Props don't counter rotate?

[Bremspropeller]

The problem with free-body-diagrams is that you need to consider the boundary-conditions corretly. Otherwise, you'll get off track very quickly.

 

An airplane is a CoG with 6DoF, suspended from the ceiling by a string. The fuselage is not fixed in mid-air. It will follow any perturbation it is subjected to, unless countered by a control-input or dampening force (e.g. by a stabilizer or stab-aug).

 

If you apply the engine torque-moments, the left wing will try to twist wingtip down, wingroot up. The right wing will try to do the opposite, resulting in an S-shaped wing.

The stiff wing and fuselage in between the engines will try to seek an equilibrium and they'll want to resolve that by rolling left, unless this is countered by an opposing input (e.g. aileron-trim).

[scoobie]

FWIW... a simple take on drawing arrows.

@Flamin_Squirrel - I think you've drawn too few arrows on this nice drawing you attached above. I've added arrows at the wingtips to your drawing (sorry for cr*ppy quality).
Now blend the two pictures into one. The fuselage wants to go up as much as go down, so it stays where it is, but what about the wingtips?
The arrows add up, right?

I suspect this contraption wants to roll.

 

[Swagger897]

Exactly this

[=475FG= Dawger]

From  ICAO Airplane Upset Prevention Training

 

https://www.icao.int/safety/loci/auprta/index.html

 The linked page has very good descriptions and illustrations of single and both engines operating aerodynamic considerations.

 

I am not sure why they explain engine torque from the perspective of behind the aircraft and then give an illustration from the front but ICAO does stuff like that regularly.

 

 

Edited August 23, 2021 by =475FG= Dawger

[mosquito64]

On 8/22/2021 at 2:48 PM, Nealius said:

"Critical engine" being the one that, if it fails, yeets you into an aircraft-ground interface? 

Failure to control the various forces under discussion, wanting the aircraft to go left on the takeoff run also leads to an aircraft ground interface. Also known as a sub optimal outcome to a takeoff event.

Here's one from 1950 - loss of control during the takeoff (B35 of 14sqdn RAF)

[mosquito64]

I think Squirrel was equating the engines as "pivot points". The important fact that has been repeated several times is that the forces all act through the center of mass not the engines. Hence the left rotation of the aircraft.

[Flamin_Squirrel]

On 8/22/2021 at 11:39 PM, =475FG= Dawger said:

I would agree with your statement except for the fact that Flaming Squirrel is arguing against some very basic physics established several centuries ago. He is not willing to consider the possibility that he does not understand simple Newtonian physics.

 

A basic understanding of Newton's three laws of motion and the practical consequences would be enough to illustrate the error of his thinking.

 

He is arguing against a century of establish aviation precedent.

 

He is arguing Kelly Johnson was an idiot when he put counter rotating propellers on the P-38.

 

The first requirement for obtaining new knowledge is the willingness to allow for the possibility that you are wrong. In the face of all evidence to the contrary, he is arguing he has some special insight into the laws of physics no one else has.

 

So, yes, until he allows that he might be in error, I don't have the patience for further discussion.

I conceded on the first page I might be wrong. But you just expected me to take your word for it. Or at least, despite being the most aggressive in asserting my stupidity, beyond linking documents, you also seemed the least interested in explaining why.

Anyway, Krupi and those who mentioned the C of G I think have the answer as to why I was probably wrong. It also makes me suspect that the image you showed above (typical of what you’d see in a text book) is actually inaccurate, leading to much (my!) confusion. 

The purple resulting torque arrows are shown rotating around the engine, but should they not be shown about the C of G (as I’ve scribbled on below)? 

 

I never intended this topic to degenerate, my mind just doesn’t work with half the picture. The fact my book shows the same (inaccurate? thoughts?) image, and describes torque being negated with both engines running, threw me. 

[Krupi]

Everyone has there preference when it comes to assimilating information, mine is definitely visually. 

 

You are kind of correct with your updated image but the original isn't wrong nor particularly concise which isn't helping. They don't show the other forces at play here working against the torque, obviously they aren't going to change the result as the force from the torque is simply too great to overcome however if you depict it like they have ideally all the forces should be shown. 

 

What you have depicted, correctly, is the resultant force acting on the C of G once all the forces have been taken into account. 

 

Edited August 24, 2021 by Krupi

[Flamin_Squirrel]

 
 

Yes you’re right, nothing wrong with the image. Each side can be imagined to have an “up” and “down” force, which just happens to be shown by the curved arrow. Because the up and down forces are not the same distance from the C of G, that leads to the resulting force shown by the arrows I drew.

 

Good grief I’m out of practice with this. Thanks for your patience.

Edited August 24, 2021 by Flamin_Squirrel

[LeCuvier]

On 8/21/2021 at 3:18 AM, Swagger897 said:

...I'd highly recommend reading this article to have a better understanding and to stop spreading further mis-information, as well as outright denying factual evidence and experience by those who thoroughly understand the topic: https://www.pilotscafe.com/engine-inoperative-principles-in-a-twin-airplane/

...

Thank you for posting this article, it certainly reduced my ignorance in this matter. But could you be a bit less harsh to those of us who are less knowledgeable than you?

[=475FG= Dawger]

4 hours ago, Flamin_Squirrel said:

I conceded on the first page I might be wrong. But you just expected me to take your word for it. Or at least, despite being the most aggressive in asserting my stupidity, beyond linking documents, you also seemed the least interested in explaining why.

Anyway, Krupi and those who mentioned the C of G I think have the answer as to why I was probably wrong. It also makes me suspect that the image you showed above (typical of what you’d see in a text book) is actually inaccurate, leading to much (my!) confusion. 

The purple resulting torque arrows are shown rotating around the engine, but should they not be shown about the C of G (as I’ve scribbled on below)? 

 

I never intended this topic to degenerate, my mind just doesn’t work with half the picture. The fact my book shows the same (inaccurate? thoughts?) image, and describes torque being negated with both engines running, threw me. 

I will admit to having very little patience with regard to the specific subject of engine torque on the propeller driven aircraft in DCS. I have argued, quite unsuccessfully, that engine torque is not correctly modeled in DCS. I have been told repeatedly that it is modeled perfectly by people who quite obviously do not understand basic physics, when it is quite obvious and easy to demonstrate in DCS.

 

So, we are all a product of our experience, myself included. Hopefully, this thread will cause people to re-examine the other aircraft with an eye for inconsistencies.

 

On to your question regarding the diagram.

 

The diagram is not incorrect. It is making some assumptions about the audience and leaving out information deemed to be obvious for the INTENDED audience. The intended audience are assumed to be well trained, experienced pilots. As such it is also assumed that they have a solid background in basic physics and basic aerodynamics. 

 

The bright pink arrows you drew describe the approximate resultant motion of the aircraft as a result of the engine torque forces acting on the aircraft, assuming no other forces acting in opposition.

 

The engine torque force cannot be thought of as linear or even curved.

 

It is a rotational force applied to the aircraft through the engine mount. 

 

So the next obvious question is why does this make it rotate around the center of gravity? The answer is it doesn't. It is very close but not quite. If the aircraft were free from the atmosphere and gravity with the torque of the two engines still acting upon it it would rotate with the center of gravity describing a small circle.

 

For atmospheric flight, it is accurate enough to assume the center of gravity as the center point of all rotation.

 

 

 

 

[Flamin_Squirrel]

29 minutes ago, =475FG= Dawger said:

I will admit to having very little patience with regard to the specific subject of engine torque on the propeller driven aircraft in DCS. I have argued, quite unsuccessfully, that engine torque is not correctly modeled in DCS. I have been told repeatedly that it is modeled perfectly by people who quite obviously do not understand basic physics, when it is quite obvious and easy to demonstrate in DCS.

 

So, we are all a product of our experience, myself included. Hopefully, this thread will cause people to re-examine the other aircraft with an eye for inconsistencies.

 

Absolutely. This has been quite embarrassing for me, as I've just shown what can happen when well meaning training material isn't worded as quite as well as it should be, combines with misinterpretation, leading to completely getting the wrong end of the stick. And I shouldn't have, as while I'm not multi-engine trained, I'm not a complete layman either.

 

As Bremspropeller said, this isn't the most complex scenario in the world, but if you mess up the initial conditions you can easily come to the wrong conclusion.

 

Out of interest, what did you find in DCS that's not right?

[grafspee]

45 minutes ago, =475FG= Dawger said:

 

 If the aircraft were free from the atmosphere and gravity with the torque of the two engines still acting upon it it would rotate with the center of gravity describing a small circle.

 

Not true, without air there would be no torque only dominant force which would stay is inertia of rotating prop and engine itself.

Gravity does not change much as well, only thing which would change that plane would act like submerged submarine with neutral bouancy 

Edited August 24, 2021 by grafspee

[Swagger897]

18 minutes ago, grafspee said:

Not true, without air there would be no torque only dominant force which would stay is inertia of rotating prop and engine itself.

Gravity does not change much as well, only thing which would change that plane would act like submerged submarine with neutral bouancy 

 

Mmm i find that a bit hard to believe? There are plenty examples of torque creating devices for spacecraft to adjust their positions for flight outside of earth’s atmosphere. 

[=475FG= Dawger]

20 minutes ago, grafspee said:

Not true, without air there would be no torque only dominant force which would stay is inertia of rotating prop and engine itself.

Gravity does not change much as well, only thing which would change that plane would act like submerged submarine with neutral bouancy 

 

Jesus, man. It was a hypothetical purely to aid explanation. Is it any wonder people get frustrated?

[grafspee]

1 hour ago, Swagger897 said:

Mmm i find that a bit hard to believe? There are plenty examples of torque creating devices for spacecraft to adjust their positions for flight outside of earth’s atmosphere. 

It is inertia devices not torque.

Edited August 24, 2021 by grafspee

[=475FG= Dawger]

13 minutes ago, grafspee said:

It is inertia devices not torque.

Really? We are going to dive down this pointless rabbit hole?

 

 

[Bremspropeller]

1 hour ago, grafspee said:

Not true, without air there would be no torque only dominant force which would stay is inertia of rotating prop and engine itself.

 

That's not true.

There would be a countering torque by the inertia of the prop (as you pointed out correctly), which in turn would runaway to destruction, as there is no resistance to halt it's acceleration above it's destruction RPM..

 

The acceleration of RPM in the absence of air would be directly linked to the rotational inertia (mass and shape) of the prop/ blades, the torque eaten by the gears and internal mechanics and the available engine-torque.  Hence, there would be an opposing torque onto the engine, mount and therefore the structure - as long as there's an acceleration going on.

 

 

 

 

[Krupi]

@=475FG= Dawger

 

I am intrigued to hear what you think is modelled incorrectly. 

[Morat]

My takeaway from all this is....

 

I going to need more throttles

[grafspee]

1 hour ago, Bremspropeller said:

 

That's not true.

There would be a countering torque by the inertia of the prop (as you pointed out correctly), which in turn would runaway to destruction, as there is no resistance to halt it's acceleration above it's destruction RPM..

 

The acceleration of RPM in the absence of air would be directly linked to the rotational inertia (mass and shape) of the prop/ blades, the torque eaten by the gears and internal mechanics and the available engine-torque.  Hence, there would be an opposing torque onto the engine, mount and therefore the structure - as long as there's an acceleration going on.

 

 

 

 

Agree, but we are talking about constant speed props, where prop or engine don't experience any rpm acceleration, so all torque effect is only contributed propeller drag.

1 hour ago, Krupi said:

@=475FG= Dawger

 

I am intrigued to hear what you think is modelled incorrectly. 

He said that.

Torque effect is not modeled correctly in DCS

Edited August 24, 2021 by grafspee

[grafspee]

1 hour ago, =475FG= Dawger said:

Really? We are going to dive down this pointless rabbit hole?

So those devices has limited momentum so i assume that they use inertia of flywheel to create this counter torque. Without large inertia of the flywheel this device would do nothing. Once it reaches max rpm it has to be reset to lower energy state.  So after all classic rocket engines must be used to allow for this mechanism to reset. Probably use of those reduce amount of required burns a lot.

 

[Bremspropeller]

25 minutes ago, grafspee said:

Agree, but we are talking about constant speed props, where prop or engine don't experience any rpm acceleration, so all torque effect is only contributed propeller drag.

 

At some time, you'll have to start your engine.

[Krupi]

37 minutes ago, grafspee said:

 

He said that.

Torque effect is not modeled correctly in DCS

 

 

.....Yes.... 

 

Clearly I read that and want him to elaborate further. 

 

Stating you don't think something is modelled correctly doesn't just end the conversation, that is the beginning

Edited August 24, 2021 by Krupi

[grafspee]

1 hour ago, Bremspropeller said:

 

At some time, you'll have to start your engine.

There is additional torque at take off, created by prop inertia while engines accelerating to max rpm, you can minimize it by slowly adding power, but in flight when rpm is settled this torque is not present.