Why the heck did they design it that way?

[pbehn]

I've read that even during the war the British had restrictions on filling and use of the rear tank on the Mustang. Is there any truth to this as far as you know.

All allied forces did. The P51 with a full rear tank is not a combat plane, you need to be sure that you wont be in combat just after take off with that full.

 

[michael rauls]

All allied forces did. The P51 with a full rear tank is not a combat plane, you need to be sure that you wont be in combat just after take off with that full.

This might sound like a dumb question( imagine that from me) but was there any way to dump that fuel should the need arise say Luftwaffe fighters met over the English channel on the way out for example.

 

[pbehn]

This might sound like a dumb question( imagine that from me) but was there any way to dump that fuel should the need arise say Luftwaffe fighters met over the English channel on the way out for example.

Not that I know of, but I think most of it would have been used by the time they had warmed up, taken off, formed up and climbed. LW fighters coming over the channel would be the job of the RAF they had interceptors lol

 

[GrauGeist]

This might sound like a dumb question( imagine that from me) but was there any way to dump that fuel should the need arise say Luftwaffe fighters met over the English channel on the way out for example.

The rear tank was used from startup through climbing to altitude - even if the P-51 had to scramble due to incoming enemy, that was used to get the ship up.

 

[michael rauls]

Not that I know of, but I think most of it would have been used by the time they had warmed up, taken off, formed up and climbed. LW fighters coming over the channel would be the job of the RAF they had interceptors lol

I didn't realize that rear tank would have been used up that quickly. I guess that kinda eliminates the need to get rid of ot in a posible unexpected situation. Really appreciate you taking the time to answer these questions. I have about a thousand more but don't want to abuse the privilege as they say. So I'll save some of those for later and hopefully others will have some interesting insights into why things were designed in a way that may look at first glance at least, like an oversight. Thanks again!

 

[pbehn]

I didn't realize that rear tank would have been used up that quickly. I guess that kinda eliminates the need to get rid of ot in a posible unexpected situation. Really appreciate you taking the time to answer these questions. I have about a thousand more but don't want to abuse the privilege as they say. So I'll save some of those for later and hopefully others will have some interesting insights into why things were designed in a way that may look at first glance at least, like an oversight. Thanks again!

Try using the forum search function, it throws up some useful stuff.

 

[michael rauls]

The rear tank was used from startup through climbing to altitude - even if the P-51 had to scramble due to incoming enemy, that was used to get the ship up.

I didn't see your post till now. Guess you already answered my next question. Thanks!

 

[pbehn]

I didn't see your post till now. Guess you already answered my next question. Thanks!

The max fuel consumption of a Merlin is about 150gals/hr.

 

[michael rauls]

Try using the forum search function, it throws up some useful stuff.

Thanks for heads up on that. I'll give it a try.

 

[wuzak]

If I have my numbers and math right( always a big if) a 10% increase in fuel capacity would be about 90 lbs for the 109. Especially once somewhat more powerful engines were in the plane doesn't seem like 90 lbs would impact performance much and think what a 10 or 15 % increase in flight time would have meant to the Luftwaffe durring the battle of Britain for example.

In hindsight we know that the Spitfire and Bf 109 got more powerful engines as they were developed. But during the design and development period they really could not rely on more power being available in the short term.

 

[michael rauls]

In hindsight we know that the Spitfire and Bf 109 got more powerful engines as they were developed. But during the design and development period they really could not rely on more power being available in the short term.

I was thinking more along the lines of a modification to a slightly larger tank when more powerful engines became available but perhaps this was impractical.

 

[michael rauls]

I was thinking more along the lines of a modification to a slightly larger tank when more powerful engines became available but perhaps this was impractical.

I guess that wouldn't be a case of original design but of later modification now that I think of it.

 

[GrauGeist]

I was thinking more along the lines of a modification to a slightly larger tank when more powerful engines became available but perhaps this was impractical.

More powerful engines are usually heavier than their predecessor, adding that and a larger fuel tank is imposing a weight penalty.

And keep in mind that the existing fuel tank is already engineered for the available space in the fuselage - there may not be any room for a larger tank.

 

[wuzak]

I was thinking more along the lines of a modification to a slightly larger tank when more powerful engines became available but perhaps this was impractical.

Supermarine did try and stuff more fuel into the Spitfire over the years.

But the amount they could add was limited by the space available in the airframe.

They added bladder tanks in the inner leading edge of the wings of the VIII and XIV. 17.5 proper gallons each, IIRC.

Rear fuselage tanks were added, with flight restrictions due to loss of stability when they were full.

But these required modifications to the wing, which may have been factory only mods (ie produced that way) that could not be added in the field.

The forward upper tank in the XIV was bigger than the Merlin Spitfires had. Had to move things around, such as the oil tank, to fit that.

 

[michael rauls]

More powerful engines are usually heavier than their predecessor, adding that and a larger fuel tank is imposing a weight penalty.

And keep in mind that the existing fuel tank is already engineered for the available space in the fuselage - there may not be any room for a larger tank.

All good points. I was thinking that even a tiny increase in wieght of fuel could have huge benefits in certain situations. I suppose once you head down that road at the drawing board you could end up in a never ending" what about 10 more gallons" and 10 more after that and 10 more after that until you end up with as someone said a flying fuel truck. I guess you have to draw the line somewhere and they drew it where the thought it needed to be but it does seem if modification for a small amount of additional fuel was possible later it certainly would have yielded large benefits. Maybe it wasn't practical or maybe it's one of those looks obvious with the benefit of hindsight but didn't seem so at the time things. One of those missed opportunities.

 

[michael rauls]

Supermarine did try and stuff more fuel into the Spitfire over the years.

But the amount they could add was limited by the space available in the airframe.

They added bladder tanks in the inner leading edge of the wings of the VIII and XIV. 17.5 proper gallons each, IIRC.

Rear fuselage tanks were added, with flight restrictions due to loss of stability when they were full.

But these required modifications to the wing, which may have been factory only mods (ie produced that way) that could not be added in the field.

The forward upper tank in the XIV was bigger than the Merlin Spitfires had. Had to move things around, such as the oil tank, to fit that.

That makes sense. If the fuselage is narrow in particular you you are limited as to how wide you can go with an additional tank and you can only go so far back before you run into serrious stability issues.

 

[michael rauls]

Supermarine did try and stuff more fuel into the Spitfire over the years.

But the amount they could add was limited by the space available in the airframe.

They added bladder tanks in the inner leading edge of the wings of the VIII and XIV. 17.5 proper gallons each, IIRC.

Rear fuselage tanks were added, with flight restrictions due to loss of stability when they were full.

But these required modifications to the wing, which may have been factory only mods (ie produced that way) that could not be added in the field.

The forward upper tank in the XIV was bigger than the Merlin Spitfires had. Had to move things around, such as the oil tank, to fit that.

It does sound like they did everything they could do within the limitations of the airframe. I guess at some point if you want more range you just need more space to put the fuel. I.e. a larger fuselage or thicker wings and both of those have downsides.

 

[buffnut453]

It does sound like they did everything they could do within the limitations of the airframe. I guess at some point if you want more range you just need more space to put the fuel. I.e. a larger fuselage or thicker wings and both of those have downsides.

And a more powerful engine to lug all that extra weight and drag around the sky at equivalent performance to the smaller aircraft.

 

[GrauGeist]

Also bear in mind that additional fuel in the fuselage (where ever it might be added) will have a direct effect on the CoG (center of gravity).

The Me262, for example, had two tanks in the fuselage, one ahead of the cockpit (at the gunbay's bulkhead) and one behind the cockpit. If the fuel was used from the rear tank first, it caused the 262 to become "nose heavy" in it's performance, so the procedure was to feed from the front tank first, then switch to the rear tank.

Many other types with similar fuselage fuel cell configurations had the same performance issues if fuel management wasn't properly followed.

 

[pbehn]

All sorts of "stuff" was done or could have been. In addition to the leading edge tanks mentioned, there was a tank used in PR spits never used on fighters it went under the pilots seat. The Griffon engine moved things about and increased the forward tank capacity. However it seems to me things were not that pressing for more range until 1942 when the RAF started to receive the first Mustangs anyway. Another thing to bear in mind was the Typhoon was supposed to replace the Spitfire but never did. Work on the Spitfire MK VIII started in early 1942 but the FW 190 force the stop gap MK IX into production. The MK VIII had a low back, bubble canopy rear fuselage tank bigger front tank however in addition to carrying more fuel the later Merlin generally used more fuel.