Missing / Inaccurate Propeller Drag

Between missing propeller drag, that is missing deceleration and auto pilot is control theory, see Control theory - Wikipedia
If you have an AP that wants to follow a control regime and you have an actuator that can not do what is needed, you have a problem. Say you drive full speed with your car straight in direction of a wall. You make an emergency brake, but the brake is not powerful enough. You crash into the wall, because your control regime (you) wanted more deceleration then your actuator (the brake) can provide.

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The whole point of control theory is that you can create feedback loops where the system doesn’t depend on X amount of drag by the propeller.

The whole point of control theory is to have deceleration and acceleration. How you get your deceleration and acceleration is not important for a working control regime. But that the control regime knows what the actuators can do (have a correct model of the actuators) is important.

We are talking past each other here. From your posts it is clear that you know little about control theory, so please stop using the word.

I am doing it for 20+ years on my job. Ever heard of Kalman filter, Acoustic Echo cancellation, alpha-beta filter or even the simple PID?
It is always the sign of the weak in the field, this “it is clear that you know little”. Tell me, teach me if you can!

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Ok mr 20 years of experience. What you are saying is that planes are incapable of slowing down enough to fly certain RNAV approaches. Obviously you can’t design a control system that makes the plane do something it cannot. This has nothing to do with control theory.

It is always the sign of the weak in the field, this “it is clear that you know little”. Tell me, teach me if you can!

I assumed you know little because you are calling drag an actuator.

Okay, I use tiny steps. Definition actuator: ’ An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a “mover”.’ Do you understand?
Now the propeller and the engine. Both parts are connected. Normally the engine drives the propeller. If the engine changes from idle to full thrust, the propeller accelerates the airplane. That is the “mover” (see definition above) propeller moves the airplane through the air. Are you still with me or shall I go slower?
Now the tricky part: What happens if - after a long time running full thrust - the engine changes to idle? Do you know the answer? The answer is: the opposite happens. We do not have acceleration, we have deceleration. Simply speaking the propeller drives the engine. The energy the propeller needs for this, the propeller takes from the moving air. Have you ever seen a windmill? It is exactly the same.
And now the finale: This “the actuator propeller takes energy from the moving air to drive the engine” we call propeller drag.
I really like people who can only understand if they are fed exactly the right way. Did I now fed you the right way?

This is exactly control theory. I wrote before:

For example: Inertia in the ship rudder. Assume you have a control regime that works very fine on a little ship. Now you take exactly this control regime without change to a much larger ship. You will see that your control regime now works very bad.
And please say now: I disagree! You do not understand! Please say so.

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<pulls up chair, listens intently>

You really pick a weird hill to die on, your own definition refutes you “An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system”.

Please do everyone a favor and stop lying on the internet, since it is abundantly clear that you do not know control systems at all.

If you say so. But look at one of my conference papers and find the control theory there. This is from 2004: http://www.andreadrian.de/kalman_filter/irs2004_adrian.pdf

Most of my work is not published outside the company I work.

The important question is: what is a system? Or, even better: what is not a system? In the control theory world everything is a system. And there are systems within systems. On a low level there is a system that controls that the rudder stays at exactly 9° deflection, even if the airplane is hit by a wake vortex. On a higher level this 9° deflection is part of flying a standard 3°/s turn and on the highest level this is your flight management system flying the 4D trajectory you programmed.

“Your” paper is about signal processing, not control theory.

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And you have the authority to say that a radar display system for air traffic control can not make use of control theory?

Dude just stop digging, you have hit rock bottom.

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@PaulFalke @RivetingFall4

If you guys are done arguing, can I have your expert opinion on this:


The prop drag issue is an interesting one.
Just did an emergency descent with the TBM and the data were pretty close to the AFM data.
FL310-FL150 within 4min.
+4000ft/min ROD couldn’t be achieved without simulating prop drag.
The problem is that it’s impossible to establish the basic airframe drag since the prop can’t be feathered.

I’ve also noticed that when you are going faster than 200kias, reducing power to idle results in a significant, most likely realistic, speed reduction rate.

The more you (try to) slow down, the less the braking effect becomes, which is logical, but it decreases way too much at low speed.
You basically can’t land the TBM (or the King Air) without flaps.

To keep explanations simple for the developers, on propeller aircraft there is a thrust setting that creates zero thrust and zero drag. This thrust setting is NOT idle.

Therefore if idle is set, especially on a turboprop, you WILL decelerate and you WILL feel like you are being pressed into your harness as you slow down. The drag that this causes is quite significant and allows turboprops to achieve very high rates of descent - this drag is the reason turboprops don’t have flight spoilers/airbrakes - They don’t need them.

Furthermore, when landing, unless an extremely short landing is required, most operators will use a propeller setting called ‘disc’, which places the blades to create maximum aerodynamic drag on the aircraft (not the drag on the propeller itself) without sending the thrust forward. Reverse thrust was very rarely used in the various turboprop operations I flew in.

Whilst taxiing, this disc angle is then adjusted slightly forward or backwards which allows taxi without using brake to slow down. Basically it gives a little bit of forward or reverse thrust to speed up or slow down on taxi.

Propeller drag is a significant factor in any propeller driven flight models. (That’s the entire reason we have to feather an engine on a multi engine aircraft if it fails). Even on piston aircraft it has an effect.

A flight model without this is simply not a flight model.


I have just tested descent in Caravan from 7000ft at 10 degrees angle at idle with and without feathered prob and the speeds are exactly the same 160 knots. Either feather does not work or there is no prop drag effect.


Nope there is none as much as there is no use for the rudder, zero adverse yaw the aircrafts (all of them mostly) fly perfect turns without it, you kick it and the airplane put herself straight without any bounce.

Clearly ASOBO made a nice game but have no clue about the very basic of flying… basic not saying they need to be as precise as Xplane 11 with the simulation, but not even as bad as now, currently is mostly an approximation / an idea, of flying not a simulation.

But hey…they keep releasing regions…


Have you tried the 172 recently?

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For SU5, lots of aircraft variables were changed to 0 - pretty much invalidating their effect. I’m guessing that was done for X-Box which is infuriating. Hopefully they’ll introduce these back in.

Prop drag though was never simulated. And I agree, currently there’s no reason for a rudder, even in a twin.

Regions are delivered by a different team BTW, so that doesn’t really affect this topic.

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