View Full Version : characteristics of performance

09-26-2007, 05:51 PM
Gleaming what I can from John Boyd's essays their are two characteristics of performance that are important in the design of a good fighter...The first is how quickly a fighter can slow itself in a turn (simplified statement).This is important for two reasons one is that it may force a pursuing fighter (boggie) into overshooting during a "hard break turn". The second reason this characteristic is important is so that you can reach optimum turn speed quickly...

The second important characteristic of performance is how quickly you can regain your speed after you slow down (simple enough)...

My question is this: I understand how to read a performance chart as far as "rate of climb" or "turn time by speed" is concerned but how do you determine "how quickly you can slow a fighter down" and "speed it back up" from performance specs.

Is it an issue of "HP to weight?" or "weight to wing area?"...

What type of test results or specifications do you need to determine these two characteristics in performance for an airplane so that you can compare two different fighters???

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09-26-2007, 09:30 PM
I'm pretty sure it's HP to Weight.

The wing area to weight is used to calculate roll rate.

09-27-2007, 01:29 PM
http://forums.ubi.com/images/smilies/1072.gif ~bump~

09-27-2007, 02:04 PM
Originally posted by ImMoreBetter:
I'm pretty sure it's HP to Weight.

Similarly with cars IMB, torque to weight ratio is a considerable factor. Perhaps some of our car aficionados can elaborate on that. I know despite being familiar with the elements of such, I'd learn something from those here with more a hands on knowledge about the subject. I know there is definitely a correlation with fighter aircraft.

09-27-2007, 02:54 PM
Power to weight would be the primary equation, but obviously drag - coefficient (aerodynamics for perhaps a more generic term) would to my mind be vital as well.

09-27-2007, 03:35 PM
Excellent subject for discussion. (http://flighttest.navair.navy.mil/unrestricted/FTM108/c6.pdf)

Turn performance is but one aspect of an airplane's maneuvering performance. To
describe the characteristics desirable in a tactical airplane, the term agility is often used.
Agility is the ability to make rapid, controlled changes in airplane motion. Included within the scope of the term agility are the climb, acceleration, deceleration, and turn characteristics of the airplane. An agile airplane is capable of performing quick and precise changes in climb angle, speed, or direction of flight. The ability to make rapid changes describes maneuverability; the ability to precisely guide the airplane through such changes describes controllability. Thus, maneuverability and controllability are subsets of agility.

I think John Boyd threw out the old Power to Weight myopic perspective for a good reason and that reason is drag loading.

The comparison between cars traveling at less than 200 (or very seldom over 200) mph when the effects of compressibility are not significant (becoming significant at around 220 mph) and airplanes traveling at over 400 mph (when drag force including compressibility effects are dominating the forces involved) isn't going to offer much insight into the picture.

Ps = T/W D/W multiplied by velocity includes D/W and at 500 mph the force of total drag is enormous and not to be ignored.

John Boyd spoke of a term called "Natural Hook" where an airplane decelerates rapidly from high speed to corner velocity quickly (agile) and therefore maximizes turn rate and minimizes turn radius because the rate of deceleration is maximized.

This should be measurable with flight test data (Loaded decelerations) plotted on an EM chart.

However, if loaded deceleration runs are continued past the accelerated stall, the data can be used to continue this curve past the maximum instantaneous turn, to the limit angle of attack. In this case, the decreasing load factor beyond the maximum instantaneous performance point causes less turn rate, even as greater energy is sacrificed. Despite the diminished turn rate, these high energy loss conditions are tactically useful for maximum rate decelerations to force an overshoot or to take advantage of slow speed pointing ability. Inspecting the shape of the curve can reveal a point of diminishing returns, a turn rate beyond which the increase in performance does not justify the increased rate of energy loss.

Figure 6.32