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Mikey-1980

JB-17G - 5 Engined Variant

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Trawling through Instagram last night I came across this very interesting variant of the B-17G.

 

Could make for an interesting subject for some one?

 

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48074464126_a5f3b02e80_z.jpg_20190616_195012

 

48074506953_49d0504516_z.jpg_20190616_195048

 

credit to Cessnateur on Instagram.

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Interesting question about the ballast. The engines concerned with their props and lubricants are all in the order of a ton and a half to two tons. Given the moments you'd probably want about a ton of balancing weight fairly evenly around the rear fuselage as I would not want to pile it all into the tail turret for structural reasons. So altogether it's about the same as the normal bomb load and ammunition weight - no real sweat for the normal piston engines to handle.

 

People tend to forget how much lighter turboprops are than piston engines of similar power, or looked at the other way around, how much power you can get for the same weight. 

An R-3350 weighs about 2700 lb . Even the experimental turboprops of this era, at a similar weight to the 2,200 hp R-3350, were able to produce around 5000 to 6000 hp. On the one turboprop alone with the four piston engines feathered, the B-17 would easily out perform the standard aeroplane.

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In reality the amount of ballast would be located approximately the same distance behind the center of lift as the engine assembly is forward of the center of lift. no?. Or to put it better, more ballast weight the closer to the center of lift and less the further aft you put it.

 It seem correct to me but then again I am a shade tree mechanic.

 

 

Dale

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I think you are trying to solve a hypothetical version of the problem that in more complex than the reality.

 

All you do is add as little weight as you can that will move the CG to the correct place, within the following practical limitations. 

1. will it fit

2. can it be secured properly

3. is it in danger of causing localised overstressing. 

 

If you are doing something intended for long term service or a new production variant you might well get into things like moving existing necessary components like batteries and oxygen bottles, but not for something one-off and purely for a specific non-operational purpose like this.

 

Obviously the further back you put the a pound of mass, the greater the effect. So in principal, for a big lump of metal up front you stuff the balancing mass as far back as possible as that minimises the amount you need. But this particular case, with such a very large additional nose mass, restoring the CG by adding ALL the required balancing mass to the tailcone, might create a significant risk of it departing through the floor every time you hit a bump on the ground on in the air.  Which is why you would spread it out into the rear fuselage.  You meed a higher overall flying weight then, but the B-17G has carrying capacity to spare when unarmed, and probably only part-fuelled.

Edited by Work In Progress

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Is it just me, or are those Curtiss Electric props and hubs in the 4th photo posted? Wondering why the Hamilton Std. props were removed. Maybe something to do with being able to feather the props and restart more easily when all four radial engines were shut down? Just curious!

Mike

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With one system you are relying on the feathering accumulators working, with the other you are relying on the electrics working. Either can fail. But given the rather patchy record of Curtiss Electric props during the war I would personally prefer the Ham Stds. 

 

That particular B-17 is testing a Curtiss engine with (as far as I know) a Curtiss prop on the front, so maybe Curtiss didn't want its rival's products in the publicity shots.

Edited by Work In Progress

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1 hour ago, Work In Progress said:

With one system you are relying on the feathering accumulators working, with the other you are relying on the electrics working. Either can fail. But given the rather patchy record of Curtiss Electric props during the war I would personally prefer the Ham Stds. 

 

That particular B-17 is testing a Curtiss engine with (as far as I know) a Curtiss prop on the front, so maybe Curtiss didn't want its rival's products in the publicity shots.

Hadn't thought of that! Betting that the maintenance on the props  would be much better by the factory crews. I seem to recall using an aircraft's battery to start rather than a cart drained it and caused problems as well as corrosion in  the slip rings that made electrical contact on the C.E. props- especially with early B-26's; which is funny, as  FM-2's with CE props weren't noted as having prop problems,  nor were later B-26's...go figure! It's hard to make a B-17 ugly, but those two five-engined examples were abominations!

Mike

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Interesting thoughts regarding the CoG question. The cockpit was actually moved further aft on this modification - note the different cockpit side window arrangement. Whether this was due to the turboprop taking up too much room or to help address the CoG, I'm not sure.

 

This airframe was subsequently converted back to standard B-17G configuration and flew as Liberty Belle until the fire that destroyed a large part of the airframe a few years ago. The Liberty Foundation are currently restoring it again - hopefully she'll be gracing the skies again in the next few years.

 

Tom

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49 minutes ago, tomprobert said:

 The cockpit was actually moved further aft on this modification - note the different cockpit side window arrangement. Whether this was due to the turboprop taking up too much room or to help address the CoG, I'm not sure.

 

All interconnected, I would say.  It's a long engine and I imagine if they'd left the cockpit where it was then the engine would have had to hang even even further forward, and that increases the CG problem, and also the destabilising pitch and yaw impacts of a far-forward cowling and prop.

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WOW!

this thread is getting more attention than my actual builds 🤣🤣🤣🤣🤣🤣🤣

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