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On 11/14/2017 at 12:09 PM, Collimatrix said:

 

I haven't found the capacity of that rear tank, but yes, by all accounts the pony was... squirrely until it was empty.  Late model FW-190s had a similar fuel tank behind the pilot, and I would bet they emptied that one first too.

 

On 11/14/2017 at 11:07 PM, Jeeps_Guns_Tanks said:

85 gallons. 

 

P-51D-fuel-tanks.png

 

On 11/14/2017 at 11:26 PM, Collimatrix said:

So that's about 500 pounds placed about four feet behind the CG.  Yep, that could cause some issues.

 

 

Not as bad as you'd think, the CG on the '51 was fairly flexible and trimmable. In addition to that 85 gallon tank, the battery of radios would sit above it, and they were not small either.  Then there was the Radiator...

 

Later, folks would remove the tank and radios and fit a second seat there, with little change in performance. 

 

The Collings foundation has a twin seat P-51 B/C.

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8 hours ago, Collimatrix said:

I think even with a seat and oxygen equipment, a passenger would weigh less than 85 gallons of fuel.

Depending on the conversion (and passenger) it often weighs more.  The one owned by the Collings foundation for example, has full dual controls and instrumentation.

Profile-

cockpit-

 

10 minutes ago, Jeeps_Guns_Tanks said:

Do civy planes warbirds have to have working oxygen systems if they can get that high?

Yes, but modern O2 systems are much lighter than the old LP systems used in WW2 aircraft. 

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4 hours ago, Collimatrix said:

 

Interesting.  Do they have to put any ballast in the front of the aircraft to offset that?

Never got a straight answer. 

I also assume there was a bit of alteration to the horizontal stab to account for the change in arm, since the bird is also slightly longer than stock in that config. 

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  • 2 weeks later...
On 12/1/2017 at 6:12 PM, Collimatrix said:

 

I figured you probably weren't a stranger to 'em.  How do they compare to modern setups for cars and the like?

Oh, completely different. Turbines were HUGE. Like 8~12" across, so spool up was slow. For a plane mill, not a huge deal,

since you were turning a huge prop, not a  30 odd inch wheel/tire.

 

Also, the biggest difference is the vast majority of WW2 era turbosuperchargers were intended to suppliment an already 

existing supercharger. As in "provide sea level atmosphere at 30K feet" and from there the existing blower would do the rest. 

Some, like the GE's on the P-38 could do far more, raising upper deck pressures to dangerous levels at sea level, much to the detriment of the 1710,

the maintenance crew, and the amusement of the pilot (My grandad told me about pilots doing this, running absurd levels of boost at TO to get to altitude fast).

 

Lastly is the size of the install. To start, you're not feeding 1500+ cubic inches in a car. So the plumbing, intercooler, turbo, wastegate, etc can be a lot smaller. 

 

Case in point, note how much space this install takes up. 

Jug Huff

 

iEZpLMy.jpg

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Modern cars have all sorts of ways of sidestepping turbo lag.  There are twin turbo setups, comprex superchargers, variable geometry, etc.  I suspect, but don't know for sure, that modern consumer car turbo metallurgy is beyond the wildest dreams of WWII turbo designers, since it's probably piggybacking on several decades of jet engine turbine development.

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I've noticed that on a lot of Western Allied radial engines, most all being of Wright and Pratt & Whitney making, there is a lack of a large nose cone or cover on the end of the propeller shaft. 

 

id_fighters_p47_06_700.jpg 

 

 

 

Meanwhile, everyone else has one

 

ww2kanin1k1j-0.jpg        

 

 

Is there a reason for this design choice?  Is it something to do with the propeller shaft?

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On 12/3/2017 at 7:40 PM, Scolopax said:

I've noticed that on a lot of Western Allied radial engines, most all being of Wright and Pratt & Whitney making, there is a lack of a large nose cone or cover on the end of the propeller shaft. 

 

id_fighters_p47_06_700.jpg 

 

 

 

Meanwhile, everyone else has one

 

ww2kanin1k1j-0.jpg        

 

 

Is there a reason for this design choice?  Is it something to do with the propeller shaft?

 

 

The piece you're looking at is called the "spinner," and yes, it is aerodynamically important.

 

There were various attempts to improve the streamlining of the spinner on radial engines.  The most radical approach was the ducted spinner used on the FW-190 prototype:

440px-Fw190V1.jpg

 

Something similar was tried on some experimental Tempests, although those had in-line engines:

EkCteWR.png

I don't know specifically why the spinner was so small on most R-2800s, but I suspect that it had to do with cooling.  The R-2800 had some pretty formidable cooling requirements.  When it was first developed it had one of the highest horsepower to cylinder count ratios of any engine in the world.  It was also the first engine from P&W with machined cooling fins.  Previously, it had sufficed to forge or cast the cooling fins surrounding the cylinders.  On the R2800, Pratt and Whitney had to finely machine each fin so they could make them as thin and densely packed as possible, for maximum cooling surface area.  Obviously, this is expensive and an enormous pain in the ass, so it gives you an idea of how hard it was to cool the monster.

 

I think ducted spinners, like V-tails did reduce drag by a measureable amount, but introduced so many other problems (the FW-190 prototype had engine cooling issues) that the juice was not worth the squeeze.

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On 12/3/2017 at 8:40 PM, Scolopax said:

I've noticed that on a lot of Western Allied radial engines, most all being of Wright and Pratt & Whitney making, there is a lack of a large nose cone or cover on the end of the propeller shaft. 

 

id_fighters_p47_06_700.jpg 

 

 

 

Meanwhile, everyone else has one

 

ww2kanin1k1j-0.jpg        

 

 

Is there a reason for this design choice?  Is it something to do with the propeller shaft?

No spinner makes for easy prop swaps, that's for sure.

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On the topic brought up previously about push-pull configurations, but sadly off topic for the thread, why didn't four engined bombers use push-pull configurations to reduce drag? They wouldn't have the bailing out, take-off, center of gravity, navigation, and vibration problems that fighters had, and the 4 engine nacelles on a bomber look like they would cause a lot of drag that could have been reduced with just two push-pull nacelles. Just not worth it on a heavy bomber compared to a fighter?

 

There seem to have been quite a few 4-engined push-pull aircraft in the Interwar but their designs cease almost entirely by 1935 after having trailed off after 1930.

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There's not a whole lot of surviving documentation on WWI aircraft, but supposing the Fokker Dr.1 represents engine horsepower (at 110HP as one source cites) of WWI aviation engines in general, I'd guess the biggest issue would be cooling. Advances in engine design, engineering processes, and material technology allowed rated power on engines to soar by the time we reached WWII. While it's possible to dump more fuel and air at once into larger and more numerous cylinders, thermodynamics aren't on vacation and you need to dump the waste heat overboard if you don't want your pistons to take a forever break. The Wright 1820 which saw use on B17s developed something like 700 rated HP depending on your model. I'm not sure waste heat scales linearly to HP but it seems intuitive to say it should, and at any rate that's a lot more waste heat than you saw in WWI planes in which could probably get by on a single cooling intake for both engines without a huge increase in front profile. So the issue for me seems to be that you just need more space in your frontal profile for cooling, which is made easy by having a full pod for each engine. There are some ways to mitigate cylinder heat, like running rich of peak and having oil coolers, but you can only mess with your F/A ratio so much before your engine can't burn fuel anymore, and oil coolers still take up some of your front profile so you're still canceling out some of your lift to cool the oil.

 

Some other things could have had an impact as well; assuming your design doesn't have the crank running through the rear of the engine so that both propellers are powered by the same engine (which means you've lost the HP of an extra engine), you need more engines, and more cylinders means the engines need more cooling air. You could run ducting for ram air into the nacelle , but that means a bigger nacelle, which probably already got bigger when you stuffed the second engine into it. Push engines in push pull configurations already lose some efficiency from operating in the disturbed airstream from the puller props, and the efficiency you gained by losing an engine pod is rapidly being reclaimed by the inescapable tendency of this world to hate fun. I'm not sure where the lines cross and whether you've gained or lost efficiency, but I'd guess the complexity you add to the system makes it easier, if not more efficient to just make the thing with four nacelles after the previous downsides have already been added up.

 

A few other possibilities; a lot of WWII bombers were conventional geared and their props already came fairly close to the ground; with the pusher props located behind the pullers, there could have been some risk for prop strikes, which would probably be the easiest engineering hurdle to fix. The mounting points would have had to have been reinforced to probably slightly less than twice their original strength to hold the new engine and all of its accessories, which may have been too much put at one location on a spar with WWII engineering (decent chance that this is bullshit!). 

 

Of course the easiest answer is that push pulls were unconventional and they may not have wanted to push something untested into production when the convention was already well tested.

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For efficient propulsion you want to throw lots of mass of air backwards at lowish speed1 ([mass per second * speed it's pushed out the propeller] gives the thrust, whereas [mass per second * speed it's pushed out the propeller ^2] gives the energy used which should correlate to the fuel used). The rearward prop is pushing on air that's already moving backwards, so needs more power to get the same thrust as the front prop. Same reason high bypass turbofans are more efficient than turbojets.

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On 12/12/2017 at 8:53 PM, AdmiralTheisman said:

On the topic brought up previously about push-pull configurations, but sadly off topic for the thread, why didn't four engined bombers use push-pull configurations to reduce drag? They wouldn't have the bailing out, take-off, center of gravity, navigation, and vibration problems that fighters had, and the 4 engine nacelles on a bomber look like they would cause a lot of drag that could have been reduced with just two push-pull nacelles. Just not worth it on a heavy bomber compared to a fighter?

 

There seem to have been quite a few 4-engined push-pull aircraft in the Interwar but their designs cease almost entirely by 1935 after having trailed off after 1930.

 

This is a good question, and I don't know the definitive answer offhand.  If I had to guess though, cooling was the biggest problem.  The HE-177 did not have a push-pull configuration, but it did have four engines with only two engine nacelles.  Each nacelle had two engines in tandem, driving a common driveshaft and propeller.

 

Cooling was evidently a problem, as the HE-177 had, according to Bill Gunston, probably the greatest propensity of any aircraft ever mass produced for catching on fire during level, cruising flight.

 

Another issue is maintenance.  WWII high-output piston engines were very maintenance intensive, and stuffing the engines together in common nacelles would have made service trickier.

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On 12/2/2017 at 9:33 PM, Collimatrix said:

Modern cars have all sorts of ways of sidestepping turbo lag.  There are twin turbo setups, comprex superchargers, variable geometry, etc.  I suspect, but don't know for sure, that modern consumer car turbo metallurgy is beyond the wildest dreams of WWII turbo designers, since it's probably piggybacking on several decades of jet engine turbine development.

 

Late to this, but "turbo lag" is a non-issue in aviation.   Modern aviation turbosuperchargers would likely blow the heads/intake of a modern car. They are simply huge, and prone to overboost ASL.  

The downside is that they are slow to drop boost.  When you design to supply sea level atmosphere at 30K feet, with few rapid throttle changes,  you divorce yourself from what is needed for automotive use. 

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On 12/12/2017 at 9:53 PM, AdmiralTheisman said:

On the topic brought up previously about push-pull configurations, but sadly off topic for the thread, why didn't four engined bombers use push-pull configurations to reduce drag? They wouldn't have the bailing out, take-off, center of gravity, navigation, and vibration problems that fighters had, and the 4 engine nacelles on a bomber look like they would cause a lot of drag that could have been reduced with just two push-pull nacelles. Just not worth it on a heavy bomber compared to a fighter?

 

There seem to have been quite a few 4-engined push-pull aircraft in the Interwar but their designs cease almost entirely by 1935 after having trailed off after 1930.

"Propwash". 

 

Running one big swirly bit in the wake of another big swirly bit yields all kinds of unfun.  This level of unfun increases with the size of the mill, and the size of the prop.

 

The Pfiel/Do335 (for example) was prone to pitch oscillation caused by the two blade-discs interacting.  It bordered on unmanageable. 

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On 12/12/2017 at 11:36 PM, Vanagandr said:

There's not a whole lot of surviving documentation on WWI aircraft, but supposing the Fokker Dr.1 represents engine horsepower (at 110HP as one source cites) of WWI aviation engines in general, I'd guess the biggest issue would be cooling. Advances in engine design, engineering processes, and material technology allowed rated power on engines to soar by the time we reached WWII. While it's possible to dump more fuel and air at once into larger and more numerous cylinders, thermodynamics aren't on vacation and you need to dump the waste heat overboard if you don't want your pistons to take a forever break. The Wright 1820 which saw use on B17s developed something like 700 rated HP depending on your model. I'm not sure waste heat scales linearly to HP but it seems intuitive to say it should, and at any rate that's a lot more waste heat than you saw in WWI planes in which could probably get by on a single cooling intake for both engines without a huge increase in front profile. So the issue for me seems to be that you just need more space in your frontal profile for cooling, which is made easy by having a full pod for each engine. There are some ways to mitigate cylinder heat, like running rich of peak and having oil coolers, but you can only mess with your F/A ratio so much before your engine can't burn fuel anymore, and oil coolers still take up some of your front profile so you're still canceling out some of your lift to cool the oil.

 

Some other things could have had an impact as well; assuming your design doesn't have the crank running through the rear of the engine so that both propellers are powered by the same engine (which means you've lost the HP of an extra engine), you need more engines, and more cylinders means the engines need more cooling air. You could run ducting for ram air into the nacelle , but that means a bigger nacelle, which probably already got bigger when you stuffed the second engine into it. Push engines in push pull configurations already lose some efficiency from operating in the disturbed airstream from the puller props, and the efficiency you gained by losing an engine pod is rapidly being reclaimed by the inescapable tendency of this world to hate fun. I'm not sure where the lines cross and whether you've gained or lost efficiency, but I'd guess the complexity you add to the system makes it easier, if not more efficient to just make the thing with four nacelles after the previous downsides have already been added up.

 

A few other possibilities; a lot of WWII bombers were conventional geared and their props already came fairly close to the ground; with the pusher props located behind the pullers, there could have been some risk for prop strikes, which would probably be the easiest engineering hurdle to fix. The mounting points would have had to have been reinforced to probably slightly less than twice their original strength to hold the new engine and all of its accessories, which may have been too much put at one location on a spar with WWII engineering (decent chance that this is bullshit!). 

 

Of course the easiest answer is that push pulls were unconventional and they may not have wanted to push something untested into production when the convention was already well tested.

Oil cooling works VERY well, in a very small range..

 

The 1820 was very understressed.  They could push/exceed the 1000 HP mark easily when feed decent fuel and cammed/timed properly.

 

Waste heat can be used.  I/E P-51, or NACA cowls. 

 

 

Also, my job sucks.

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  • 1 month later...

Hi y'all.

 

Some interesting views put forth, here in this thread.

Do allow some corrections, though, won't you fellas...

 

1,The rear "Aux" fuselage tank in the P-51 wasn't actually a "drop tank" - it was for transit flying & was used 1st,

rather than in combat.

2, P-38 did ok against the lesser forces of Nippon, but could not technologically* hack the pace in the ETO,

  & was replaced by the P-51.

3, P-47, like the other P&W R-2800 powered fighters  - was a real gas-hog,  even at a slow cruise speed,

& like the P-38, was dumped by the 8th AF, on the advent of the P-51 - arriving in numbers.

4, P-51 did not need the "dive flaps" belatedly attached to both P-38 & P-47 to recover control,

& pull out safely - from 'Mach crit' high speed dives.

5, Victory stats collected by the USAAF in the ETO, confirmed their top choice of the P-51 as air-superiority fighter,

- was the right one**.

 

* P-38 was a real handful of control complexity to simply fly, let alone operate in combat - esp' against the faster diving 109/190, whose pilots could spot the big twin-boom Lockheed from a distance,

& then choose an attack profile - to suit themselves. 

 

**The RAF wanted all the Mustangs they could get, (even the Allison powered ones, which they used 'til war's end),

but didn't want P-38's, & relegated the hundreds of lend-lease P-47's they received - solely to combat against the lesser forces of Nippon, too. 

 

 

 

 

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