Operational Performace Comparison: Viper, Beagle, and Stubby

The F-35 compared with other modern jets.
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sprstdlyscottsmn

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Unread post08 Feb 2018, 14:41

A quick review of what is shown vs what isn't shown. Lift curve slope hits nearly .15. that's amazing! We don't know the actual CL or Alpha shown in the graph. Still, I think it's worth updating the lift curve in my model.
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sprstdlyscottsmn

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Unread post10 Feb 2018, 15:07

Hey everyone. Quick question. In an air-to-air agility metric I am trying to find something useful to measure. Instead of listing a doghouse plot, right now I am ploting time, radius, speed, and altitude to complete a standard rate 90 degree turn, followed by a "half-pull" (linear portion of CL alpha curve) 90 degree turn, followed by a CL Max 90 degree turn, followed by a max Alpha nose position change. The maneuver starts at 0.8M at 30,000ft and afterburners are engaged the whole time. My thoughts are that these four things measure energy gain during gentle turns, energy loss during modest turns, energy loss during hard turns, and low speed nose pointing. I then measure how much energy has been lost.

So my question is, do these seem like good things to measure? Any ideas of something else that should be measured?
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rheonomic

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Unread post10 Feb 2018, 21:40

sprstdlyscottsmn wrote:So my question is, do these seem like good things to measure? Any ideas of something else that should be measured?


I haven't really been following this thread, but have you looked at any of the fighter agility metrics work done in the early to mid 90s?
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Unread post11 Feb 2018, 03:11

rheonomic wrote:
sprstdlyscottsmn wrote:So my question is, do these seem like good things to measure? Any ideas of something else that should be measured?


I haven't really been following this thread, but have you looked at any of the fighter agility metrics work done in the early to mid 90s?

I have now. Looks like what I am tinkering with is similar in nature to the CCT study with the F-5, F-16, and F/A-18. Thanks for the tip.
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Unread post11 Feb 2018, 03:44

gta4 wrote:This is the lift curve slope from the same article. You can see that Super hornet has VERY STEEP lift curve slope (greater than 0.1/deg), significantly higher than any existing fighter jets.


No surprise there, since the SH has the lowest wing sweep of any modern fighter.
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Unread post11 Feb 2018, 04:13

I think the traditional agility metric (CCT) is good enough:

1) bank 90 degree
2) turn until 180 degree nose pointing change (so the angle changed in flight path is smaller than 180 deg, given the the presence of AOA increase)
3) bank 90 degree to level flight
4) accelerate back to initial speed

Calculate the total time spent. The shoter the better.

At 4500 m, the best performer (I remember) is F-16, which spends only 21-22 seconds. This is because it bleeds less energy in Phase 2.
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Unread post11 Feb 2018, 05:04

sprstdlyscottsmn wrote:. Instead of listing a doghouse plot

I personally like doghouse plot more as we can see turn rate at various speed
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Unread post11 Feb 2018, 05:59

The doghouse plots make themselves at this point, so leaving them in is no big deal. As for the 180, a sustained turn will take the longest to turn but will have 0 acceleration time, while a max AOA turn will have the smallest flight path change but the largest acceleration time, and all the variations in between. There is a lot to consider.
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Unread post11 Feb 2018, 14:04

The main disadvantage of doghouse plot:

You can not get any "dynamic" knowledge of the aircraft, e.g. how long does it take to turn a given angle?

So it may cause some illusion: two jets with similar peak ITR will give you the impression that they have similar first turn capability. However, the one who bleeds energy slower will deplete its turn rate slower, thus has angular advantage right from the start.

A more extreme example: the one who bleeds energy slower, even has lower peak IRT, may still finish the 180 deg turn first.
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Unread post11 Feb 2018, 14:57

gta4 wrote:The main disadvantage of doghouse plot:

You can not get any "dynamic" knowledge of the aircraft, e.g. how long does it take to turn a given angle?

So it may cause some illusion: two jets with similar peak ITR will give you the impression that they have similar first turn capability. However, the one who bleeds energy slower will deplete its turn rate slower, thus has angular advantage right from the start.

A more extreme example: the one who bleeds energy slower, even has lower peak IRT, may still finish the 180 deg turn first.

Don't you get the bleed rate Ps from the chart too?
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Unread post11 Feb 2018, 17:42

garrya wrote:Don't you get the bleed rate Ps from the chart too?

Not from the charts I can produce. What I have looked at is what ten seconds of turn looks like comparing max CL vs mid range pull to see what the final differences are in rate, radius, and speed.

Another thing that I do put in with my doghouse plots is a deceleration plot representing the maximum CL pull seen in the doghouse.
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Unread post13 Feb 2018, 15:51

Anyway, do not use 90 deg turn. Use 180 deg turn. I will explain later.
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Unread post13 Feb 2018, 17:50

I've already gone back and re-done the turn as a max CL 180 and a recovery acceleration in a straight line back to the starting speed. I had to re-do one of my algorithms to allow for the "I'm pretty much stalled and can't use all my lift to arrest my decent or I will stay stalled". My previous algorithm caused almost any stall to result in a floating leaf departure all the way to the ground or then AB thrust was enough to arrest the fall.

I am now recording the total CCT, the time to reposition the nose 180 (not the flight path) and the theoretical minimum time to reposition the nose 180 (max AoA pull at the last second to shave a few seconds off the nose position time). I am also recording the total radius of the turn and the altitude lost during recovery.

So far I am seeing some interesting and unexpected results.

The Hornet loses no altitude. I determined this is because it has a wing tank which limits it to 20 AoA, not a max CL pull.
The Super Hornet loses no Altitude. I determined this is because it's stall speed is so low that does not require a dive to recover. It also seems to have more efficient lift generation than the legacy hornet (less drag per lift) from the data I have seen.
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Unread post14 Feb 2018, 06:51

For giggles and grins, not as a true aircraft agility metric, but more of a system metric, what about calculating minimum time necessary to obtain a HOBS missile solution? This would include minimum time to crank the nose around, combined with HMDS parameters, and the particular HOBS missile carried (e.g. AIM-120, AIM-9x, ASRAAM, IRIS-T, Python 5 etc). 90° off boresight is probably way too little -- some systems may permit the pilot simply to look at 90° and shoot. Perhaps place the bogey at 135° or 180° for such a system agility metric.
Take an F-16, add a dollop of A-7, a big gob of F-22, sprinkle on some AV-8B, stir well, then bake. What do you get? An F-35.
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Unread post14 Feb 2018, 11:39

Two agility metrics I came across year ago:
Assume an aircraft starts a turn at M0.8 and maintains a constant AOA in a turn.
Using traditional CCT metric, and assume the AOA limit of an aircraft is alpha_l,
Traverse through all values from 0 to alpha_l with 0.1 deg increment. Calculate the corresponding T1, T2, T4 and T4.

T1: bank 90 degree
T2: turn until 180 degree nose pointing change (so the angle changed in flight path is smaller than 180 deg, given the the presence of AOA increase)
T3: bank 90 degree to level flight
T4: accelerate back to initial speed

Metric 1: Find the AOA that minimize T2. T2 is the metric.
Metric 2: Find the AOA that minimize T_total=T1+T2+T3+T4. T_total is the metric.

Metric 1 evaluates missile fight capability, especially with HOBS missiles.
Metric 2 evaluates gun fight capability.

Fun fact: Those who have good peak ITR may not perform well in Metric1. :mrgreen: :mrgreen: :mrgreen:
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