730]

I can't seem to find anything on that...

 

[Greyman]

Eric Brown says no automatic boost control (he tested the A-5).

Supposedly the specification called for 60 degree dives.

 

[Sid327]

From memory,

Didn''t the RLM (?) do this for the Ju 88 too? (i.e. making this aircraft dive bomber capable).

 

[Capt. Vick]

I believe the A-4 version at least had Dive Brakes

 

[Zipper730]

Supposedly the specification called for 60 degree dives.

I thought the specification called for some degree of glide bombing; then after 1938 it was changed to a 60-degree dive bombing requirement.

 

[pbehn]

The issue with a military aircraft having two or four engines is that unless the two are massively more powerful than the four where is the advantage? A hit or a breakdown on one engine means you lose 25 or 50% of your power, what would you rather have?

 

[Greyman]

I thought the specification called for some degree of glide bombing; then after 1938 it was changed to a 60-degree dive bombing requirement.

Demands/requirements were changing constantly throughout development. I was probably too loose with my application of 'specification'.

 

[GregP]

Virtually none of the W-24 engines were all that reliable. They developed the Allied Allison V-3420, which consisted of two V-1710 Allisons in one crankcase. It was a decent engine but, like all the W engines, if it developed an oil leak, the oil quickly went down to a hot exhaust pipe with predictable results.

The German DB 610 that powered the He 177 had it's own share of oil fires, largely caused by the fact that the inside and outside valve covers and other bolts were hard to reach, making keeping them tight with regular torque checks extremely difficult. Since the aircraft HAD this issue, the expedient of going to a 4-engine layout as in the He 277 seems to me SHOULD have been rather obvious. It had the disadvantages of slightly greater profile drag and slightly reduced roll rate due to the inertia of wing-mounted engines, but this was a bomber after all, not a fighter, and the decrease in roll rate should have been a non-starter as an issue. The advantages were a hugely-reduced chance of inflight fires and a large jump in engine reliability compared with the DB 610. Otherwise, it SHOULD have been about a wash.

The thing is, engine reliability and in-flight fire events were MAJOR in the He 177 operational lifetime.