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Some questions about flight characteristics


JanTenner

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Hi Twin Otter Experts :)

I have a few questions about the Twotter Extendeds behaviour during flight. This is not complaining about the flight dynamics. As I really love this aircraft I want more than just flying it, I want to understand how it is working. So this is aiming also for the real aviators.

So what I try to figure out is going into descend or climb from level flight. I have a perfectly trimmed aircraft flying with 140 kts IAS. Now I want to go into descend. Therefore I pull the power lever a bit.

Now the Twotter is pitching up. I could imagine that this is because of the thrust missing on top of the aircraft where the engines are leading the pitching up, right (or wrong) ? It also looses speed going down to nearly 100 kts IAS before it pitches down. Then it is going into descent stabilizing at an IAS of 120 kts. I got it trimmed to 140 kts. Why is it 120 kts in descent? I havent touched the trim (or any controlls other than the power levers).

When I then push the power levers to go into level flight it first pitches down. Is it also because of the engines on the upper side of the aircraft? Then it is showing the same bahaviour as when going into descent but in the oposite direction (IAS goes up etc.).

If someone here is willing to tell me if thats right, at best with some explanations, I would be very thankful :)

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  • Aerosoft

Indeed the engines are mounted way above the center of the aircraft and any change in power setting will change pitch. So reducing power will make the aircraft slow down and pitch up, and that is indeed a bit strange when you are not used to it. How ever you can use the effect as well when you flare. Makes for very soft touchdowns.

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Here's a technique I am just getting used to during my training for powered planes (I was a glider-only pilot before):

Basiclly to change the altitude you use the power. To change the speed adjust the pitch.

So if you want to climb you will add some power and then use the pitch (pull on the yoke, it won't happen on it's own!) to keep the speed where you want it.

It's the same for the descend. Pull back the throttle and then push the nose down to maintain your speed.

What is important is that it won't maintain the speed when you pull on the throttle. Surely it will start to climb/descend, but that mainly happens due to the change in lift with the different speeds.

This technique should be used in most powered aircraft (actually I don't know any in which it shouldn't), not only the Twin Otter.

I hope I could help you.

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Thanks, Emi.

As I said I have not really real aviation experiences. Most of my knowledge about flying i got from the internet, especially from FSX or others simulators tutorials. I am very interested in General aviation and mostly flew the small ones (the Cessnas, the Cub ...) and learned (somewhere ;) ) that I do the landing in best case without using the yoke but with the throttle. This technique I have more or less used for all climbs and descents. And it worked pretty good, well, on those with piston engines. With the Twin Otter I have experienced exactly what you have told me. So, another thing learned.

I am really happy about this great aircraft. It is not only good looking, sounds awesome but made me learn more about flying :) Thanks again for your answers.

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To correct your assessment of trim. You materially changed the relationships that lead to a trimmed state, so what was trimmed for level flight at 140 knots constant power state does not imply that after a reduction in power the aircraft will pitch down to a trimmed 140 knot state. You will need to re-trim.

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  • 4 weeks later...

I was told the APT method for climb and descent - Attitude, Power, Trim, in that order. So, to descend = push yoke (Attitude), reduce throttle (power), then trim (Trim). In this way you go smoothly from level flight to a nose-down descent attitude without any messy nose-up which might gain some altitude. Same for ascent = pull yoke, add power, then trim.

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I was told the APT method for climb and descent - Attitude, Power, Trim, in that order. So, to descend = push yoke (Attitude), reduce throttle (power), then trim (Trim). In this way you go smoothly from level flight to a nose-down descent attitude without any messy nose-up which might gain some altitude. Same for ascent = pull yoke, add power, then trim.

Makes sense...

Trimming is the last one to adjust, since all other aircraft parameters must have been stabilised before zeroing out any control input.

Finn

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Power change... need to re-trim

speed change ... need to re-trim

attitude change... need to re-trim

Configuration change... need to re-trim

Co-pilot farts ... need to open the window.

The window changes the airflow pattern... and you may need to re-trim.

Trim is a consequence, not a cause.

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Power change... need to re-trim

speed change ... need to re-trim

attitude change... need to re-trim

Configuration change... need to re-trim

Co-pilot farts ... need to open the window.

The window changes the airflow pattern... and you may need to re-trim.

Trim is a consequence, not a cause.

Easy to understand explanation Simon :)

Finn

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I was told the APT method for climb and descent - Attitude, Power, Trim, in that order. So, to descend = push yoke (Attitude), reduce throttle (power), then trim (Trim). In this way you go smoothly from level flight to a nose-down descent attitude without any messy nose-up which might gain some altitude. Same for ascent = pull yoke, add power, then trim.

Exactly.

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