Aircraft motion for platform software

[Roland]

Can anybody give me some details on actual small / medium GA aircraft motion during the following events:

1.When extending flaps: what do you feel during transition and after extended?
2. When lowering gear: what do you feel during transition, lock, and flying with gear down?
3. When extending airbrakes / spoilers: what do you feel during transition and after extended?
4. What is the actual duration of the buffeting after rotation takeoff? What does the buffeting amplitude depend on?
5. Does anyone have a good trigger event during flight (vertical speed?) for activating motion platform turbulence?

As you can see, I'm getting to the detailed programming part of my motion platform, but I'm lacking some real flight experience.
Although my motion cues drive only pitch and roll, with the right algorithm you can get some nice effects. See http://www.simprojects.nl/motion_software.htm and the subchapters. Some videos are included.

Thanks for your help!

Roland

[NicD]

Hi Roland - I've flown a number of singles and light twins (pilot in real life).

Keep in mind that there are significant differences between aircraft in respect to these.

So...

#1 During transition - a ballooning effect with both lift and deacceleration, and a slight rumbling. After that it's just an attitude change in the aircraft.

#2 During lowering - a definate rumbling and slight buffeting, slight bump on lock in some a/c. After lowered - increased wind/rumble noise (but not alot)

#3 No experience with these in GA aircraft

#4 Not sure what you mean there - most takeoffs I've experienced have no buffet. But there are definate wind and low-level turbulence effects which produce mild bumps and yaw at low level, especially in windy and/or hot conditions.

#5 Suggest you ask Peter Dowson on this one ... FSUIPC might have software triggers. Otherwise it can be identified by the rate of climb/descent in feet per second going beyond a certain limit, or g-force accelaration/deacceleration beyond a certain limit (don't know what those limits are off-hand sorry).

[AndyT]

1.When extending flaps: what do you feel during transition and after extended?
Pretty much what Nic said on this one. Once you stabilize the ballooning, you have a slower flight along with the attitude change. The planes' attitude SEEMS to 'Sit up' and 'pay attention' more, although it actually gets more sluggish and slower to respond.

2. When lowering gear: what do you feel during transition, lock, and flying with gear down?
Go with Nic on this one too. Also the gear makes the plane a little more sluggish.

3. When extending airbrakes / spoilers: what do you feel during transition and after extended?
I've only ever felt this on a Mooney with speedbrakes. A definate buffetting as they come out which keeps buffetting when fully extended. Remember, you are disrupting the flow of air over the wing. This would depend heavily on the type of speedbrake / spoiler and its location on the plane.

4. What is the actual duration of the buffeting after rotation takeoff? What does the buffeting amplitude depend on?
Can you clarify this? If you mean the bouncing around when you take off, thats due to thermals rising off the runway and the tempature transition at the end of the runway.

5. Does anyone have a good trigger event during flight (vertical speed?) for activating motion platform turbulence?
If you have it copy the attitude indicator, you should have all the turbulance you'll ever need.

[Roland]

Hi Guys,

Thanks a lot, exactly the info I'm looking for.
Concerning # 4: In the big ailiners (I sat mostly in 747's), there is significant vibration when the wings just start to produce lift, just after rotation and initial climb. The whole cabin shakes, and the effect will slowly dissapear.
I'd like to know more details how this effect happens and what flight parameters influence this effect in GA aircraft.
Concerning #5: I have tried to directly feed attitude indicator info to the platform, but then I'm missing the high frequency turbulence effects, but instead I get some annoying on/off residue noise. Most likely because FS does not deliver enough resolution and update speed. Therefore I have opted to generate my own waveforms, but I would need to trigger the effect by detecting some specific changes in certain flight parameters (attitude, vertical speed, G's etc.

[AndyT]

Concerning # 4: In the big ailiners (I sat mostly in 747's), there is significant vibration when the wings just start to produce lift, just after rotation and initial climb. The whole cabin shakes, and the effect will slowly dissapear.
I'd like to know more details how this effect happens and what flight parameters influence this effect in GA aircraft.

I gave you the explanation for this already but I guess if you dont know much about aerodynamics then it doesnt make much sense.

The wings of a plane do not produce lift evenly across the surface of the wing. Lift is generated most strongly at the root (point where it connects to the plane) and as the wing tapers in, the generated lift changes in volume. Also, at the end of the wing the lift actually rolls off the wing sideways and loops under the wing effectively changing the lift characteristics of the airfoil. This is where wingtip vortices come from. Pilots commonly call this 'Wake Turbulance'.

Now, Add in the different air densities that the plane is slamming thru as it climbs. These different densities affect different size planes differently. They are normally caused by two factors; Barometric density and temperature. The runway is made of asphalt or concrete which has a different temp gradient than the grass or dirt right next to it and at both ends. The change in temps creates highly localized thermal action on the airmass and as the plane flies thru it, it 'bounces' off the higher density air. The higher the density, the more lift it creates AND also the more parasitic drag it creates.

So.... A plane taking off encounters different air masses moving at different speeds across the surface (due to wind) which have varying charactistics of temp, density and motion all of which affect the attitude of the aircraft. Think of this.... You take off and pass over the runway, the threshold, the grass strip at the end of the runway, the road on the other side of the fence, the canal or river or lake, a depression or a hill, different types of vegetation, buildings.... Each of these creates its own effect on the airmass which in turn affects the aircraft. The closer you are to them, the greater the effect they have on your plane. As you gain altitude, their effect lessens.
And lastly, very early in the morning, the turbulance is almost zero. It does not occur until the sun rises and begins heating the ground.

Concerning #5: I have tried to directly feed attitude indicator info to the platform, but then I'm missing the high frequency turbulence effects, but instead I get some annoying on/off residue noise. Most likely because FS does not deliver enough resolution and update speed. Therefore I have opted to generate my own waveforms, but I would need to trigger the effect by detecting some specific changes in certain flight parameters (attitude, vertical speed, G's etc.

Ok, I need more clarification here...
What do you mean by 'High freq turbulance'?
Any and all turbulance is caused by the factors I listed above.
Nic gives a few good indicators for this and I'd also add height AGL (Above Ground Level) as it can change nearly instantly in strong turbulance.

Hopefully, my short lesson in turbulance helps. I hope you dont think I was 'talking down' to you. I posted it because you said you are not a pilot and you did not seem to understand what was causing the plane to shake rattle and roll. The above is a LOT of info that may or may not seem to come together for you. I have some sweet 3D pix around here somewhere from my flight training days that help illustrate this. If you think you need them, let me know and I'll dig them up and scan them to post here. But it would be easier if you went down to your local flight school and watched the Aerodynamics video. They probably wont charge you for it either especially if you take the intro flight.

If you want a more technical explanation of all this then try this website by NASA;
http://www.grc.nasa.gov/WWW/K-12/airplane/short.html

[Roland]

Thanks for the detailed explanation on take-off turbulence, Andy. I have some basic knowledge on airodynamics, but nowhere as detailed as what you just told me.

With "high frequency" I mean the ability to send fast transitions to the platform. It means that FS needs to generate fast update speeds to the interface. The turbulence / shaking that I have experienced in airliners is in the order of 10Hz, which would require parameter update speeds of at least 20Hz / 30Hz. Resolution is another problem, as the aircraft vibrations tend to have very small amplitudes, that don't show up in the readouts of the attitude indicators.

Actually I'm planning to take your advice and pay a visit to the small airports nearby, talk to some of the pilots and maybe take a intro flight.

[AndyT]

I do not know about where you live, but most schools when you take the intro flight, they let you fly the plane. The licensed pilot does the takeoff and once he stabilizes the plane, he lets you fly it. Then at the end of the flight, he does the landing. I know that you will enjoy the flight and, it will answer many of your questions about the takeoff turbulance. Especially if you fly in a small plane like a C172.

If possible, try this for your intro flight;
1. Take off
2. Steep turns
3. Touch and Go
4. Around the pattern
5. Full stop landing.

This is a very nice program that will give you some of the most intense sensations of G-forces and turbulance you can get in a short flight.
Frankly, I think you will love this flight so much you will want to do it again!

[AndyT]

Roland,

Ever take that intro flight? How was it? Share your experiences with us.

[Roland]

Nope, not yet, as my old colleague that has a PPL did not have time yet. But I may have an opportunity somewhere in August.

Right now back in Taiwan for couple of weeks holidays.

Roland