Performance data is based with what, when, how?

Discussion in 'Flight Test Data' started by Frantish, Aug 13, 2010.

  1. Frantish

    Frantish Member

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    Look in any major book on WW2 aircraft and they say "Max. Speed: xyz"

    Anyone here probably knows that information is very subjective as it depends on a myriad of variables.

    So what IS the usual parameters the aircraft is under when the speed is calculated?

    Just to fill in the possibilities:

    Aircraft (excluding loadout):
    Conditions of aircraft and engines
    Cleanliness
    Fixed Equipment (radio antennas, hard-point mounts, exhaust suppressors)
    Empty weight (includes the oil, hyd-fluid, coolant, O2, other)

    Load-outs:
    Fuel (how much?)
    pay load (crew, munitions, mission items (maps, nav tools) )

    Atmosphere:
    Alt
    Temp
    Humidity

    And did each county used different loadouts?


    When it comes to bombers, is the MAX speed calculated with no bombs and half fuel?
    Or is it with just 1 crewman, no bombs, and only 30 min of fuel?? ;)

    Thanks
     
  2. Shortround6

    Shortround6 Well-Known Member

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    I will go out on limb here and get it sawed off behind me.:lol:

    Most contracts are going to specify that the plane will do a guaranteed speed. This will be specified at certain altitudes. There will be a certain % + - tolerance written in (mostly minus). Failure of aircraft to reach these guarantee speeds will result in financial penalties to the manufacturer, at least in the US.
    From this we can guess that the condition of the aircraft and engines will be good to excellent, the aircraft will be clean but not overly polished (factory standard finish) and normal external fittings for it's "primary job" fitted, like radio antennas fitted.
    Fighters may very well not have bomb racks fitted unless specified. Special fittings, like flame suppressors for night fighters may also not be counted unless specification is for that model ONLY. If flame suppressors are fitted to ALL aircraft of a type or model (Night bombers) then performance will be with with the service (flame suppressor) exhaust.
    Empty weights are also written into contracts/specifications. In the US failure to meet weight limits (again with tolerances) would lead to financial penalties. how over weight a particular aircraft could be before it was rejected by the inspectors I don't know.

    Fighters performance figures will be with armament (and ammunition) and everything needed for the fighter mission (radios, navigation equipment, oxygen etc.) but without external loads.

    Atmospheric conditions are standardized and probably show more uniformity between nations than some other conditions. Performance was "corrected" using formulas to the "standard" conditions. 59 degrees F/15 degrees C at sea level and standard pressure etc.

    Bombers top speed is very often given with either NO bombs or a nominal bomb load but with full crew and equipment. It does the User (air force) no good to know what the bomber will do without guns and only a pilot aboard if they are never going to fly it that way is service. Knowing what the bomber will do with full crew, guns and part fuel will tell them how fast the bomber can go when running from fighters after dropping it's bombs.

    Many books only give sketchy performance specs because it is so much easier. Bombers especially are hard to get good numbers on because of the trade-offs between bomb load and range even if there is no trade off between bomb load and fuel carried. Once "optional" tanks and "overload" weights enter the picture a bomber can have pages worth of 'max speeds' 'max ranges' 'service ceilings' and so on.

    Book editors don't have the patience, interest, or budget for extra pages to list all the combinations. :)
     
  3. lingo

    lingo Member

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    I think I'm right in saying that for the British system it was always tested after a maximum weight take off in a clean airframe. Of course as time went by and modifications were applied the approved maximum weight tended to increase. If wing racks were fitted drag reared its ugly head. However, there are some notable exceptions as the Lancaster appeared to suffer no ill effects with the H2S radar scanner in place. They even reckoned that retracting the tail wheel could have harmed the performance - and it certainly added to the rear surface area to improve stability. Strange!
     
  4. drgondog

    drgondog Well-Known Member

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    #4 drgondog, Aug 15, 2010
    Last edited: Aug 15, 2010
    A Flight Test report basically states all the conditions and configuration (i.e engine type, propeller type, etc) of the aircraft before flight, rarely talks about the specific temperatures and barometric pressure conditions.

    So, the Test will usually state the amount of fuel (i.e full internal tanks), ammo or ballast, if a fighter whether or nor gun ports are covered, any unusual difference in normal GFE left out of test (radios, etc).

    If anything other than factory acceptance conditions have been performed such as wing surface treatment to smooth surface, additional sensors, pitot tubes or instrumentation, presence or absence of external stores racks, then these should also be noted.

    With this information you usually have to back out fuel, oil, pilot, ammo/ballast out - then add to empty weight to arrive at gross takeoff weight for the airframe/crew.

    When a high speed run is performed, there will be altitude, power/boost settings, rpms, etc for each tested altitude run in tabular form.

    Problems with the Test affecting results are noted (i.e failure to achieve boost or rpm to standards)

    The format was pretty much the same fr simple reason that tests would often be repated with new equipment, airframe mods, change in weight, etc and the variables needed to be as visible as possible for comparison.

    The net of the reported performance noted in publications is that they rarely state the critical facors under which the tests were performed and you can not rely on the data to be reasonably accurate, in context, with the load and configuration under which the tests were performed. Many book/article authors do not know them and do not understand the issues and conditions of the test.

    Range, for example, combined with bomb load for bombers is one of the best examples of widely ranging 'it depends' factors. Range with a 5,000 pound bombload for a B-17G varies significantly depending on whether the airplane is free to take off, climb at economical power settings, to the altitude with the best combination of engine performance and winds aloft - versus as part of a 54 ship combat box of B-17G's assembling over an airfield or splasher, climbing to altitude, staying in formation at the lowest speed (~150IAS) of the slowest ship with tired engines, and flying at a less than optimal altitude to maximize the range.

    Top speed in a dash run depends on good engines, prop pitch and rpm per specs, well running engine and carburation, clean ship, known weights nd presence or lack of external features such as bomb racks which will affect drag (and top speed). The data WILL have to be reduced to STP from the atmospheric conditions of the teat.
     
  5. Frantish

    Frantish Member

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    Thanks for the details!

    Would be nice if aircraft books did at least have a prefix describing how the speed was measured, and the given speed was as a loaded war-ready speed, and it could go faster as ammo, fuel, and other items where used up.

    BTW, with the Ju-88, most of its bombs, especially the big ones, was on eternal racks. Is the speed given with racks on, and loaded with the 250 Kg bombs (typical load)?
     
  6. drgondog

    drgondog Well-Known Member

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    Classic case in point! If you find any official LW test report it will specify the details.
     
  7. GSENN

    GSENN New Member

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    Agreed, the 'standard atmospheric conditions' is one of the few areas where all nations agreed upon years before WWII started, especially at altitudes below 40kft.

    Yet I see a lot of folks attributed anomaly in performance data due to countries having their 'own' standard, which is just not the case.

    Granted it wasn't until after WWII with the space race kicking into high gear that they started looking at altitudes well above 60kft. Take a look at the std atm values from the NACA in the 20s and 30s and comp them to the std atm values of 1976 and they are for the most part spot on up to 40kft.

    Another point to keep in mind is they didn't allays convert the test results to std atm, but, if they didn't they typically labeled the result as such, i.e. data not converted to std atm
     
  8. drgondog

    drgondog Well-Known Member

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    Shortround - your comments were dead on - only add that the Customer (USAAF, USN) typically picked one of the first 10 production models so that the Contractor could not prepare a specially prepared ship for flight test.

    There were examples of delivered airframes where major components (such as carburation systems) in a Packard 1650-9 was not up to specs and the flight test results were below the Contractor guarantees for that specific ship... or in the case of the F-102, faulty aerodynamic design preventing any HOPE of meeting the specs.
     
  9. Snautzer01

    Snautzer01 Well-Known Member

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    Calculate.
     

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