Pitch-Roll input axes

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Tulkas

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Unread post07 Nov 2013, 14:35

Hi.

This question goes for Viper drivers/engineers working on the jet.

The force sensor under the stick is mounted twisted 12 degrees outboard regarding the stick grip on top, which is aligned with the long. axes of the aircraft (see attached picture)

Does this mean that to achieve a pure pitch input you actually have to pull a bit inwards? Sounds right from the ergonomics point of view (you can exert more force pulling towards you than parallel to your body) but weird regarding controlling a plane.

Rephrasing my question in a more direct way: If you put a finger in front of the stick and push, the plane will just pitch up or pitch up mixed with right roll?

Thanks

Marcel
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johnwill

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Unread post07 Nov 2013, 18:12

The original YF-16 stick, in addition to being rigid, was aligned with the airplane axis system. It was found that pilots had a tendency to input some small roll command while trying to do a pure pitch pull up. Not sure when the change was made, but production airplanes have a stick with a small amount of motion in addition to the 12 degree rotated axis.

So, yes, if you pull straight back on the stick (airplane axis) you will get a small right roll command.
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Tulkas

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Unread post08 Nov 2013, 08:44

Thank you very much for the answer.

We have quite an ongoing discussion on this at the cockpit builders forum. Just to quote you there, I hope you don't mind I ask you how did you get the information, Are you pilot, ground crew...?

Thanks

Marcel
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JoeSambor

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Unread post08 Nov 2013, 11:52

Tulkas wrote:Thank you very much for the answer.

We have quite an ongoing discussion on this at the cockpit builders forum. Just to quote you there, I hope you don't mind I ask you how did you get the information, Are you pilot, ground crew...?

Thanks

Marcel


Can't wait to hear John's answer to this one...
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JetTest

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Unread post08 Nov 2013, 12:49

Yes, you never know who someone might be....
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Tulkas

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Unread post08 Nov 2013, 13:23

Hey guys! I hope I did not offend anybody with my question. If I did I apologyze, it was not my intention.

When you are not habitual in a forum you don't really know who is who... and you all know how inet forums are.

But now I notice johnwill is a very senior member here and is obvious that you guys respect him so I am more than satisfy, I don't need him to answer if he doesn't wish to (but now I am curious :) )

Regards

Marcel
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JetTest

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Unread post08 Nov 2013, 14:12

I don't think anyone is offended by your question.
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lamoey

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Unread post08 Nov 2013, 20:28

In my time I did replaced a few stick grips and also the box it is mounted on. I never really noticed its alignment.

However, during the mandatory tests, after certain flight control maintenance events, full test of control surface range of movements had to be done. This test involved a person sitting in the cockpit operating the stick while a bunch of people were located around the aircraft with tape measures to measure how far each control surface moved under various simulated speed, altitude, gear up/down and AOA conditions. It turned out to be a challenge to do a pure pitch command as well as a pure role command. Since the aircraft was securely located on jacks in a hangar there were no horizon to help tell you if you pulled in the wrong direction. You had to rely on feedback from the guys with the tape measures to tell you if you did it correctly, and more often than not, it was not.
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sprstdlyscottsmn

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Unread post08 Nov 2013, 22:56

Boy if Gums chimed in we would have our Trifecta of F-16 gurus.
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lamoey

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Unread post08 Nov 2013, 23:24

There were an adjustable arm rest, but I can't remember of it was adjustable sideways as well as up and down. Most likely only up and down, and it would then align your arm to the sticks off-center alignment.

In principle the off-center alignment is similar to the way some helicopters main rotor is often mounted with a few degrees roll angle to compensate opposite to the tail rotors sideways push, hence avoids having the pilot trim for this correction all the time.
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johnwill

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Unread post09 Nov 2013, 05:47

Marcel,
Welcome to the forum and thanks for asking a good question. I hope you will visit and contribute frequently. To answer your question about how I knew about the stick axis rotation, I was a structural flight test engineer for General Dynamics on the YF-16 and F-16 programs from 1973 until 1985. On modern airplanes, everything affects everything else, so even structural engineers have to be familiar with how the flight control system affects the structure.

I'm not sure of the details of how the axis rotation was accomplished, but it would be simple to do it with software only, using sine and cosine functions to rotate the axis.
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Tulkas

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Unread post09 Nov 2013, 22:44

Thank you for your answer and the welcome! So I just met one of the F16 fathers? You can be proud of your child sir!

I don´t own a real F16 SSC but there are 4-5 persons in the cockpit building community that I know own them. They have opened them and showed pictures. I can tell you that the sensor is physically rotated so that´s how the axis rotation is achieve. What we were not sure -and you just answered it- is if that rotation meant a rotation in the inputs axes as well or just a rotation in the sensor for some obscure engineering reason. To complicate more things, some of those who owned real SSC claimed theirs were not rotated. Of course following -your answer- now I assume the belong to the models pre-modification.

This is, by the way, why I asked. I am building my own F16 and I am trying to get it as real as possible:

Image

Image

I will try to achieve the rotation of the axes by software since my commercial force sensor goes straight with the stick.

Another question just popped up in my head, the roll axis, is still perpendicular to the pitch axis, right? I mean, for a pure left roll you need to move the stick left and a bit forward?

Regards

Marcel
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johnwill

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Unread post10 Nov 2013, 02:25

Marcel, thanks for the explanation of the problem and the photos. I was aware some people build replica cockpits, but did not realize the great lengths they go for accuracy, both functional and appearance. Are the cockpits used as flight simulators with computer screens for visual feedback to the operator? If you have any problem with the rotation equations, I can post them here for you. And, finally, yes the roll axis is rotated to still be 90 deg from the pitch axis.
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Tulkas

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Unread post12 Nov 2013, 10:12

Yes the cockpits are going to be use as full flight simulators. Visuals depends mostly on what the builder can afford but you can find monitors, tv screens, multiple projectors setp... In my case I have a 47" TV screen and plan to add two more when the cokpit is finished. Maybe upgrade to projectors far in the future.

We use the simulation Falcon BMS which give us I'd say around 85-90% functionality of the whole cockpit, both flight and combat systems. The effort they have put just to get the flight model and FLCS inner workings right is nothing short of overwhelming:

(the following document is not about the real F16 but about the simulated in BMS FLCS)
http://www.benchmarksims.org/forum/atta ... 1267814283

Regards

Marcel

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