Rolling at 1 g is an barrel roll?

Always wondered why the F-16 has a tailhook, or how big a bigmouth F-16's mouth really is ? Find it out here !
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saberrider

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Unread post17 Mar 2017, 22:59

If a/c is trimmed for 1g,and you're rolling at half of max . performance and true speed is in the middle of 200kts the plane is doing a barrel roll or an aileron roll?
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Unread post18 Mar 2017, 04:45

In this explanation, keep in mind that g is not acceleration, it is load factor, n = lift/weight, in airplane coordinates. Let's go through a roll to the right at 90 deg/sec rate. The load factor, n, is constant at 1. Gravity is pulling down on the airplane. As the airplane rolls 90 deg, the lift is now horizontal in earth coordinates, so the airplane is accelerating to the right. Gravity is pulling the airplane down, so it is losing altitude, accelerating down in earth coordinates. Then roll to inverted, 180 deg bank angle. Lift is now down in earth coordinates and gravity is down, so the airplane is accelerating down at 2g, losing more altitude, but zero accel to the horizontal(earth). Then bank to 270 deg, and the lift is to the left (earth) with gravity still pulling down (earth). The airplane is moving to the left and still sinking. Then roll erect again back to wings level. The lift is now up and balances the down gravity, so you are back to level flight, but you have lost 128 feet of altitude. (Average down velocity is 32 ft/sec for 4 seconds).

So is that a barrel roll or an aileron roll? Looks like a sloppy barrel roll to me. Actually a crisply flown barrel roll is done in a shallow climb, so the loss of altitude is negated by the climb. Next time you see a roll at an airshow, chances are the airplane will enter a shallow climb just before the roll.

I assume you mean an "aileron roll" is a level flight roll, with no loss of altitude, like a zero radius barrel roll. To do that the g command must be modulated through the roll, 1g at initiation, 0g at 90 deg bank, -1g at 180 deg bank, back to 0g at 270 deg bank, and back to 1g at termination wings level. Practice that on your simulator sometime, not easy. The easiest way to do what looks like an aileron roll is to fly very fast and roll very fast, so the apparent loss of altitude is minimized.

I hope this is clear. If I screwed up somewhere, I'm sure someone will correct me.
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sprstdlyscottsmn

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Unread post18 Mar 2017, 05:45

No no, johnwill, you are spot on.
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saberrider

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Unread post18 Mar 2017, 07:19

Thanks, for your time to explained that to me . My Sims .contain propelled a/c,multi engine's, so the P factor it is part of the game when pulling, or a preference to do a better roll to the left/ right ,first jet era quirky engine Me262,Sabre,Mig15 ,17,19 or F104 and most advanced Tornado, F-15 A,C,E F16A,C , F18 legacy's Hornet' s, Harrier jump jets...etc. To do a realistic calibration for stick/rudder i read a lot of technical data's then tactics to use . My quest is what if ? this... two different a/c meeting in the sky. Many jet engine performance are derate by their user to extend life of them.
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vilters

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Unread post18 Mar 2017, 14:12

Step by step.

From your start position : Level flight at 200kts. =>You have an alfa to maintain level flight at 200 kts IAS.
That alfa is to compensate earths gravity pulling you down. (level flight => lift = gravity)

Roll to the right.

To maintain forward direction? => You have to kill that alfa => Push DOWN elevator to keep the nose pointing straight forward.

And you have to bring in TOP rudder to bring the aircraft's nose up so that the fuselage can generate the lift required to NOT loose altitude.

At the inverted position? =>You push DOWN elevator to generate a negative alfa to maintain inverted level flight while the rudder comes back to neutral.

At the 270°? => Reduce the nose down push to maintain nose position, and bring in the (now opposite) rudder again to get the nose up so the fuselage can again generate the lift to maintain altitude.

At the 360° => You are back in trimmed position at the same alfa as you started with.

Look at some YouTube video's form some aircraft performing 4 point rolls. You can clearly see the surfaces movements during all 4 stages of a 4-point roll.

Rudder action can clearly be seen when an airshow aircraft performs a knife edge flight.
The rudder puts the fuselage at an alfa to maintain the nose high position required to generate the lift to compensate for the gravity.
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vilters

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Unread post18 Mar 2017, 14:14

If : From your stating position of level flight and 200 kts?

If?
All you do is crank in the aileron?
=> You will barrel roll around (sloppy) and loose altitude, and end up nose down.
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saberrider

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Unread post18 Mar 2017, 15:35

Thank you,but it is not the ARI on F16 actively contribute to not drop the nose at 90/270 degrees bank angles ,which make my question ?My mistake the ARI is there for prevention of adverse yaw and rudder action result in proverse yaw . But if you applying top rudder then you have canceled (negates) ARI function don't you?
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Unread post19 Mar 2017, 12:25

The primary function of the aileron-rudder interconnect is to make coordinated turns. (Keep the ball in the center) and prevent yaw departures at high alfa.

Something you do not want in a 4-Point roll or a "pure" aileron roll.

There is a nice video on YouTube where aileron rolls go out of control on an F-18.

Flight control systems are designed to protect pilots during normal operations.
If you let them "do their thing" during multiple aileron rolls?
You get this.

https://www.youtube.com/watch?v=wZNaQuUGluk

The above is the video of the F-18. => Roll departure even at zero G.

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