Question regarding descend behaviour

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paradoxagi
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Joined: 10 Jan 2018, 11:32

Question regarding descend behaviour

Post by paradoxagi »

Hi...

... now I have a question related to the flight model (P3D V4.1)

I noticed that when I fly straight and level, for example at 100 KIAS and reduce throttle for a descend (not touching the trim setting), the C172 lowers the nose ans starts to descend, as expected. But it also accelerates and airspeed increases significantly while keeping nose down attitude and sink rate. From my theoretical knowledge the increase of airspeed should result in more lift, then reduced airspeed and finally the plane should keep the speed I originally trimmed for (when still flying straight and level) while descending.

So, as I don't know how a C172 really behaves in this regard, my question is:

Is this right for the C172?

I also read about problems installing updates that can result in odd behaviour, so possibly there's something wrong with my Installation (update 10-12-2017 is installed) or simply my knowledge of flight theory is wrong or does not apply to this aircraft.


Kind regards,

Agi

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Lewis - A2A
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Re: Question regarding descend behaviour

Post by Lewis - A2A »

How long did you leave the aircraft and what was you trimmed at, if you are trimmed down and are going down you'll end up going down down down a long while before popping up, but any popping up is short lived before you'll be dipping and down again.

Technical reasons also come into play, if your controller has a little noise its fully possible a slight nose down is being inputted.

And then of course theres the weather, air and wind etc which all play there own part.

thanks,
Lewis
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AKar
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Re: Question regarding descend behaviour

Post by AKar »

paradoxagi wrote:I noticed that when I fly straight and level, for example at 100 KIAS and reduce throttle for a descend (not touching the trim setting), the C172 lowers the nose ans starts to descend, as expected. But it also accelerates and airspeed increases significantly while keeping nose down attitude and sink rate. From my theoretical knowledge the increase of airspeed should result in more lift, then reduced airspeed and finally the plane should keep the speed I originally trimmed for (when still flying straight and level) while descending.
Not quite. You'd end up with classical phugoid cycle with that experiment. Airplanes are not speed stable (against what your instructor sometimes might tell you), but more "angle-of-attack stable". A subtle but important difference in what comes to the dynamics.

-Esa

paradoxagi
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Joined: 10 Jan 2018, 11:32

Re: Question regarding descend behaviour

Post by paradoxagi »

So... I did a few test for checking if I have some controller interferences. First, my T16.000 Joystick seems to be completely neutral when not touched, windows calibration confirms absolutely no directional input in centre position for all axes. Inside the airplane the virtual yoke also is perfectly centered when I don't touch the joystick. I also know what weak inputs with lots of noise can be from my xbox 360 gamepad that I use for gaming (and for flying at first) - the T16.000 is much better in this category.

Also, I set up a test flight with 'clear skies' weather theme from P3D that did not render any winds (confirmed by the shift-z display upper left side of the screen, always 0 kts). I did the flight in one direction and in reverse, climbing to 8000 ft MSL and leveling out at 105 kts, 2300 rpm, 55% mixture setting and let the plane fly for about a minute after it got stable. Then I reduced throttle, decreasing rpm to 2200 (at first, rpm went up again later when gaining airspeed), initiating a descend and the C172 increased speed and ended up with 113kts when it got stable (at about 7000 ft MSL).

I did this two times heading west, one time heading east, the result was the same everytime.


Is this correct for a C172?

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Scott - A2A
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Re: Question regarding descend behaviour

Post by Scott - A2A »

I just did a quick and dirty test, flying level at about 4000 ft, 103 kts. Pulled the throttle back, speed drops, nose comes down, speed increases, but when I approach 103 I stabilize the airplane. Once stable, I'm seeing 104kts.

The other thing to consider is, as you descend your power is increasing.

Scott.
A2A Simulations Inc.

paradoxagi
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Joined: 10 Jan 2018, 11:32

Re: Question regarding descend behaviour

Post by paradoxagi »

Hi Scott,

I assume 'stabilize' means when you are at 103 kias you trim to hold that speed? That works for me too, but does the real airplane also need trim or elevator correction for stabilizing the speed in a descend?

Thank you,

Agi

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Scott - A2A
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Re: Question regarding descend behaviour

Post by Scott - A2A »

No not trim, but use the elevator to stop the oscillations, as the up / down oscillations can be pretty big, and by the time it stabilizes, you would be thousands of feet lower. So adjust, let the plane drop, watch the airspeed and as it approaches the former cruise speed, pull back the elevator to soften it's descent then slowly release the elevator pressures.

Scott.
A2A Simulations Inc.

paradoxagi
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Joined: 10 Jan 2018, 11:32

Re: Question regarding descend behaviour

Post by paradoxagi »

I ended up at 7.000 ft MSL with slight elevator corrections for less oscillation (faster stabilization) in one of my tests, stable diving at about 113 KIAS (originally trimmed for 105 KIAS at 8.000 ft MSL). With no elevator correction the plane takes some longer time to get stable, of course some thousands of feet lower, ending up at about 116 KIAS. But, in any case, if I want to get my original airspeed on descend, I always have to trim the elevator - if I don't, I always see airspeed increase significantly.

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