P-51D external tanks

[72modeler]

Wow! Just now found this topic discussion while doing research for another member. Very interesting! The P-51D alluded to, Happy Jack's Go Buggy, is based here in San Antonio, where I live and I get to see and hear her all the time! Very interesting that some flying examples still use the fuselage tank, which caused serious cg problems when full, and IIRC, any maneuvering was prohibited until a good amount had been used out of that tank. I have seen the tanks and plumbing that are on HJGB, and they are very true to the original and function. I hope my fellow Mustang Maniacs will find this post of interest.

Mike

 

https://a2asimulations.com/forum/viewtopic.php?f=93&t=49064

[Work In Progress]

The fuselage tank is definitely something requiring a switched-on pilot and not suitable for anything other than airline-style flying. But in modern conditions, having ditched the heavy radio gear aft o the pilot and replaced it (in most cases) with an empty seat, it's less of a problem than it was in ww2 (providing you have no-one in the rear seat, of course) 

 

 

[72modeler]

1 hour ago, Work In Progress said:

The fuselage tank is definitely something requiring a switched-on pilot and not suitable for anything other than airline-style flying. But in modern conditions, having ditched the heavy radio gear aft o the pilot and replaced it (in most cases) with an empty seat, it's less of a problem than it was in ww2 (providing you have no-one in the rear seat, of course) 

 

 

WIP- hadn't thought of that, but makes a lot of sense. I'm also thinking the Mustang, with all of the armor plate, guns, feed chutes, etc. removed on almost all warbirds, it would still have a pretty darned good range, even without the fuselage tank. Most likely more endurance than the pilots who are flying them. Now....I'm wondering if any of the airworthy P-51's still have functioning relief tubes! As in Pee Fifty One? (Whoo boy- I've still got it!)

Mike

[Toryu]

1 hour ago, Work In Progress said:

But in modern conditions, having ditched the heavy radio gear aft o the pilot and replaced it (in most cases) with an empty seat,

 

I don't think the fuselage tank and a second seat would go along space-wise, would they?

[Work In Progress]

Good thought. You might well be right there, I've never actually had eyes on the fuselage tank in situ, only ever had my nose in aircraft with a rear seat fitted. Of course it used to be fitted simultaneously with the big old radio setup, but that is less vertical than a human. So it might be that the second seat swaps in and out with the rear tank and that it is not actually possible to have both masses in place at once - which would be a relief as it enforces common sense.

Edited September 24, 2019 by Work In Progress

[Work In Progress]

50 minutes ago, 72modeler said:

WIP- hadn't thought of that, but makes a lot of sense. I'm also thinking the Mustang, with all of the armor plate, guns, feed chutes, etc. removed on almost all warbirds, it would still have a pretty darned good range, even without the fuselage tank. Most likely more endurance than the pilots who are flying them. Now....I'm wondering if any of the airworthy P-51's still have functioning relief tubes! As in Pee Fifty One? (Whoo boy- I've still got it!)

Mike

I don;t have the pilot notes handy as I'm on a train, but according to a quick pit of googling it's 92 USG each side and 85 USG in the fuselage tank.  I think normal no-rush cruise fuel burn was about 60 USG per hour in military service. You can get it lower per hour if you really try but then you will be at "best endurance" speed  (normally close to best climb speed, say around 160 mph IAS) and you usually get best range at a slightly higher speed.  So really you would tend to fly at best range / max MPG speed. The real benefit of a lighter and cleaner airframe will be that you can climb quicker for a given power setting, probably climb quite nicely in an auto-lean cruise climb, and then when you get to the height you want to cruise at you will pick up a fair few knots compared to a military spec aeroplane. So you will get better MPG and better range.  

 

At a guess I would say with just the wing tanks you are on for 2.5 hours plus a 30 min reserve at max duration, at say 180 mph cruise, so let's call it 420 to 450 miles range in still air.  That is as long as I like to fly for at any given sector in a warbird type of aeroplane anyway. I would not have enjoyed having to do Berlin trips week in, week out.

Add the fuselage tank and you have say four hours. Add a pair of 110 USG drops and you have perhaps seven hours (you will burn more fuel while heavy with fuel). 

 

If I remember I will look in the books later in the week.

[72modeler]

WIP,

Sounds right- I seem to recall Bud Anderson saying his longest mission in the Mustang was 6.5 hours, and I seem to recall the shuttle missions to Russia went at least seven hours. I sure wouldn't want to have been sitting on that concrete block of a seat parachute sucking oxygen for that long! They were truly manly men back in the day! If I remember correctly, the fuselage tank was mounted below the radio stack, so if that's right, then I don't see how a Mustang with two-seat capability could still retain the tank- unless maybe the capacity was reduced by fitting a smaller tank? 

Mike

 

I found this link that is very interesting and has some great Mustang photos and diagrams; if you scroll 'way down, you will see a cutaway drawing that shows the location of the fuselage tank below the radios- don't see how a tank could still be fitted if  a second seat was installed behind the pilot, I hope this site will be useful to many of you. Awesome photos of Princess Elizabeth- the prettiest  B/C model of them all- in my biased opinion!

 

https://steamcdn-a.akamaihd.net/steam/apps/223770/manuals/DCS_P-51D_Flight_Manual_EN.pdf?t=1509665417

[Jimharley]

Mustangs burn around a gallon a minute in cruise, 35-36"map at 2700rpm. TP-51C Betty Jane had a full set of dual controls and a fully outfitted rear instrument panel...add that weight to a 200lbs passenger and you have close to a full aft gas tank. It flies just fine, however, I noticed it likes to carry an extra 10mph over the fence than the Ds I've flown. Acro was the same as a D, but the C seemed to cruise faster at the same power settings as the D...good aerodynamics I'm thinking. 

 

Jim

[72modeler]

36 minutes ago, Jimharley said:

the C seemed to cruise faster at the same power settings as the D...good aerodynamics I'm thinking. 

Yep- the B and C were faster than the D, at 440mph- due to the smoother airflow and greater stability imbued by the increased keel area and smoother transition of air created by   the canopy being faired in to the fuselage. The loss of keel area when the bubble canopy was fitted to the cut down rear  fuselage is why the D/K was fitted with a dorsal fin extension to improve stability.

Mike

[Jimharley]

11 minutes ago, 72modeler said:

Yep- the B and C were faster than the D, at 440mph- due to the smoother airflow and greater stability imbued by the increased keel area and smoother transition of air created by   the canopy being faired in to the fuselage. The loss of keel area when the bubble canopy was fitted to the cut down rear  fuselage is why the D/K was fitted with a dorsal fin extension to improve stability.

Mike

Exactly Mike, I will say that today, you are limited to 250kts below 10,000ft.. Every time I've flown formation with a D I'm running around 32-33in map versus them up around 35-36, doesn't seem like a lot but the couple times I was flying a D in formation with Betty I was up around 38-40" just to keep up, and that's down low. Amazing the difference. I'd bet that's why Paul Mantz favored the C for his cross country racing...and his entire wing was wet, in addition to the fuse tank.  I've never flown one with drops but in talking with those that have it kills your airspeed quite a bit...20kts or more in cruise. 

 

jim

Edited September 25, 2019 by Jimharley

[Work In Progress]

Great input there from Jim which is far more authoritative than my approximations, so I am pleased my reckoning is in the same ballpark as his practical experience. 

 

Coming back to the CG issue in modern P-51 ops, it is worth bearing in mind also that the the weight of a rear passenger is far, far less than the weight of a full 85 USG tank. The empty seat and the empty tank are probably around the same weight as each other, but 85 USG of fuel weighs a hefty 510lb, which is close to three pie-eaters. Plus during the war you had the heavy radio fit on top of that. 

 

So in today's ops, with no heavy rear radio and no rear fuel tank, squeezing in a rear-seater of normal proportions leaves the CG in a far better place than for a long range wartime mission.

 

Jim's additional point "32-33in map versus them up around 35-36": absolutely may not look like a lot as a number on the screen, but those three inches of manifold pressure are probably about 90 hp difference. And 90 hp may not sound like a lot but it is the same output as normal cruise power in a lot of two-seat aeroplanes, and full power in the original Pitts Special. If you stand behind a 90hp Continental at a full power run-up, it's a whole lot of breeze. 

 

[Bedders]

I'd love to see a real live filletless D-model on the warbird circuit but I understand that FAA/CAA rules dictate that all D-models must have the fin fillet in place. Spoilsports!

 

Justin

[72modeler]

12 hours ago, Bedders said:

I'd love to see a real live filletless D-model on the warbird circuit but I understand that FAA/CAA rules dictate that all D-models must have the fin fillet in place. Spoilsports!

 

Justin

I have been wondering why none of the airworthy restored D's  have been done as D-5! Now I know why- thanks for that tidbit! Bureaucrats-  you gotta love 'em! Since the fillet was added to the D models mostly to compensate for the lack of keel area due to the cutdown rear fuselage, I don't understand why the FAA requires the fillet on D models; Both The P-51B/C and P-51D were fitted withe the 85 gallon fuselage tank, which caused instability and a dangerous aft cg until the some of the contents were used. NAA supplied field kits  to upgrade the D-5's with the fillet, and many were also fitted to P-51B/C's, but they didn't need them for loss of keel area, maybe for better stability with the fuselage tank fitted? That being said, if the 85 gallon fuselage tank isn't fitted to a restored Mustang, I wouldn't think the fillet would be necessary. I'm thinking handling torque with overzealous application of throttle  at low speeds and altitude is much more dangerous for warbird pilots not experienced with high-powered warbirds with big props. than the  presence or absence of a fin strake! I bet @Jimharley can shed light on this subject- I'm sure no Mustang pilot, (Except in my dreams!)

Mike

[72modeler]

Where in my post did I say I was smarter than the groups/organizations mentioned? I think I stated that I was no Mustang pilot, which I thought  would indicate I was no authority. @Jimharley appears to be one, as I think from his comments that he has flown Mustangs. I don't recall in any of the references I have that pilots who flew the early D's had any real issues with  stability due to the loss of keel area, except maybe because it was not as  stable a gun platform as the earlier versions,, but certainly there seems to be a lot of  documentation about  losses of pilots/aircraft  due to the cg issues brought about by any serious maneuvering with a full fuselage tank.

 

What I  really meant, and should have stated, I guess, was if an actual P-51D-5 was restored or  a later version was modified back to that fit, would the FAA require that a dorsal fin strake be fitted? I guess what the FAA requires regarding the Mustang was put in place after the CAB, which  would have been the regulatory agency in place during the time the Mustang was in service? I think I read somewhere in an FAA publication  on certification of aircraft that an aircraft can be granted an airworthy certificate only if it adheres in all respects to drawings, diagrams, dimensions, etc. existing for the type, and if the aircraft is in airworthy condition. I know very little about the certification procedure and guidelines, or about STC's or whatever they are called, except maybe those concerning Beech  18/C-45 wing spars, which I was researching for a fellow modeler recently and discovered the FAA STC's concerning serious wing spar corrosion issues. That being said, would it mean that to the FAA a P-51D-5 in its original configuration, if one was discovered  and restored, would be required to have a dorsal fin strake fitted before it could be certified for flight? Bottom line, this topic ain't no big deal to me, as I'm  no pilot or aeronautical engineer, just a modeler, who also has a curiosity about how and why airplanes are built the way they are. I defer to your expertise.

Mike

[Work In Progress]

4 hours ago, 72modeler said:

What I  really meant, and should have stated, I guess, was if an actual P-51D-5 was restored or  a later version was modified back to that fit, would the FAA require that a dorsal fin strake be fitted?

I can completely see why this question emerges from a mind-set that comes naturally for a modeller or the enthusiastic but extremely small minority of spectators at air shows who know about such variations and notice them. Before I started an involvement in aviation I would very likely have had a similar question in my mind.

 

But it's not a typical aviation mind-set. I have no specific information on what the FAA requires of a bubble-top P-51 today but, thing is, for an aviation operator even to ask that a question would imply someone whose mindset is looking for loopholes in order to take an unnecessary risk for a gain that is of no value to them.  I honestly find it hard to imagine that anyone out there. with the resources, training and network of relationships required to operate a P-51 in today's environment, would consider a minor cosmetic difference like that worth the potential downside.

 

A bubble-top P-51 with no dorsal can undoubtedly be flown with reasonable safety in certain parts of the loading and flight envelope, but there is real value in having more parts of the envelope safely available. All risk is in percentages and any time you operate in an aerobatic envelope you are considerably increasing the risk of departure from controlled flight. And in those circs there is zero doubt that the dorsal fin is a big help.  Nothing re-attaches airflow over a stalled fin like a low aspect ratio dorsal. 

 

Then there's the training issue. Everyone training to fly a P-51 does so in a far more rigorous way than they did during WW2, because your insurer requires it, and at somewhere like https://www.stallion51.com you do a lot of dual departures from controlled flight. And you need the muscle memory that your training develops to be as representative as possible to the muscle memory that will help you in subsequent flying.

 

It's all a balance of risk and reward but from the point of view of the sort of person operating the aeroplane today you'd just be creating entirely voluntary problems for yourself, and for what? The dorsal is easy risk mitigation with no practical downside.  Same sort of reason if you were restoring say, an early B-26 Marauder, there's no way you would reproduce its original configuration where the emergency landing gear deployment required use of the crash axe to cut through the structure. A lot of bad corners were cut in WW2 by rushing things into service before they were properly sorted, risk to the operator or uninvolved bystanders frequently disregarded if it were thought to be offset by the advantage of greater military effectiveness in the short term. Arguably necessary in war, not really a conversation that is encouraged in full size aviation today. 

 

We do see occasional outbreaks of putting the 'cool factor' ahead of sober flight safety considerations - Thunder City's Lightning operations were an example of that - but when the associated bad things happen it tends to reinforce the conservatism of the overwhelming majority of warbird operators.

Edited September 27, 2019 by Work In Progress

[Graham Boak]

The B/C was faster than the D at altitude but slower lower down - the reason lies in the engine fitted which is the US brother to the LF Spitfire Mk.IX rather than the F or HF variety - the supercharger peaks at a lower altitude but had less losses below that.  I'm sure that the B would have been faster than the D given the same engines, but production ones didn't.

 

The aft tank was unstable with the B or a D the solution was to use that fuel first.  Which was fine on a long range mission unlikely to encounter the enemy too soon.  The pilot had to work hard to fly the machine with the aft tank full, but that's ok where no significant manoeuvring is required.  The D was just less safe - there's a sliding scale of safety not an on/off switch, at least until you get too far - and I suspect this could be related to specific fuel states but maybe not.  Not all stability issues are to do with aft cg.  Given that the fillet was not and is not compulsory with the B then I suspect that it is a matter of flow eddies from the rear of the bubble canopy, but it may have been something less obvious.

 

There's always a trade-off between agility and performance in fighter design - the Camel was a killer and like many WW1 aircraft had to be flown continuously.  Things had moved on a bit by WW2 but the same basic tradeoff applied.  Aircraft in civilian hands don't need to fight.

 

Re "cool" factors.  For "associated bad things" read "inevitable bad things".  What's acceptable for pilots of the quality of Keith Hartley just isn't for less capable beings. 

[Work In Progress]

11 minutes ago, Graham Boak said:

.Given that the fillet was not and is not compulsory with the B then I suspect that it is a matter of flow eddies from the rear of the bubble canopy, but it may have been something less obvious.

 

Highly likely to be a factor, especially since we know the bubble canopy reduces speed and therefore must be worsening the airflow over the fin, but also there's just a whole lot less side area aft of the CG when you take down the rear fuselage for bubble canopy and that gross area reduction has to play a part too.

[Jimharley]

The fin fillet was installed on the D to improve yaw stability as mentioned, it didn't have to do with the fuel tank, it was strictly aerodynamics. The B/C model was retro-fitted with the fillet for the same reasons...although, after flying Betty Jane well over 1400 hours I never had a yaw issue. 99% of flying Mustangs are registered in the Limited Category, which, in a nutshell means they are stock airframes. Racers, such as Strega, Voodoo, Dago Red and a few others are in the Experimental Category since they do not conform to the NAA Type Certificate Sheet. Nowadays it is possible to fly a Mustang without a fillet BUT you would have to register the airplane in the "Experimental" category since no D-5s were ever run through the FAA system. As an example, Jack Roush had his P-51B restored as a P-51B, not a P-51C, which is strictly a paperwork issue. The categories were standardized after WWII and no P-51B was registered in the Limited Category until Jack had his airplane built. He did not want an Experimental category Mustang so he jumped through the hoops for his, and future B model restorations by getting all the FAA paperwork required to make it a Limited category airplane.  If I remember right it is one of the reasons he put the fillet on his airplane as well. That is a grey area and not really enforced on other B/C airframes. My understanding is that Paul Mantz and his C models were the basis for the Limited category  P-51C  model. The paperwork did not include the B. There were literally no flying B/C airframes after Mantz and Charles Blair... until Pete Regina restored his during the early 80's.  The trend in the last few years was to use only paperwork and serial numbers (data plates) for the C since all the pencil pushing was already in place, now it's possible to call a B a B with the proper registration. The FAA is actually very warbird friendly, which allows shops to literally build a Mustang from scratch using existing data plate provenance.  As an interesting side note, E.D. Wiener was a prominent racing personality in the late 60's and early 70's and removed the fillet from one of his racing Mustangs to maybe pick up a few extra miles an hour. After flight testing the airplane he found it was too "squirrely" for his liking and put the fillet back on. I hope this helps! 

 

For more info for those interested...when you restore a P-51, or just about any other antique airplane in Limited Category, let's say a P-51, it comes with Supplemental Type Certificates. This, in a nutshell, is a list of FAA approved modifications that differ from the NAA Type Certificate as it left the factory. As an example, the addition of a second seat has paperwork and technical specifications for such an installation. They have been approved by FAA Designated Airworthiness Representatives that deemed them safe and airworthy. This goes for any modification or improvement that has been made to the airframe over the last 70 years. 

 

Jim

Edited September 30, 2019 by Jimharley

[72modeler]

Good points all, and I have learned a lot about this topic from those who have posted, who obviously have a great deal  more knowledge of aerodynamics than I will ever attain. What brought this to mind for me is the restored P-51D-20 Happy Jack's Go Buggy, which is based here where I live, being a D-20, it has the DFF (On the P-51 SIG, it's referred to as the dorsal fin fairing, and there were evidently two different types- one with a straight leading edge and the other with a curved leading edge, known known as the 'swayback' fairing; one was fitted at the assembly plant and the other was supplied as a field kit and installed on B/C's and  early D's) when the actual mount of Jack Ilfrey, the pilot of the real HJGB, was a D-5. I never thought to ask the owner, Bruce Winters, why he chose a scheme that was not accurate for the airplane, but after reading all of the responses, was most likely because the FAA does not allow P-51D's to be flown without the DFF. Thank you gentlemen- one of the reasons I love  this website, is  the vast collective knowledge of the members which is so freely shared; I never fail to learn something about either  modeling or aviation each day! Much obliged. podnuhs, as we say here in sunny south central Texas!

Mike

 

P.S. I know all about Crazy Horse Aviation! I couldn't afford flight time in their 2-seat Mustang, but I did get an hour's stick time in their T-6 several years ago, so I could get a feeling for what my  father experienced when he flew them during his training at Randolph Field  in 1944. I found out real quick what he meant when he told me you had to be sure the tailwheel was locked- said the instructors always checked the wingtips for grass or scrapes to see if you had groundl;ooped!

 

http://www.crazyhorseap.be/Mustangs/Mustangs/N74190HappyJack/N74190HappyJack.htm

Bruce flies her almost every Friday, and she always comes over my house! (Who needs surround sound?)

[72modeler]

@Jimharley,

 

Thanks so much for taking the time for such an extensive reply- answered a lot of nagging questions I have always had on the subject. I remember when Pete and his brother did their P-51B- a real melange of B and D parts, some of which were found in Israel, IIRC. Jack Roush is a real friend of the Mustang community, especially with his Merlin engine rebuilding program. Your explanation of the STC's for the Mustang was very interesting- had no idea that the C model was the one that was used draft the certification paperwork, but that makes sense, as I think all the airframes surplussed and purchased were most likely low-time C's and F6C's. Thank you again, Jim, and welcome to Britmodeller! 👍

Mike

[Adam Maas]

The dorsal fin fillet was also a structural mod, to prevent a known failure mode of the horizontal stabilizer during a slow roll with excessive rudder (which could become a snap roll and overstress the horizontal stab). See TO 01-60J-18 which covered the DFF and TO -1-60J-6C which banned slow rolls in B, C, D &K models until fitted with the DFF and a reverse rudder trim tab (which added control force on the rudder to prevent pilots from accidentally snap-rolling the Mustang when performing a slow roll)