Performance Comparisions, discussion of the science behind it

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Crumpp

Banned
330
5
Sep 5, 2006
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
 
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
 
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.
 
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.

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.

We had 8 aircraft in our flight company and no two would perform the same based off of condition alone.

That's real aircraft for you!

I will continue tommorrow with some of the aerodynamics.
 
That's real aircraft for you!

I will continue tommorrow with some of the aerodynamics.

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.
 
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
 
This thread has been dormant for 13 years, but I think its worth resurrecting it, as the original question has not really been answered.

I think the answer is Energy-Maneuverability Theory, developed by Col. John Boyd and Thomas P. Christie of the USAF. It was developed to provide a means of quantifying and comparing aircraft maneuvering performance. A few years ago, I spent a 3-day weekend studying it and figuring out how to construct the diagrams involved. And since then, I've been obsessed...

To properly construct the diagrams, you need to know:
- how the aircraft's maximum lift coefficient varies vs. Mach number
- how the aircraft's drag changes with Mach number. I break this down into trends of zero-lift drag and "e" vs. Mach number.
- how the aircraft's maximum engine power changes with altitude
- details of the aircraft's weight build up, to determine what a likely combat weight would be
- an idea of how prop efficiency changes with power and speed
- the aircraft's limit load factor (how many Gs is the structure designed to tolerate)
- the aircraft's dive speed limits

I get a lot of this data from wind tunnel test reports and flight manuals. For some of the trends against Mach number, I have to apply some guesswork, based upon experience.

Below is my comparison of the Fw 190A-8 and P-51D at 15,000 feet altitude, each with 50% fuel and full ammunition. All flight within the dashed lines is sustained performance, which can be maintained with no loss of altitude. Within the dashed lines, the aircraft have extra energy, which can be used climb or accelerate. The dashed lines touch the X axis on the right at the maximum level flight speed of the aircraft and intersect the minimum speed (stall) line on the left. The solid lines show the limits of instantaneous performance. All flight between the dashed and solid lines involves a cost - the aircraft will bleed off energy and descend to recover energy. The Y axis shows the turn rate possible at the flight condition. I've also overlaid lines of constant turn radius and constant g loading.

As can be seen by comparing the two aircraft, they are very evenly matched in terms of sustained performance. The P-51D has an advantage at low speed, where it can out turn the Fw 190, achieving a 1,000 ft radius turn at 15 deg/sec, at 2.5G and about 132 KCAS. The best the Fw 190 can do is a 1,500 radius turn, at about 13 deg/sec at 2.5G and about 152 KCAS. The P-51D can also instantaneously pull about 7.75G, while the Fw 190 can only pull 6g. The two aircraft are very evenly matched in dive performance, out at the right edge.

A lot can be told by studying these plots. They tell you a lot, but they don't take into account pilot capability and tactical factors. A pilot diving out of the sun will have an initial advantage. If he/she is a good shot, they will win the engagement.

1611981798027.png
 
Not a critique, but suggesting a different viewpoint about dogfighting.

Cool chart, but dogfighting is mostly about the starting position. Most of the WWII planes were all faster in a dive than any of their enemy's top speed, at any altitude. The Zero was the exception here. Built light for maneuvering, it wasn't structurally sound in a dive, and couldn't roll in a dive anyway.

Ditto when comparing sustained turn rates. Altitude can be traded for turn rate, so once again, the initial position was more important than engineering numbers. A prudent pilot, seeing he is starting from a disadvantageous position, or even an even position, breaks off, to fight another day.

i suspect that in many cases, the mission, for instance, tasked with attacking high flying bombers protected by high altitude fighters, meant the pilots of Germany sometimes flew into situations where they were deliberately at a disadvantage. Just one example of many. The Cactus Air Force at Guadacanal also seemed to fight from a disadvantage at times.

while its intriguing to compare basic numbers, as if in dogfighting, both pilots should start from an even position, it turns the serious act of dogfighting into a sport, simlar to any other martial art. Whereas dogfightng is truly a fight of survival, and there are no points to becoming the dead guy.

Perhaps too philosophical. The one good point of the energy maneuverabilty data is to know when one might have an advantage, and when one doesn't.
 
Not a critique, but suggesting a different viewpoint about dogfighting.

Cool chart, but dogfighting is mostly about the starting position. Most of the WWII planes were all faster in a dive than any of their enemy's top speed, at any altitude. The Zero was the exception here. Built light for maneuvering, it wasn't structurally sound in a dive, and couldn't roll in a dive anyway.

Ditto when comparing sustained turn rates. Altitude can be traded for turn rate, so once again, the initial position was more important than engineering numbers. A prudent pilot, seeing he is starting from a disadvantageous position, or even an even position, breaks off, to fight another day.

i suspect that in many cases, the mission, for instance, tasked with attacking high flying bombers protected by high altitude fighters, meant the pilots of Germany sometimes flew into situations where they were deliberately at a disadvantage. Just one example of many. The Cactus Air Force at Guadacanal also seemed to fight from a disadvantage at times.

while its intriguing to compare basic numbers, as if in dogfighting, both pilots should start from an even position, it turns the serious act of dogfighting into a sport, simlar to any other martial art. Whereas dogfightng is truly a fight of survival, and there are no points to becoming the dead guy.

Perhaps too philosophical. The one good point of the energy maneuverabilty data is to know when one might have an advantage, and when one doesn't.
Some good points but some comments - when you say the Zero wasn't structurally sound in a dive, what specifically are you referring to?

"Energy Maneuverability" - absolutely, also consider aircraft acceleration.

"A prudent pilot, seeing he is starting from a disadvantageous position, or even an even position, breaks off, to fight another day." 100%


Mentioned many times, most aerial kills occurred with the "victim" never seeing their opponent. I think the more successful fighter pilots avoided "dogfighting,"

Find the enemy and shoot him down. Anything else is nonsense.

Manfred von Richthofen
 
BTW, this compares a F-16 and a F-86F-25:
View attachment 672791
Looking at this graph I get the following provided I calculated it right...
  1. F-16A
    • Maximum L/F
      • Mach 0.95-0.96: 9.0g
      • Mach 0.90: 8.75g
    • Maximum Sustained L/F
      • Mach 0.90: 5.29g (radius of 4912')
  2. F-86F-25
    • Maximum L/F
      • Mach 0.77: 7.5g
    • Maximum Sustained L/F
      • Mach 0.90: 1.73g (radius of 14969')
      • Mach 0.85: 2.2g (radius of 10528')
      • Mach 0.77: 2.26g (radius of 8494')
      • Mach 0.52: 1.72g (radius of 5364')
Honestly, I thought the F-16's sustained turn-rate would have been 9g to at least 25000' to be honest.
 
Hello,

Just because i'm totally clueless in aerodynamics :)
How can you determine the instanteneous pull acceleration? ...please use simple words :D
Thanks
I think it is the structural limit of the airframe before the wings or fuselage permanently deforms (yield) or breaks (ultimate tensile strength), as in pulling out of a high speed dive. WW2 aircraft had a comparatively low power/ weight or thrust /drag ratio, what they could do as far as turning goes was much different pulling out of a dive compared to at a constant altitude because as soon as you start turning the drag increases. At low altitude a Spitfire XIV could out turn or stay with most adversaries, at 42,000ft it could hardly turn at all, but could still out turn most adversaries.
 
Good luck finding a simple answer for such a complex issue
 
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