Performance Comparisions, discussion of the science behind it

Discussion in 'Flight Test Data' started by Crumpp, Dec 31, 2007.

  1. Crumpp

    Crumpp Banned

    Joined:
    Sep 5, 2006
    Messages:
    330
    Likes Received:
    0
    Trophy Points:
    0
    I see so many performance comparison threads in this forum.

    For some of you, this is a review at best as I know you are working aerospace or have retired from such.

    I just thought it might be beneficial to discuss some of the science of flight so that the community can make better informed comparisons. This could be sort of a place to gather all of that knowledge and a reference for the forum.

    Keep in mind that aircraft are a system and must be examined as one. Looking at one characteristic will not give a true picture as to an aircrafts performance.

    Definitions are essential. All aerodynamic texts will include a glossary of definitions applicable to that text. Why? Not all the terms have exactly the same meaning. Most certainly do translate well and are common, however many do not. Let's make sure we understand each others terms and conditions of flight.

    First of all, it does absolutely no good to compare aircraft that are not under the same condition of flight. Condition of flight is everything in aeronautics. It defines the possible behaviors and yes, things do change based on the condition of flight. What is true for one condition may not be true for another. So let’s make sure we understand the condition of flight.

    What does that mean? Well you cannot for example say, my airplane has a higher CLmax, therefore it will outperform yours.

    Lift force only meets the amount required. There is no such thing as extra lift no matter what condition of flight. Lift will always meet the exact amount for force required. A parametric study will show that a heavier aircraft is going to have a higher Coefficient of lift all things being equal at the same condition of flight. It requires more lift force.

    Having one aircraft operating at a different density than another is good example of different conditions of flight. Now we are trying to compare an aircraft flying through well, thin air, with one trying to muck through pea soup. The faster airplane may very well be stuck in the pea soup but we will never know if we don't put both aircraft under the same conditions.

    Variations, all aircraft performance is a percentage variation over a mean average. There is no such thing as an absolute.

    Some airframes are lemons while others are hotrods: just depends as the saying goes, if it was built on a Friday or Monday. Sometimes a fuselage just isn't built as straight as another or a rivet line is not as tight. A friend of mine just removed 9 lbs of aluminum shavings from the bilge of his Aztec!

    I did a rivet count on the aircraft I bought, yeah there were rivets missing. That introduces pressure drag and flex that can effect performance.

    Just look at the normal operation of an air-cooled engine. None of cylinders receive the same amount of fuel or air. Every time the engine fires, the cylinders produce just slightly different amounts of power based on specifically how much fuel/air the cylinder received.

    You can point to an engine design as say, that motor produces X amount of horsepower. Well that probably isn't true. The reality is that engine produces in the ballpark of that power figure if the conditions are standard.

    Put some time on the motor and we lose horsepower. A motor will begin at break in with a peak power production that will slowly degrade over the life of the engine. If something breaks, it will degrade even faster. This is why compression checks are considered so important in real aircraft. You will always here owners or buyers discuss their compressions at last annual when referring to the health of their engine.

    These reasons and others induce variation in our airplanes performance. A manufacturer will publish their percentage range for a given design.

    have things to do and will check back lateer. Perhaps others will come by and add to this.

    In closing for now:

    1. Get your definitions down and make sure your both understand the condition of flight
    2. The condition of flight is key and we cannot compare unless both designs are under the same conditions. We can include configuration changes under condition as well if that is acceptable.

    3. View the design as a sytem. Don't look for example at the turn radius and proclaim what a great turning aircraft you have found. The rate can be and if our configuration has a lower L/D ratio usually is much worse. In the BGS system, Rate of Turn is not base on our radius, it is based off our Angle of Bank and velocity.

    We can't say, for example, this aircraft is high drag, it is a bad design. High drag means speed control especially if the power is appropriate for the design. Look at your STOL aircraft, STOL is all about precise speed control on the backside of the curve.

    The aircraft must be viewed as a design system.

    Take Care All!

    All the best,

    Crumpp
     
  2. drgondog

    drgondog Well-Known Member

    Joined:
    Jun 28, 2006
    Messages:
    7,359
    Likes Received:
    561
    Trophy Points:
    113
    Gender:
    Male
    Occupation:
    Executive, Consulting
    Location:
    Scurry, Texas
    Great comments Gene.

    Interesting to note the price points today when 'manufacturabilty' must take a back seat to Performance (B-2, F22, F-17) and rivet/sheet metal yield to composites.

    Rivets are Heavy.

    Bill
     
  3. FLYBOYJ

    FLYBOYJ "THE GREAT GAZOO"
    Staff Member Moderator

    Joined:
    Apr 9, 2005
    Messages:
    23,200
    Likes Received:
    786
    Trophy Points:
    113
    Gender:
    Male
    Occupation:
    Aircraft Maintenance Manager/ Flight Instructor
    Location:
    Colorado, USA
    Great comments!!!!!!!!!
     
  4. Nikademus

    Nikademus Member

    Joined:
    Dec 14, 2007
    Messages:
    525
    Likes Received:
    0
    Trophy Points:
    16
    Occupation:
    Drone
    Location:
    Seattle
    definately awesome.....now about that P-47 thingy....... :lol:
     
  5. DerAdlerIstGelandet

    DerAdlerIstGelandet Der Crew Chief
    Staff Member Moderator

    Joined:
    Nov 8, 2004
    Messages:
    41,768
    Likes Received:
    684
    Trophy Points:
    113
    Occupation:
    A&P - Aircraft Technician
    Location:
    USA/Germany
    Yeap good post there.

    I certainly agree with you on aircraft condition. We had 8 aircraft in our flight company and no two would perform the same based off of condition alone.
     
  6. Crumpp

    Crumpp Banned

    Joined:
    Sep 5, 2006
    Messages:
    330
    Likes Received:
    0
    Trophy Points:
    0
    I helped a friend of mine building a Lancair. IIRC a 4X8 sheet of Carbon Fiber cost him ~4500USD. Go look at a Diamond too. It is just a tube really, a $335,000 tube for a DA40 with 60 year old engine technology.

    For WWII era fighters, some of the wood construction techniques was the carbon fiber of the day. The laminate construction was surprisingly strong, light, and offered some very good drag reduction qualites. Look at the Mosquito or some of the VVS designs.

    In an effort to reduce drag from rivets Focke Wulf actually produced several all welded construction FW190A's. They felt the changeover would be too costly in terms of lost production, had some logistical trouble with the welding rods/machine, and the pilots just did not like the idea!

    About 10 aircraft were constructed and sent to operational testing.

    That's real aircraft for you!

    I will continue tommorrow with some of the aerodynamics.
     
  7. DerAdlerIstGelandet

    DerAdlerIstGelandet Der Crew Chief
    Staff Member Moderator

    Joined:
    Nov 8, 2004
    Messages:
    41,768
    Likes Received:
    684
    Trophy Points:
    113
    Occupation:
    A&P - Aircraft Technician
    Location:
    USA/Germany
    I agree. Having experience as a A&P Mechanic, some private flying (not much), and over 1500 flight hours in rotory wing aircraft I do not base all of my conclusions off of paper stats and I certainly dont base any of my conclusions on computer flight sims.
     
  8. pbfoot

    pbfoot Active Member

    Joined:
    Apr 14, 2005
    Messages:
    7,636
    Likes Received:
    4
    Trophy Points:
    38
    Location:
    niagara falls
    I agree that each aircraft is it's own little entity . Just a thought the comment at the beginning by Crump should almost be a preamble to each best of thread
     
Loading...

Share This Page