Long range, high speed Spitfire fighter: the best approach?

[wuzak]

The ultimate LR armed Spitfire would have been the Mk XVIII

This was a Mk XIV with some strucutral reinforcements and 188 gallons of internal fuel. That's 221% of what the Mk I through IX had.

Of course, the Mk XVIII also weighed more empty than the Spitfire Mk I did fully loaded.

What was the range of the XVIII?

The XIV and XVIII had engines angled to give the pilot a better view over the nose. Because of that they had to use a smaller diameter prop, and thus went for 5 blades. I wonder how much of an effect this had on propulsive efficiency? Also, I believe the prop tips would be supersonic near the aircraft's top speed.

For the 21 and later the cockpit was raised slightly and the engine straightened. The landing gear was also longer, I believe. So they were abl;e to use a bigger prop again.

 

[OldSkeptic]

All sorts of stuff.

Minor changes:

1. Fully enclosed wheel wells (tested in the Mk III)
2. Retractable tail wheel (happened in the Mk VIII)
3. Fully flush riveted construction (happened in the Mk VIII)
4. Leading edge fuel tanks (happened in the Mk VIII)
5. Larger forward fuel tanks (happened in the Mk VIII)
6. Fit Hispano Mk V cannon (happened in the Mk 21)
7. Fit inboard .50 cal (happened in the Mk IXe)

Moderately easy changes:

7. Re-angle front windscreen and fit curved 'Speed Spitfire' window fairing (tested on the Mk XIV, showed 8 mph improvement in speed)
8. Fit 29 imp gal rear fuselage tank (happened in the Mk V)
9. Re-design the aircraft to use US-style aircraft fasteners (much lower panel gaps on US fighters)
10. Re-profile the nose skinning to Griffon-engine style 1 piece skin
11. Lengthen the nose profile slightly to add more fuel and aid the CoG issue

Difficult changes:

12. Reprofile the radiators to take better advantage of boundary layer and Meredith effect
13. Partially laminar flow wing

Pretty much bang on. except the wing.
The Spit wing very, very good and had lower drag than the Mustang's in several flight regimes (overall the Spit was better on the climb and worse on the dive, roughly similar on level flight(ie a low Cl), better at high speeds with better mach induced drag (where drag stops being linear with the square of the speed, and become exponential).

Therefore the lower speed for the same power came down to radiator drag and detail drag.

Small details matter. The 20mm cannon Mk Vs were 5mph slower than the Mk Vs with 8x0.303.
The bulletproof flat windscreen cost (with a rear mirror) about 10mph (ditto the Mosquito fighters, fighter/bombers, vs recon and bombers).
It got worse with speed as it had a critical mach limit of only about 0.6 (and a bit from memory), therefore the windscreen drag got worse at high speeds.

Radiator drag. The Spit, like the Me-109 used the first versions of utilising the Meridith effect to reduce radiator drag. The Mustang was a generation ahead.
It's placement was far better, being in the fuselage meant less turbulent air and thus better flow control and cooling efficiency. And the design was better, with variable shuttering vs the simple limited settings for the Spit and 109.
Yes the Mustang still had some net radiator drag, but it was very low.

Even one of the designers of the Mustang discounted the wing (which Supermarine found out when it made its own 'laminar' flow wing) as being the reason for its speed. Wind tunnel tests confirm this.

If you take, on similar powers, a 30mph difference in speed (yes it varied a bit for model to model and configuration to configuration, that's a rough average) then 50% was from radiator efficiency, the other 50% was details. I'd take 25mph as a gap to aim for, because the Mustang would also have lost 5mph with similar cannons.

The prototype Spit Mk III, with better radiators and lots of all those details cleaned up was rated for about 400mph compared to 375 for a similarly powered normal Spit. That's that magic 25mph region again.

Supermarine actually proposed moving the radiators to the same location as in the Mustang, and estimated (for a 60 series engine) 430mph. MAP rejected it due to production disruption issues and then came along the Griffon.

So they could have done it. If RR hadn't been able to keep increasing Merlin power and thing bringing on the Griffon then they would probably have been forced to. But they took the easy way of more power being available as it was easier to re-engine the Spit (a wonderful thing about its design) than do other changes.

 

[nuuumannn]

Yep, Old Skeptic, this makes an interesting what-if, because so much was and could be done with the Spitfire, but I guess there was no need for such a thing from 1943 on (and being anticipated earlier), because of the Mustang - and the fact the British had no requirement for a long range escort, but that's another thread entirely...

 

[stona]

Redesign the wings for less drag, carry fuel similar to P51, redesign the cooling scoops similar to P51, clean up the fuselage, if possible add aft fuel tank?

The Spitfire is to a large extent its wing. It is a very complicated shape indeed.

Redesign the wing and you don't have a Spitfire. The truth is that they went about as far as they could with the Spitfire at the time, you won't be able to achieve anything that wasn't done historically without producing another aeroplane.

How would you clean up the fuselage? It is the shape it is for complicated aerodynamic reasons, particularly the very large wing root fillet (look at a picture of an uncamouflaged Spitfire if you can). Later Marks were flush riveted throughout. There's only so much you can do to the finish of an aircraft in high volume war time production and much of that was done.

Laminar flow wing ? Show me a true laminar flow wing on an aircraft designed in the 1940s,never mind the 1930s like the Spitfire.

Cheers

Steve

 

[davebender]

Mustang was a British fighter aircraft and it was in service by 1942. Isn't that a better starting point for a long range British fighter aircraft?

 

[tomo pauk]

Well, the thread is about it

All sorts of stuff.

Minor changes:

1. Fully enclosed wheel wells (tested in the Mk III)
2. Retractable tail wheel (happened in the Mk VIII)
3. Fully flush riveted construction (happened in the Mk VIII)
4. Leading edge fuel tanks (happened in the Mk VIII)
5. Larger forward fuel tanks (happened in the Mk VIII)
6. Fit Hispano Mk V cannon (happened in the Mk 21)
7. Fit inboard .50 cal (happened in the Mk IXe)

Moderately easy changes:

7. Re-angle front windscreen and fit curved 'Speed Spitfire' window fairing (tested on the Mk XIV, showed 8 mph improvement in speed)
8. Fit 29 imp gal rear fuselage tank (happened in the Mk V)
9. Re-design the aircraft to use US-style aircraft fasteners (much lower panel gaps on US fighters)
10. Re-profile the nose skinning to Griffon-engine style 1 piece skin
11. Lengthen the nose profile slightly to add more fuel and aid the CoG issue

Difficult changes:

12. Reprofile the radiators to take better advantage of boundary layer and Meredith effect
13. Partially laminar flow wing

Agreed pretty much, though the laminar flow wing might not be necessary if the boundary layer (above radiator intakes) is better controlled, and the wheel well covers are installed. The Hispano V is too late for deadline (mid 1943)
The engine should be maybe the Merlin 70 - the Spit VIII IX were making 416 mph @ 27,500 ft with it, vs. 404 mph @ 21,000' ft for the Merlin 66 engined sub-versions. The reduced drag from above-mentioned items might add further 10 mph.

 

[drgondog]

Simple boundary conditions - Spitfire IX (or VIII) to approximate Mustang LR capability

1. Decrease Profile drag by ~40% accompanied with a.) slighly less internal fuel, and b.) install wing pyons and plumbing to replenish additional fuselage tanks, or increase internal fuel approximately to 120% Mustang and match external fuel capability to attach 108 gallon tanks and plumbing.

2. The Wing, despite low inherent Induced drag due an elliptical wing (higher theoretical oswald efficiency), must dramatically enable the most improvement to the Profile drag of the Wing as the wing is by far the biggest contributor to Drag.

3.) The biggest contributors to the Total Drag of the Spit, in cruise condition, in order
Wing Profile drag due to airfoil type and aspect ratio, then the Parasite Drag, including surface roughness including grainy camo paint and including gaps for wheel and flap/aileron hinges
Fuselage Parasite Drag including canopy
Tail surfaces Parasite Drag including roughness
Engine, Radiator, exhaust stacks Parasite Drag
Appendages (masts, armament, tail wheel) Parasite Drag
Induced Drag
Compressibility Drag (V~ >55% M)

From memory the Wing Drag Alone including Profile Drag plus all the components of Parasite Drag of the Wing are ~50% of the Total Profile Drag/Parasite Drag of the airframe.

I would have to do the calcs to be more precise but IIRC the Cdo of the Spit IX is approx .0023.

Spit Induced Drag was inherently low compared to Mustang so all the focus should be on the Wing, and specifically on the wing section to reduce Profile Drag and still maintain high CL

The Mustang has an incredible CL/CD ratio>14:1 where CL=CD at V= best cruise.

If "All that could be done was Done" on the airfoil itself there are very few areas of investigation that will drive 40% out of CDo or even close. I speculate that stripping 20mm and replacing with .50 cal, closing wheel well, surface prep at factory to fill and sand (like Mustang), polishing wind surface, would take 3+% Parasite drag off the airframe and increase top speed 10-15mph at top speed - but stiil not match Total Drag of the Mustang.

Leaving you with necessity of driving internal fuel supply to exceed the Mustang AFTER you did everything else above except improve the wing airfoil...



At High Speed/High altitude Compressibility Drag increases dramatically for a Spit in the .84-.86 range.

 

[OldSkeptic]

Well, the thread is about it
Agreed pretty much, though the laminar flow wing might not be necessary if the boundary layer (above radiator intakes) is better controlled, and the wheel well covers are installed. The Hispano V is too late for deadline (mid 1943)
The engine should be maybe the Merlin 70 - the Spit VIII IX were making 416 mph @ 27,500 ft with it, vs. 404 mph @ 21,000' ft for the Merlin 66 engined sub-versions. The reduced drag from above-mentioned items might add further 10 mph.

The wing position for the radiator was about the worst pl;ace you put it. It not only had issues of turbulence in the rad intake affecting radiator efficiency (and angle of attack issues as well), it also affected wing efficiency too.

Supermarine stuck with the wing position for the Spiteful, my god did they work at it to try and get comparable rad efficiency to the Mustang. They probably pulled it off in he end (just), but it would have been a lot easier to got a for a fuselage mount right from the beginning.

What would have helped was a bit further forward in the wing design, into clearer air as per the Spit Mk III, that would have allowed a more efficient design to be employed and thus cutting rad drag.

Basically you wanted the radiator to be inset into the body of the plane (leading edge was another alternative to the fuselage) and be deep, not hanging off the outside of the wing.
I think, from memory again, Hawker with one of the Tempest designs had a deep leading edge radiator and that they claimed zero net drag from it (the Mustang still had some positive drag, just less than anything else).

Amazing the significant impact of small things at the leading parts of the aircraft. When they were desperate for more low level speed for use against the V1, they found that things like chipped paint and poor filling (and so on) could have 5-15mph speed impacts. Significant improvements were often made just by cleaning up the plane.

That was another advantage of the Mustang, Art Chester (an unsung hero). Though not a formally taught engineer, his experience in flying and building racing aircraft gave him great practical knowledge of the combined drag impacts of very small design elements. Schued put him in charge of the power plant design group (am inspired decision) and all that experience paid off with a very smooth front end with meticulous attention to drag inducing details. That plus the radiator were almost certainly responsible for the speed difference.

What the 'laminar' flow (it wasn't) wing did though was to provide a good critical mach limit, despite being fairly thick (enough for both fuel and guns), due to its pressure distribution.
The P-38 showed the problem of having a fast aircraft with a low mach limit, so all the low drag work on the Mustang would have been useless if its mach limit was not high enough for combat.
And it was good, not as high as a Spit, but as high or higher than the German fighters and that was what counted.

 

[tomo pauk]

Bill, wouldn't the addition of 2 wing racks be a more draggy thing than the existing single attachment point (capable to hold 204 USG drop tank)?

 

[drgondog]

Yes - but a 200 gallon tank on a Spit is a monster and you still have to get internal fuel above 300 to reasonaly match a 51's range.

 

[gumbyk]

The laminar flow wings designed for the Spiteful/Seafang were a pig to get right, with lots of airflow problems around the ailerons, and the bugs were never really ironed out because by 1945 jet aircraft promised better performance. It may well be that successful laminar flow wings could have been designed for the Spitfire had Supermarine consulted with NAA and NACA, but that's pure speculation.

That's not a fault with laminar flow wings, that's a fault with the designers.

 

[OldSkeptic]

Yes - but a 200 gallon tank on a Spit is a monster and you still have to get internal fuel above 300 to reasonaly match a 51's range.

That's right. best, absolute best you could achieve would be about 2/3rds of the P-51. Still very useful in '43.

 

[tomo pauk]

+1 on that. 450+ miles of combat radius should provide problems for LW defenders.

Yes - but a 200 gallon tank on a Spit is a monster and you still have to get internal fuel above 300 to reasonaly match a 51's range.

It was the monster that worked
I was not trying to get the Spitire making 700 miles of combat radius, but some 500, ie. 'beyond Ruhr', per post #1 here. If we arrive to laminar flow wing, then also the U/C gear retracting can be changed, so the gear well is located in front of the main/front spar - that would leave the space between spars for fuel tanks.
Hmm, wonder if the original Spitfire's wing was bale to be reworked for such U/C retracting.

Question for the Spitfire experts: could you please share details about the fuel tank located under the pilot, as found in (early?) PR Spitfires? Was it protected/self-sealing?

 

[wuzak]

Hmm, wonder if the original Spitfire's wing was bale to be reworked for such U/C retracting.

Don't know if the original wing was strong enough to do that. More to the point, the structure would probably need totally redesigning.

Supermarine had the opportunity with the Mk 21 wing - which wasn't laminar flow. I think it had the saem profiles as the original, but had a slightly different plan form.

The 21 had the fully enclosed landing gear (outward retracting still) amd was much stronger. I wonder if there weren't so many interruptions (ie interim models, such as the IX, XII XIV) if Smith could have got the new wing onto production aircraft sooner?

 

[tomo pauk]

Don't know if the original wing was strong enough to do that. More to the point, the structure would probably need totally redesigning.

Fair enough; maybe it would be less of the task than making a whole new wing?

Another random thoughts:
-coolant and oil radiators in the 'beard 'position, like P-40F, Lanc or Beau, should've made boundary layer induction into those a non-issue? (less drag) Intercooler radiator remains in the place for now.
-hig-alt Merlin 70 + clipped wings - less drag (hopefully), better roll rate, but also less lift - wonder how such a plane would've behaved (speed and RoC wise) between 25-30000 ft?
-leading edge radiators, the British introduced those in a number of planes. The fuel can go where the radiators were 'eating into' wings.

Comments?

added: I've dug out the 'Spitfire - History' book; an interesting tidbit might be this: the Spit V was tested in August of 1942, with 170 IG blister tank and 29IG rear tank. Cruising at 15000 ft, the 'still air range' was 1625 miles. The fuel from blister tank was used for climb and initial part of level flight, then use the rear tank (drop the empty blister tank), then switch to main tank(s). Total fuel load 284 imp gals. Corrected (for fuel used for take off and landing?) range was 1550 miles.
my comment: Spitfire VIII should add further 35 IG on this, while the 'torpedo' drop tank of 170 IG cuts drag to the level of the 90 IG blister tank. All available in early 1943...

 

[OldSkeptic]

+1 on that. 450+ miles of combat radius should provide problems for LW defenders.



It was the monster that worked
I was not trying to get the Spitire making 700 miles of combat radius, but some 500, ie. 'beyond Ruhr', per post #1 here.

Modified Mk VIII with a 66 (imperial) gallon rear tank and a 90 gallon (or maybe just a 45) drop tank will do 500 mile combat radius.

All consumption numbers taken from Spit VIII pilot notes:

SpitFire VIII Long Range

Mission Range 500
Combat Time mins 15
Min Rear Tank Level -

Spit VIII Capacities (UK Gals)
Front Tanks 96
Wings 26
Rear Tank 66
Drop Tank(s) 90
Total Fuel 278

Combat Range Calculation

T/O Climb Fuel Use
Main Fuel -
Rear 26
Drop -
Total 26

Cruise to Target Fuel Used
Main Fuel -
Rear 40
Drop 37
Total 77

Cruise to Target Range (Miles)
Main Fuel -
Rear 260
Drop 240
Total 500

Combat (15 mins) Fuel Use
Main Fuel 34
Total 34

Return Fuel Left
Main Fuel Wing 88
Rear Tank -
Total 88
Left 19
Reserve 10.2%

Return Range miles
Main Fuel Wing 640
Rear Tank -
Total 640
Reserve Range 140
% Reserve Range 21.8%



Notes:
All cruise speeds at most economical, 20,000ft at 220mph.
Rear tank emptied first.
When going to target (meeting bombers at some specified point and escorting) the spare capacity in the 90 gallon drop tank allows faster cruise speeds for all or parts of the journey.
External tanks dropped before combat.
Moderate reserve fuel for returning, for periods of faster speeds, otherwise at most economical cruise.
Extra (up to) 26 gallons possible if .303 guns removed and additional leading edge tanks added, though they would impact roll rate.
15 mins combat would more than exhaust ammunition, but allows for climbing back to optimum height and fast disengagement.
Combat fuel allowance taken from pilots notes.
Careful tactics needed so that not all escorts run out of fuel/ammo too fast at the same time leaving bombers vulnerable.
Combat with drop tanks against twin engined Luftwaffe planes (110s and JU-88s) acceptable. Not acceptable for 109s or 190s (obviously).

Other thoughts
Merlin 70 (ie HF version) may be more appropriate than Merlin 66, as it allows higher altitude cruise performance, figures based on LF (Merlin 66) version.
Climbs close to bomber rendezvous (especially for the high cover squadrons) can be done on the drop tank, which has a fair reserve).

If Spit IX used, then leading edge wing tanks need to be added ... obviously.

Pilots need thermal underwear .... and a large bottle....

 

[Jabberwocky]

Fair enough; maybe it would be less of the task than making a whole new wing?

Another random thoughts:
-coolant and oil radiators in the 'beard 'position, like P-40F, Lanc or Beau, should've made boundary layer induction into those a non-issue? (less drag) Intercooler radiator remains in the place for now.

Chin radiators were probably marginally less efficient that the Spitfire's existing radiator set up. Cooling drag is slightly higher overall, and then you have the issues of increased frontal area and, crucially for the Spitfire, nose heaviness.

-hig-alt Merlin 70 + clipped wings - less drag (hopefully), better roll rate, but also less lift - wonder how such a plane would've behaved (speed and RoC wise) between 25-30000 ft?

The clipped wings on the Spitfires actually increased drag at high altitudes, due to much more turbulent wing vorticies and induced drag.

Plus, they have less than desirable effects on climb rate, turn and handling above 20,000 ft.

-leading edge radiators, the British introduced those in a number of planes. The fuel can go where the radiators were 'eating into' wings.

LE radiators were slightly more efficient than either the chin or embeded wing radiators in terms of drag, but only marginally so. Napier put them somewhere between 4.5 and 6.5% better in terms of overall drag.

 

[vinnye]

Didn't the Hurricane have a central radiator scoop?
I would have thought that the designers could have had a look and see how to do so on the Spitfire quite early in its development?

 

[Aozora]

That's not a fault with laminar flow wings, that's a fault with the designers.

And I didn't make the point by saying Supermarine should have/could have worked with NAA and NACA?

 

[wuzak]

Where did PR Spits keep all their fuel?

IIRC they had full width leading edge tanks. Is there some what to use them and add 2 cannon, and 2 0.50s or another 2 20mms?