|Technical Note No.Aero.1501(Flight)||Technical Note No.Aero.1501(Flight)|
Improvement of Performance of Fighter Aircraft
Operating Against the German Flying Bomb
- by -
W. Stewart, B.SC.
R.A.E. Ref: 1465R/WS/140
Performance measurements of top level speeds at low altitudes have been made on Tempest V, Mustang III and Spitfire XIV aircraft, as received from Squadrons operating against the German flying bombs.
Improvements on the top level speed of the Tempest V were 5 ½ m.p.h. due to improved quality of finish and 20 m.p.h. due to increasing the boost pressure from +9 to +11 lb./sq.in.
On the Mustang III the wings were cleaned up and the bomb racks and a small bracket at the base of the whip aerial removed giving a total increase of 21 m.p.h. It is estimated that the separate effects were 12 m.p.h. for improved finish and 8 and 1 m.p.h. for the bomb racks and aerial bracket respectively. Replacing the exhausts by Spitfire type stubs gave a further increase of 1 ½ m.p.h.
Improved quality of finish on the Spitfire XIV gave 8 m.p.h. increase in level speed. The boost pressure was increased from +19 to +25 lb./sq.in. but only two level speed measurements were obtained. These indicated an increase of 28 m.p.h.
Fighter aircraft of A.D.G.B. have been in operation recently against the German flying bomb. It was realized that in this type of work, optimum top level speed performance at low altitudes was one of the main requirements for successful fighter interception.
Tempest V, Mustang III and Spitfire XIV aircraft were sent to the R.A.E. Farnborough from operational squadrons to determine the possible improvements in performance that could be obtained on the average operational aircraft.
The top level speed performance of each aircraft was measured in the condition as received. Improvement of performance was obtained in one or more of the following ways:-
(a) by cleaning up the wing surfaces and generally improving the finish of the paintwork on the aircraft.
(b) by the removal (or improvement) of operational equipment not required on patrols against German flying bombs.
(c) by increasing the available engine power.
3. Method of Test
As the operational height of the majority of the German flying bombs is of the order of 3,000 ft., the level speed performance of each aircraft was required at low altitude only and has been measured over a range from 0 to 8000 ft.
The results have been corrected for instrument errors, compressibility correction to the A.S.I., position error, variation of temperature from standard and for any slight differences in boost pressure from the stated values. The position error for each aircraft was obtained by measurement on the speed course.
Top level speed measurements were made on each aircraft as received from the squadron and after various modifications. Details of the modifications carried out on each aircraft are described, and the effect of each is given in the results.
4. Description of Aircraft and Modifications
4.1 Tempest V (Sabre II.)
Considerable difficulty was experienced on this type of aircraft due to unserviceability. Three aircraft were received from Squadron.
The effect of improving the condition of the paintwork was measured on Tempest JN.783. This was a standard series I fighter aircraft (four 20 m.m. cannon projecting from the leading edge of the wing). The gun muzzles were sealed. There were blisters on top of the wings over each gun. A debris guard was fitted in the air intake.
The paintwork was in fairly poor condition. It was badly chipped along the leading edge of the wing. The wing surface was stripped for a distance of about 2 ft. back from the leading edge and repainted. The rest of the wing and aircraft surfaces were rubbed down only. The aircraft could have been maintained in this final condition without any great difficulty under squadron conditions.
During the test, the engine in this aircraft was giving only +7 ½ lb./sq.in. boost and as a defect was subsequently found, J.N.738 was rejected for further tests at higher boost.
This aircraft was replaced by Tempest J.N.735 but, during the first flight, the engine caught fire and the aircraft had to be abandoned.
The effect of increasing the boost pressure was measured on Tempest J.N.763. This aircraft was a standard series I fighter aircraft, similar in quality of finish and in external equipment to J.N.738, described above, except that it did not possess a debris guard.
A new boost cam and capsule were fitted allowing an increase in boost pressure to +11 lb./sq.in. The 150 octane fuel was used.
Flights were made at +9, +10 and +10 ½ lb./sq.in. boost respectively (3,700 r.p.m.) One flight at +11 lb./sq.in. boost was made but engine trouble was experienced and in the subsequent inspection, parts of the pistons and piston rings were found in the oil filters.
4.2 Mustang III (Merlin V-1650-7)
Performance measurements were made on Mustang III F.B.377. This aircraft, as received from Squadron [No. 316, Brent], was flying at +25 lb./sq. in. boost, 3,000 r.p.m. The enrichment jet and the balance pipe had not been fitted. These modifications were carried out.
The aircraft was a standard mark III type. Bomb racks were fitted under the wings. The whip type aerial was fitted behind the hood and offset to starboard.. The adhesive patches over the gun ports were loose and were replaced by fabric patches before flight. Except for this minor altercation, the aircraft was flown as received.
The paintwork was in a very poor condition. The paint on the leading edge and inboard surfaces of the wings was badly chipped. This is most serious in the case of the Mustang due to the thickness of the paint layer. At least six separate coats of paint had been applied.
In the cleaning up tests, the leading 2 ft. of the wing surfaces had to be stripped of paint and repainted. The rest of the aircraft was rubbed down only.
The bomb racks were removed and also a small projecting bracket at the base of the whip aerial.
At low altitudes and at 25 lb./sq. in. boost, the original exhausts are inadequate in providing the optimal exit area. The exhausts were therefore replaced by Spitfire type stubs representing approximately the correct area required. These were the only type available at the time. No exhaust shrouds were fitted in either case.
4.3 Spitfire XIV (Griffon 65)
Spitfire R.B.176, as received from Squadron, was operating at +19 lb./sq.in. boost and 2,750 r.p.m. (Griffon 65; 5-bladed Rotol propeller). It had a circular external rear view mirror with hemispherical fairing; no ice-guard on the air intake; a whip type aerial behind the hood; a radio mast projecting from the lower surface of the wing forward of the starboard aileron; another mast set in a fairing under the fuselage; small type bulges over the 20 m.m. cannon and the cannon stubs faired; the machine gun ports in the leading edge sealed. The 30 gal. auxiliary fuel tank (slipper type) was removed for the purpose of the tests.
The paintwork was in poor condition. Parts of the leading edge and inboard surfaces of the wings were very badly chipped and scored. The leading edge was stripped of paint and repainted. The rest of the aircraft was rubbed down only.
The engine was then modified to give +25 lb./sq.in. boost and one flight was made. Only two level speed measurements were obtained, as the engine became suddenly rough after about two minutes of the high boost on each level. Subsequent inspection showed that a blow back had occurred, damaging the air intake. The reduction gear was also found to be cracked and no further tests could be made with this engine.
It will be noticed, in the above descriptions, that the paint was stripped from the leading 2 ft. of the wing surfaces of each aircraft. This was done merely because of the facilities available for this type of work under the existing conditions at Farnborough. Under Squadron conditions, the same results could have been obtained by treating the wing surfaces in sections. In the case of the Spitfire in particular the inboard sections of the wing are much worse than the outboard sections, due to careless reloading of the ammunition belts, etc. and in the limited resources of Squadron conditions only the worst sections need be repainted. The rest of the wings can be successfully treated by stopping any cracks and careful rubbing down. Again it must be emphasized that thorough rubbing down is the most important feature in producing a smooth surface and that the smoothness of the surface is entirely independent of the glossiness of the finish.
The level speed – altitude curves showing the increase in speed due to improved surface finish and due to increase in boost pressure are given in figs. 1 and 2 respectively. The position error measurements are given in fig.3.
As pointed out in the description, the improvement of finish of the wing surfaces was done quickly and only carried to a condition which it was thought could be maintained under squadron conditions. This gave an increase of 5 ½ m.p.h.
The measured increase in speed due to increasing the boost pressure from +9 to +10 and +10 ½ lb./sq.in. was 10 and 15 m.p.h. respectively. The dotted lines (fig.2) show the increase that would be expected by increasing the boost pressure from +9 to +11 lb./sq.in. is 20 m.p.h. and this gives a true air speed of 412 m.p.h. at 2,750 ft.
This aircraft had a similar finish to J.N.738 in its original condition and it is expected that with similar treatment, the speed of J.N.763 could have been increased to the same extent. This would have resulted in a true air speed of 418 m.p.h. at 2,750 ft.
5.2 Mustang III
The level speed – altitude curves for the various conditions are given in fig.4 and the position error correction in fig.5.
Replacing the exhaust stubs by Spitfire stubs, representing approximately the correct area required, gave a further increase of 1 ½ m.p.h. No exhaust shrouds were fitted in either case.
The final speed of the aircraft was 413 ½ m.p.h. at 3,400 ft.
5.3 Spitfire XIV
The level speed – altitude curves for the various conditions are given in fig.6 and the position error correction in fig.7.
The increase in level speed due to improved quality of finish was 8 m.p.h. This test was done at +19 lb./sq.in. boost, 2,750 r.p.m.
Due to engine trouble, only two points were obtained at +25 lb./sq.in., 2,750 r.p.m. These indicate an increase in speed of 28 m.p.h. which is slightly higher than the predicted value as shown by the dotted curve (fig.6).
Further increases in speed could be obtained by removing the rear view mirror, clipping the wings and fitting the conical type windscreen. The effects of those items have been measured previously on Spitfire IX aircraft.
Due to the urgency of the tests the bomb racks and the bracket at the base of the whip aerial were removed at the same time as the finish of the wing surfaces was improved. This gave a total increase of 21 m.p.h. and the effects of the separate items are estimated as 8 m.p.h. due to the bomb racks, 1 m.p.h. due to the aerial bracket and 12 m.p.h. due to improved finish.
It is worth while comparing the level speed performance that one be obtained on the above types of aircraft. This comparison is given in fig.8.
The Tempest V is represented by J.N.763, operating at +11 lb./sq.in. boost, 3,700 r.p.m., and allowing a similar increase in speed for the improvement in surface finish measured on J.N.738.
The Mustang III is represented by F.B.377 and measured in its final condition (5.2) operating at +25 lb./sq.in. boost, 3,000 r.p.m.
The Spitfire XIV is represented by R.B.176 in its final condition, using the points obtained at +25 lb./sq.in. boost and 2750 r.p.m. Allowances have been added for the removal of the rear view mirror and for clipping the wing tips.
Fig 1. Tempest JN.738 Improvement of Level Speed Performance Due to Finish
Fig 2. Tempest JN.763 Level Speed Performance at Increased Boost
Fig 3. Tempest Position Error Correction
Fig 4. Mustang III FB.377 Improvement of Level Speed Performance
Fig 5. Mustang III FB.377 Position Error Correction
Fig 6. Spitfire XIV RB.176 Level Speed Performance
Fig 7. Spitfire XIV RB.176 Position Error Correction
Fig 8. Comparison of Top Level Speed Performance