TFDi Design 717

[SpiritFlyer 987]

3 hours ago, Christopher Low said:

The lift does decrease, and the nose drops. However, the airspeed then increases, and the nose should start to rise again. At least, that is what happens with the PMDG airliners (and most of the other aircraft that I have). What are your thoughts regarding the refusal to change pitch with variable airspeed?

Good grief Christopher, some just keep talking over you as if they did not read a word you wrote.

Just to confirm:

When you make that previously described left turn, and the nose drops, the plane picks up speed and descends all the while not responding properly to positive trim commands. Right?

What happens when you pull back on the yoke? Any variation of the same problem at different flap settings, landing gear deployment etc.?

 

Once thing that we should all agree on:

+As speed increases, so should lift. As lift increases the nose should raise and altitude altered accordingly (all else being equal)! Right or wrong?

Anyway, holding off purchase until confident of the flight model. 

Don't need another abandoned dud sitting in a field out back behind my hanger!

Kind regards,

[Christopher Low 5,352]

Quote

When you make that previously described left turn, and the nose drops, the plane picks up speed and descends all the while not responding properly to positive trim commands. Right?

Not quite. I make the turn without altering the trim at all. I do this all the time in the (for example) PMDG 737 and 747, and they react as I would expect. In short, the nose starts to drop, the airspeed increases as a consequence, and then the nose either starts to rise again or the aircraft settles into a new state of equilibrium. They do not keep on diving towards the ground picking up airspeed. Similarly, if I am in level flight in one of the PMDG aircraft....increasing thrust results in an increase in airspeed, and the nose starts to pitch up. If I reduce thrust, the nose pitches down, and I start to descend. All nice, stable and predictable.

In contrast, the 717 seems to struggle with any kind of pitch changes when I increase or decrease thrust. The airspeed changes dramatically, but the pitch just refuses to comply. This is what I do not understand. Please note that this is with a set trim position. That is probably not how it would be done in the real world, but that is not the point. The pitch of the plane should react to changes in airspeed much more dramatically than it does.

I am no expert, so someone please feel free to tell me that this is how the 717 is supposed to behave.

[Chock 17,769]

19 minutes ago, SpiritFlyer said:

Once thing that we should all agree on:

+As speed increases, so should lift. As lift increases the nose should raise and altitude altered accordingly (all else being equal)! Right or wrong?

Not necessarily right. As speed increases there is more lift generated, but that doesn't necessarily mean the nose will raise, it just means the climb rate should increase owing to the greater amount of lift, especially since this also means an increase in drag too. An increase in speed can actually pitch the nose down in many aeroplanes because as the airspeed over the wings increases, the centre of lift moves rearwards, meaning the wing will be pushed up from more toward its trailing edge, and since the force of lift may then no longer be balanced with the longitudinal CoG, it will largely depend on the trim settings as to what happens with regard to pitch. The tail down force of the horizontal stabiliser will counter that shift in the position of the centre of lift somewhat, since the horizontal stabiliser too will be more effective as the speed of the airflow over it increases, but don't be surprised to see the nose go down when the speed goes up, it is a function of the centre of lift changing its position. This is most obviously seen in aircraft when you drop the flaps, the massive increase in lift shifts the centre of lift rearward and down goes the nose, the increase in drag only adding to that too. How well all this is modeled in a flight sim is another matter of course.

[SAS443 1,723]

Suggestion: insert the tfdi 717 in a phugoid oscillation, this is one of the basic aerodynamics modes. What happens? (Pro tip: might want to disable any computer assisted inputs in the aircraft.)

You should see a change in pitch (and altitude...) as airspeed increases/decreases. But important to point out is that AoA is almost constant here. The aircrafts longitudinal axis is aligning with the flight path.

Not sure if the tfdi 717 has an AoA indicator - I know the 737ngx has it. But this should be a fairly easy assessment how the aircraft handles dynamic stability?

[Ebs 114]

Christopher, what does it say on your PFD (with regards to modes) when this unexpected stuff is happening? 

[sidfadc 1,344]

On the fence about the 717 for a while...is it worth a punt?

[JLuis 409]

YES! Amazing addon!

[JRBarrett 3,070]

23 hours ago, Christopher Low said:

The lift does decrease, and the nose drops. However, the airspeed then increases, and the nose should start to rise again. At least, that is what happens with the PMDG airliners (and most of the other aircraft that I have). What are your thoughts regarding the refusal to change pitch with variable airspeed?

While I have never flown any kind of airliner, I have several hundred hours piloting experience in GA aircraft ranging from Cessna 152 to Bonanza. In all of them, a bank requires at least some “up” elevator.

If the nose starts to drop in the turn, and the pilot does not correct by increasing pitch, the nose will continue to drop even as airspeed increases. I’ve never flown any aircraft where the nose will rise due to an increase in airspeed in a turn. In fact, in an aircraft which is designed to have low drag (like a Bonanza or Mooney), the aircraft speed and rate of descent will continue increasing in an uncorrected turn to the point that it could easily exceed redline. (The infamous “death spiral”).

The steeper the turn, the more up elevator is required. The increase in angle of attack will also increase the “G” forces felt by the pilot - which cannot be emulated in a desktop sim of course. 

An elementary flight maneuver every beginning flying student has to learn is to perform left and right 360 degree turns at 45 degrees of bank while maintaining initial altitude within plus/minus 100 feet. Doing so will require continuous “pull” (up elevator) throughout the turn. As the aircraft is rolled back to level, the elevator input is relaxed to prevent a climb from starting.