Jump to content

Props don't counter rotate?


Recommended Posts

23 minutes ago, =475FG= Dawger said:

Both engines are always applying a rotation force to the airplane opposite the direction of the propeller rotation. If they are rotating in opposite directions, the forces are in opposition and cancel each other out. If they rotate in the same direction, the forces are cumulative.

 

You can't just add the forces together because the torque from each engine acts through the centreline of the prop, not the centreline of the fuselage. Image single engine flight on each engine, non-counterrotating. From the point of view of rotating around one engine, it'll try and rotate the fuselage up. With the other engine operating, down.

 

Both engines are trying to rotate in the same direction in, but because the points of rotation are offset they work in opposition. 

Link to comment
Share on other sites

6 minutes ago, Flamin_Squirrel said:

 

You can't just add the forces together because the torque from each engine acts through the centreline of the prop, not the centreline of the fuselage. Image single engine flight on each engine, non-counterrotating. From the point of view of rotating around one engine, it'll try and rotate the fuselage up. With the other engine operating, down.

 

Both engines are trying to rotate in the same direction in, but because the points of rotation are offset they work in opposition. 

You need to take a basic physics course. 

 

 

  • Like 2

475th Fighter Group Discord https://discord.gg/xkKsApD

 

 

 

EDsignaturefleet.jpg

Link to comment
Share on other sites

grafspee: Th anks. So the P-factor bit makes perfect sense and can see why that would lead to yaw; but the torque bit not so much. 

 

Consider the following single engine scenarios, and the resulting effects on the airframe:

 

sLpRFuo.png

 

In both examples, the engines cause an anti-clockwise rotation about the engine itself, but because the pivot points are on different sides of the aircraft, the effects on the fuselage are in different 'directions' for want of a better word.

 

If these two moments don't cancel, why?

Link to comment
Share on other sites

easiest way to think about p factor and torque is that torque causes roll in the opposite direction to the engine, while p factor occures perpendicular to the point where the pressure is applied on the prop plade. 

 

So a clockwise engine causes left roll 

a clockwise prop in a climb causes right yaw.

 

edit: i literally just had a principles of flight exam for my atpl studies and propellors are big section of it, i really need to get a life lmao.


Edited by zcrazyx
  • Like 1
Link to comment
Share on other sites

5 hours ago, =475FG= Dawger said:

It wasn’t an attack. It was a genuine recommendation. You are lacking fundamental understanding that makes further discussion of this specific issue fairly pointless. 

 

You don't know my background.

 

Given that you seem to think the centre point of wherea torque acts is irrelevant, I'm not sure why you think anyone should take you seriously. 

Link to comment
Share on other sites

10 hours ago, Flamin_Squirrel said:

grafspee: Th anks. So the P-factor bit makes perfect sense and can see why that would lead to yaw; but the torque bit not so much. 

 

Consider the following single engine scenarios, and the resulting effects on the airframe:

 

sLpRFuo.png

 

In both examples, the engines cause an anti-clockwise rotation about the engine itself, but because the pivot points are on different sides of the aircraft, the effects on the fuselage are in different 'directions' for want of a better word.

 

If these two moments don't cancel, why?

I can see that, but final result is that airframe is rolling left no matter which engine fails. So when both engines works left roll is not a simply 2 times amplified, but final result is left roll as well.


Edited by grafspee

Windows 11, I9 9900KF @5.0, Gigabyte Z390 Aorus PRO, Ram 32 GB G.skill, Palit Gamerock OC 3090, Hotas Warthog, Thrustmaster Pendular Pedals, OLED 48" 120Hz.

Link to comment
Share on other sites

12 hours ago, Flamin_Squirrel said:

If these two moments don't cancel, why?

It is basic physics, two vectors added together create a resultant sum greater than the natural aerodynamic forces imposed on the airframe. In a non counter rotating multi engine aircraft, two engines assumed to be operating under similar conditions both create a torque action opposite of the path of blade rotation. These two torques are added together. This is the simplest form of explanation.

 

For counter rotating multi engine aircraft, the two torque vectors DO cancel each other out, providing a more stable airframe. The descending blades are then also on opposite sides of the engine centerline, and airframe centerline, thus also decreasing p-factor to a minimal or near zero effect. 

 

All of the examples posted in this thread demonstrate that, as well as Mr. Week's referenced video from the first page. 

  • Like 2
Link to comment
Share on other sites

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

Z390 Aorus Elite | i5-9600k @5.0Ghz | RTX3060 12GB | 32GB DDR4 | Windows 10 | Quest 2 | Warthog HOTAS | Sahaj 20cm Extension

Link to comment
Share on other sites

the critical engine is the engine, that if fails, creates the largest yawing moment. therefor with a twin engine aircraft, with clockwise rotating propellors, it follows that the left hand engine is the critical one, as when it fails it then results in the least ability to oppose the yawing moment. 

 

Criticalengine1.jpg

 

edit for clarity: so in essence the right engine has a longer arm from the Centre Of Gravity to the downgoing blade and hence creates a greater moment, therefor making the left hand engine the critical one.  if the right one had failed with left still operating, the yawing moment would be less as the thrustline is closer to the CofG


Edited by zcrazyx
  • Like 4
Link to comment
Share on other sites

So that changes depending on the direction the prop turns, right? 

Windows 10 Pro | ASUS RANGER VIII | i5 6600K @ 4.6GHz| MSI RTX 2060 SUPER | 32GB RAM | Corsair H100i | Corsair Carbide 540 | HP Reverb G2 | MFG crosswind Pedals | Custom Spitfire Cockpit

Project IX Cockpit

Link to comment
Share on other sites

58 minutes ago, Krupi said:

So that changes depending on the direction the prop turns, right? 

correct, with countra (counter rotating) props that are inwards, there is no critial engine.   if the props are anti clockwise then the critical engine should be the right one. 

Link to comment
Share on other sites

The Air Ministry didn't want different handed engines for the sake of simplifying the supply chain. 

 

Another example is when the RAF wanted the P-38 they demanded Lockheed make them with both engines rotating in the same direction. This coupled with the deletion of the turbochargers pretty much castrated the Lightning.

 

Mossie pilots would stagger the throttles on take-off to help tame the yaw tendency until the fin and rudder became effective.

Link to comment
Share on other sites

https://www.aircraftspruce.com/catalog/pdf/13-09032.pdf

 

https://en.wikipedia.org/wiki/Couple_(mechanics)

 

Most of the yawing forces come from the propeller slipstream anyway. IIRC the original P-38s (not counter rotating) were quite a handful on takeoff for this reason, but it is a slightly different case as the twin boom tail means the Rudders are behind the prop discs and in the slipstream. This is less so the case for the Mossie.

 

Anyway as others have said there is no way 2 moments acting in the same direction can cancel out. And the point through which the torque acts plays no role here.

9./JG27

 

"If you can't hit anything, it's because you suck. If you get shot down, it's because you suck. You and me, we know we suck, and that makes it ok." - Worst person in all of DCS

 

"In the end, which will never come, we will all be satisifed... we must fight them on forum, we will fight them on reddit..." - Dunravin

Link to comment
Share on other sites

@DefaultFace difference in mosquito is the tail is located between slipstream from engines, unlike P-38 which has tail stab directly behind the engines.

In case of cross wind it definitely make things more difficult to control.

not heaving any kind of tail locking mechanism, take offs and landings will be quite challenging. 

Windows 11, I9 9900KF @5.0, Gigabyte Z390 Aorus PRO, Ram 32 GB G.skill, Palit Gamerock OC 3090, Hotas Warthog, Thrustmaster Pendular Pedals, OLED 48" 120Hz.

Link to comment
Share on other sites

Yes thats what I was trying to say.... Will be interesting to see. The Me 210 also had controllability issues in the beginning IIRC and it is closer in configuration to the Mossie. Although that was due to a few other things as well.

9./JG27

 

"If you can't hit anything, it's because you suck. If you get shot down, it's because you suck. You and me, we know we suck, and that makes it ok." - Worst person in all of DCS

 

"In the end, which will never come, we will all be satisifed... we must fight them on forum, we will fight them on reddit..." - Dunravin

Link to comment
Share on other sites

12 hours ago, Flamin_Squirrel said:

Given that you seem to think the centre point of wherea torque acts is irrelevant, I'm not sure why you think anyone should take you seriously. 

 

It IS irrelevant though, when considering the phenomenom discussed here, ie. the reaction of the whole airplane, as a rigid body, to system of forces and moments applied to it. That's what guys here are trying to explain to you in this thread. The way I see it, you're confusing yourself by dividing the airplane into segments and introducing multiple pivot points unnecessarily. Which might not be a bad thing, but not in this study case.

 

What matters in our case is reaction of the plane as a whole, and as one of Newtonian physics axioms says, you can move the moment attachment point around and the body will still be forced to rotate in the same manner (a similar case to the situation where force acts along a line and which point exctly it's attached to is irrelevant - what matters is that line and vector direction).

 

Check the drawing I made in MS paint below. It's a gross simpification, but one can imagine the beam representing the airplane seen from tail, supports representing main landing gear and the moment M representing one of the engines running. If you write a basic set of static equations (sum of all forces equals zero, sum of all moments equals zero) and calculate the reactions forces Ra and Rb you will notice that result is the same no matter if moment M was attached to point A / left engine or point B / right engine.

 

Moreover, in this thread you sometimes seem to be talking about aircraft reaction in the yaw axis, and sometimes in the roll axis - which is a different kettle of fish and I think it adds to confusion  / mutual misunderstanding even more.

 

beam.jpg


Edited by Art-J
  • Like 3

i7 9700K @ stock speed, single GTX1070, 32 gigs of RAM, TH Warthog, MFG Crosswind, Win10.

Link to comment
Share on other sites

1 hour ago, Art-J said:

 

It IS irrelevant though, when considering the phenomenom discussed here, ie. the reaction of the whole airplane, as a rigid body, to system of forces and moments applied to it. That's what guys here are trying to explain to you in this thread. The way I see it, you're confusing yourself by dividing the airplane into segments and introducing multiple pivot points unnecessarily. Which might not be a bad thing, but not in this study case.

 

What matters in our case is reaction of the plane as a whole, and as one of Newtonian physics axioms says, you can move the moment attachment point around and the body will still be forced to rotate in the same manner (a similar case to the situation where force acts along a line and which point exctly it's attached to is irrelevant - what matters is that line and vector direction).

 

Check the drawing I made in MS paint below. It's a gross simpification, but one can imagine the beam representing the airplane seen from tail, supports representing main landing gear and the moment M representing one of the engines running. If you write a basic set of static equations (sum of all forces equals zero, sum of all moments equals zero) and calculate the reactions forces Ra and Rb you will notice that result is the same no matter if moment M was attached to point A / left engine or point B / right engine.

 

Moreover, in this thread you sometimes seem to be talking about aircraft reaction in the yaw axis, and sometimes in the roll axis - which is a different kettle of fish and I think it adds to confusion  / mutual misunderstanding even more.

 

beam.jpg

 

You are wasting your time. He doesn't know what he doesn't know.

  • Like 2

475th Fighter Group Discord https://discord.gg/xkKsApD

 

 

 

EDsignaturefleet.jpg

Link to comment
Share on other sites

1 hour ago, Flamin_Squirrel said:

There ARE two pivot points. You cannot get away from that.

 

If you're saying that can be resolved into a single co-ordinate system then sure I'm sure it can, but you've not done that in your example.

 

Sorry but what do you mean by pivot points?

 

To put it very simply there is only one pivot point which is the centre of gravity of the object, this point is where all the forces act around.

 

 


Edited by Krupi

Windows 10 Pro | ASUS RANGER VIII | i5 6600K @ 4.6GHz| MSI RTX 2060 SUPER | 32GB RAM | Corsair H100i | Corsair Carbide 540 | HP Reverb G2 | MFG crosswind Pedals | Custom Spitfire Cockpit

Project IX Cockpit

Link to comment
Share on other sites

9 minutes ago, =475FG= Dawger said:

You are wasting your time. He doesn't know what he doesn't know.

 

What a ridiculous attitude, if you don't have the patience to try and explain then simply don't get involved. 

  • Like 1

Windows 10 Pro | ASUS RANGER VIII | i5 6600K @ 4.6GHz| MSI RTX 2060 SUPER | 32GB RAM | Corsair H100i | Corsair Carbide 540 | HP Reverb G2 | MFG crosswind Pedals | Custom Spitfire Cockpit

Project IX Cockpit

Link to comment
Share on other sites

55 minutes ago, Krupi said:

 

What a ridiculous attitude, if you don't have the patience to try and explain then simply don't get involved. 

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.

  • Like 2

475th Fighter Group Discord https://discord.gg/xkKsApD

 

 

 

EDsignaturefleet.jpg

Link to comment
Share on other sites

3 hours ago, Flamin_Squirrel said:

There ARE two pivot points. You cannot get away from that.

 

If you're saying that can be resolved into a single co-ordinate system then sure I'm sure it can, but you've not done that in your example.

Okay so in attempt of trying to understand your POV, what is causing the “pivot points”, where is each point located at, and are you referring to either the longitudinal axis or lateral axis of the aircraft? 
 

Also for these pivot points, how does their center(s) relate to that of the airframe, and what are their affects on it under normal operation (takeoff, cruise).

Link to comment
Share on other sites

 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...