Flight test variances

[tomo pauk]

Looking at the flight test figures, seem like many new aircraft, despite being of same type (ie. same airframe, with same engine, with about as same weight, same armament, antennae etc.) were displaying significant differences in speed.
Like the F2F (Brewster) having like 30 mph of speed difference, despite engines of only 50 HP difference - weight creep is guilty here? Or, P-40 - differences of like 20 mph (335-355 mph), for same sub-types. Then, Spitfire V - between 350 and 375 mph at 19-20000 ft, the 8-gun versions being the fastest (I'm not counting here the low-alt versions with cropped supercharger). Two-cannon versions got some times only as good as 350 mph? The versions with Merlin 46 are spread between 352-368 mph at 20200-22500 ft.
What should be the main 'culprit' here?

 

[Juha]

Hello Tomo
Spitfire Story Alfred Price:
According to tests made at Farnborough in 1943
A standard Mk VB was tested and developed a maximum speed of 357 mph (height, engine type not given)
Multi stack exhausts fitted = gain of 7 mph
Removal of carburettor intake ice-guard = gain of 8 mph
Fitting of faired rear view mirror = gain of 3 mph
Whip aerial in place of mast = gain of .5 mph
Cutting cartridge case and link ejector chutes level with wing = gain of 1 mph
Sealing, rubbing down painting and polishing wing leading edge = gain of 6 mph
Polishing remainder of airframe = gain of 3 mph

Top speed now = 385.5 mph

 

[CORSNING]

Hi Juha.

Hi tomo,
As Juha is pointing out, you must read flight performance tests carefully. The only item I can see that Juha did not mention was the engine boost levels. Most engine boost levels were raised as knowledge increase with time about a particular engine.

Jeff

 

[tomo pauk]

Hello Tomo
Spitfire Story Alfred Price:
According to tests made at Farnborough in 1943
A standard Mk VB was tested and developed a maximum speed of 357 mph (height, engine type not given)
<snip>

Top speed now = 385.5 mph

Puzzling thing is that a test of Spitfire Mk. VA X.4922 (armament 8 Brownings; test made on 29th April 1941, here) says: " The top speed of the aeroplane is 375 m.p.h. at 20,800 feet." Unpolished, with regular exhaust stacks etc. Once in 1943, the run-on-the mill Spit VB is slower by 18 mph. The ice guard and cannons could be the main culprits?

Hi Juha.

Hi tomo,
As Juha is pointing out, you must read flight performance tests carefully.

Yes, sir. BTW, you have PM.

The only item I can see that Juha did not mention was the engine boost levels. Most engine boost levels were raised as knowledge increase with time about a particular engine.

Jeff

Anything that sheds light to the topic is welcomed.

 

[GregP]

I have seen a lot variance in test data when engine boost was limited for some particular reason.

The biggest differences in top speed I have noted are whether or not racket launcher rails are fitted and the particular maximum rpm / boost used.

Other than that, the test seem quite similar to one another.

 

[nuuumannn]

Hi Tomo, hazarding a guess here, but individual aircraft can and did differ from each other considerably, particularly in wartime. There are many examples of aircraft in squadrons that, although following off the production line after or before their nearest neighbour failed to match the other in some aspect of their performance. pilots often had their favourites owing to differences in performance from one aircraft to another. These things can be put down to build quality, which often suffers in wartime, quality of componentry - the same, different manufacturing methods in different factories, finish of each aircraft etc.

In testing, different factors also enter the equation and in reality, no two test scenarios are ever going to be identical for whatever reason, weather causing variations in temperature, anomalies in barometric pressure at altitude, conditions on the ground of the field the aircraft operates from, etc.

In my limited experience in working with aircraft, each one is different and issues that pop up on one do not necessarily occur on another. The British blamed this on 'Gremlins'.

https://en.wikipedia.org/wiki/Gremlin

 

[Aozora]

Hi Tomo, hazarding a guess here, but individual aircraft can and did differ from each other considerably, particularly in wartime. There are many examples of aircraft in squadrons that, although following off the production line after or before their nearest neighbour failed to match the other in some aspect of their performance. pilots often had their favourites owing to differences in performance from one aircraft to another. These things can be put down to build quality, which often suffers in wartime, quality of componentry - the same, different manufacturing methods in different factories, finish of each aircraft etc.

Completely in line with the above, here are some interesting excerpts from:

describing how the use of flush riveting from one Seafire builder altered the handling and also describing why the fit of the engine cowlings could be a problem:

 

[Neil Stirling]

Tomo,

Copy and paste from Spitfire Performance Testing.

An Air Ministry document from 29.10.43 details the protocol to use in arriving at performance figures for aircraft:

NOTE ON THE USE OF PERFORMANCE ESTIMATES

1. There are 4 main stages in the evolution of performance for a new aircraft:-

(1) Original Estimate - based on drawings and expected engine powers.
(2) Flight Tests - Performance of actual aircraft - (a) Prototype, (b) Production, when built
(3) Provisional Performance Curves - Adjustments of (1) in view of (2)(a).
(4) Final Performance Curves - Based on (2)(b) and subject to variation with the condtion of the aircraft.

2. The limits within which the Stage (1) estimate can be guaranteed depend on whether the new aircraft is -

(i) A development of an established type whose characteristics are well known (e.g. Spitfire XIV); or
(ii) A new type, about which little or no confirmed information is available, (e.g. F2/43, with Centaurus engine).
(Note:- The addition of new factors, e.g. Contra-rotating props. to an aircraft in Category (i) would probably transfer it to Category (ii).)

3. With "Development" aircraft, original estimates should be accurate to within 1-2% in speed, and 50-100 ft./min. in rate of climb. With "New Type" aircraft, however, the error might be up to 6-7% in speed, and 200-250 ft./min. in climb.

4. Flight Test figures, by themselves, should be treated with considerable reserve, since they may be obtained under non-standard conditions, and the aircraft flown may differ from production machine.

5. From the prototype trials, Provisional Performance Curves can be obtained, modifying them, if necessary, to allow for the effect of the difference between the prototype and production machine.

6. Only when trials with representative production machines have been carried out under known conditions can the Final figures be issued. These figures then represent the performance expected of an average production machine of this type.

7. Any particular machine off the production line, however, may vary from the average in top speed and climb because of differences in engine power and general finish. The usual variation for single-engined fighters is up to 3% in top speed, and 150-200 ft./min. in rate of climb; heavy bombers vary up to 4% in top speed, and 150-200 ft./min. in climb. (...)

Neil.

 

[tomo pauk]

Thanks for feedback, gentlemen
If we use the 3% variation, the speed for a 350 mph airplane (manufacturer's guarantee) would be some 10-11 mph, ie. up to 5.5 mph in either way. The P-40 seem to be a main 'sinner', the versions with V-1710 many times maxed out at 330 mph on military power (deficit of 20 mph out from many times quoted 350 mph). The Merlin were usually around 360 mph, though a max of 374 mph was achieved in one test (mil power at 18100 ft)

 

[GregP]

We (Planes of Fame, that is, not me) operate a P-40N with the Allison V-1710 and I have spoken many times with John Paul who owns and flies two P-40's with the Allison V-1710. None of this population of three aircraft fail to achieve 350 mph at military power, which is very occasionally used.

John Paul raced one of his P-40's at Reno and a few years back, beat a Corsair in the Bronze race with it at Military power being used on the last 2 laps.

 

[fastmongrel]

We (Planes of Fame, that is, not me) operate a P-40N with the Allison V-1710 and I have spoken many times with John Paul who owns and flies two P-40's with the Allison V-1710. None of this population of three aircraft fail to achieve 350 mph at military power, which is very occasionally used.

John Paul raced one of his P-40's at Reno and a few years back, beat a Corsair in the Bronze race with it at Military power being used on the last 2 laps.

To be fair comparing a pampered hangar queen to a front line aircraft maintained in the open and treated roughly by various pilots isnt a good comparison. I can well believe a P40 could beat a Corsair at low level but I reckon the Corsair would probably beat it over 10 races.

 

[GregP]

I just knew somebody would say something like that. Bunk.

A decent Allison runs just fine. Now if the plane and / or engine are just plain old worn out, I can see it dropping 20 - 30 mph. But if the engine and prop are NOT worn out, they kept them as best they could at front line airfields. The enemy had to do the same. Performance meant the life of the pilot.

I'm sure there were worn out P-40's that could just make 330 mph just as there were worn out Spirfires that did the same. If there were more worn out P-40's, it is perhaps due to that fact that the P-40 was supplied to the theaters with less priority and they had to make due. If a V-12 gets too worn out, it will cost you a plane and a pilot at some point. Better to maintain it or ground it than lose a pilot to a piece of junk aircraft.

 

[tyrodtom]

Even during Vietnam supply sometimes couldn't meet demand and we had to make do with what we had . Calling off a mission because of lack of supplies just is not going to happen.

I can remember running out of tailcones for 750 lb bombs during the TET of 68. We had to go around to all the chowhalls, and squadron areas and get back all the tailcones used for ashtrays. We sent out a lot of 750 lb bombs with bent tailfins, or less than the full number of setscrews that held the tailcone attached to the bomb body. There were plenty of bombs, just not enough tailcones. This was thruout SEA for about a week.
Later when I was in the Army I can remember having fly more than once with a less than perfect tail rotor ( erosion) but had to fly because the other aircraft had even worse faults.

And I'm sure that happened in WW2 also.

 

[GregP]

I know it happened, but the majority of the P-40's out there weren't worn out or failed to meeet spec performance.

Some? Sure. Most? No way. At least not any worse than other plane with other engines.

The Allison V-1710 was NOT a substandard engine. The USAAF got exactly what it ordered, and it performed just fine ... and still DOES.

Question Tom, how much yaw capability did a worn tail rotor lose you? The Hueys I rode in weren't pre-flighted by me or anyone I was with, but they seemd to fly just fine except when we got hit in the transmission. Then we had maybe 3 - 5 minutes to get over water and get out or find a clearing away from the action before the treanmission temperature gauge reached critical mass. We chose the water both times. At least it wasn't cold!

 

[tyrodtom]

The tail rotor was a thin aluminum skin over a honeycomb inner matrix, the skin was deeply scratched by a excursion thru brush and small limbs.
It didn't loose noticeable efficiency, but it had lost structural strength , and could fail. plus the blades weren't tracking with each other, and were beyond our ability to correct. The aircraft was red Xed, not flyable, but not as seriousely red xed as the other aircraft. So we ( me, the pilot, and the maintenance officer) made a judgement call and flew it twice that day. The buzz thru the pedals from the bad blade track feedback numbed the pilot's feet.

I replaced the tail rotor that night with one from another grounded OH-6.

I'm sure those Hueys would have been preflighted at least at the beginning of the day by their crew chiefs, and in all units I was around, the crew chiefs stayed and flew with their aircraft. That tended to keep the crew chiefs careful.

 

[drgondog]

lose a tail rotor and you may kiss your ass goodbye - as you know

 

[tyrodtom]

lose a tail rotor and you may kiss your ass goodbye - as you know

If you just lose one blade, the imbalance can take the tail off. You'd better hope the other blade comes off first. Then you could autorotate with some control.

I don't remember exactly the glide ratio a OH-6 had in autorotation, but it was better than most, like 5 to 1 or so, but inside the chopper going down that wouldn't bring much comfort.

 

[tyrodtom]

But war stories aside, even in Vietnam, sometimes we didn't have the replacement parts we needed, or the munitions required for a particular mission at the time.

If I experienced it in my limited time in Vietnam, I'm sure it occurred even more often during WW2, especially in some of the more far flung areas.

 

[drgondog]

On flight tests and results expressed - for the US, the bench tests for HP were always 'assumed' and we know that like airframes, the power plant is not always matching factory specs.

Further a well planned and executed test will a.) describe the airframe condition (polished/not polished, racks/no racks, ports covered/uncovered, etc), b.) loading and weights at takeoff and c,) estimated fuel consumption to estimate weights for multiple tests (Vvs altitude, boost, rpm; ROC at different altitudes, etc)

Examining different tests for the same airframe will usually highlight the differences - but when I see same airplane/power plant and gross weights with material speed and ROC differences for same boost/rpm the rat I smell is the engine performance that day.

 

[drgondog]

If you just lose one blade, the imbalance can take the tail off. You'd better hope the other blade comes off first. Then you could autorotate with some control.

I don't remember exactly the glide ratio a OH-6 had in autorotation, but it was better than most, like 5 to 1 or so, but inside the chopper going down that wouldn't bring much comfort.

I'm trying to imagine any control during auto rotation when the tail rotor is lost. There is no cockpit combination of collective and cyclic that I can think of that will prevent immediate and accelerating rotation of tailboom/fuselage around the mast axis. With some forward velocity the 'rudder' may provide a smidgeon of yaw damping - but not near enough to overcome the rotor/engine system torque.

What has your experience/knowledge been relative to survival or control techniques?