Please un-torque my neurons.... Prop effects et al...
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Please un-torque my neurons.... Prop effects et al...
This very same questions have been asked certainly many times, on many forums, and I guess I even might have already raised the problem in other forms here at PPrune, but I really have to understand what's going on between Real Life, Theory and Flight Simulation....
I am a glider pilot since 1980. Have flown many times on prop aircraft, and even had the chance to "drive" a few, but I do not have a PPL for motorized aircraft other than SSG / SLG.
I have long been using flight simulators, and have found in almost all of them the prop effects: Torque, P-Factor, Gyroscopic Precession and Spiraling slipstream) translated mostly into roll instead of yaw, while, IRL, it is mostly yaw, and not bank, that results in the first place, on most GA aircraft ( naturally, a WW2 fighter is a different matter...) causing the nose of the aircraft to drift to the left ( on a CW rotating prop ) and the pilot to use right rudder to counter it. When he does it, the ball centers, there is no sideslip, and the wings are level.
Let's assume the aircraft in this post have all CW-rotating props.
Now, the books, and many texts for aviation name the torque effects as causing a rolling moment ( to the left ). The simulators I have used translate mostly into roll the sum of the 4 above mentioned prop effects, and you really have to use aileron, or aileron trim, because if you use only rudder, there will be a lot of it required to level the wings, and when they're level, "your ball" will be almost fully to the left (???) I always found this representation of the prop effects in most flight simulators not precise.
When I talk to fellow pilots and ask them about what they feel on their Cessnas, Pipers, Robins, etc... they all say that it's mostly yaw.
When I try to refute the arguments of flightsim designers they, most of the time, come up with a justification based on the fact that the pilot almost unconsciously adds yoke / manche inputs to counter the roll from torque, and levels the wings, but RL pilots tell me this is not the case, and that they can even let the hands off their yokes while they use pure rudder to counter the prop effects at high power / high AoA, keeping the ball centered and the wings level, without an hint of aileron or aileron trim use.
Also, most GA aircraft do not even have trimable rudder or ailerons and some have only a rudder trim. Yes I am aware that they have fixed trim tabs, some on the rudder others on the ailerons too.
Now, can you help me understanding why:
- In RL apparently the prop effects translate mostly into YAW, and of course roll will naturally be induced by if you do not take action to counter the yaw and stop the sideslip;
- In Theory, and many aviation syllabus the roll due to prop/engine torque effects is mentioned as existing and playing it's role;
Thanks in advance for any help/suggestions :-)
I am a glider pilot since 1980. Have flown many times on prop aircraft, and even had the chance to "drive" a few, but I do not have a PPL for motorized aircraft other than SSG / SLG.
I have long been using flight simulators, and have found in almost all of them the prop effects: Torque, P-Factor, Gyroscopic Precession and Spiraling slipstream) translated mostly into roll instead of yaw, while, IRL, it is mostly yaw, and not bank, that results in the first place, on most GA aircraft ( naturally, a WW2 fighter is a different matter...) causing the nose of the aircraft to drift to the left ( on a CW rotating prop ) and the pilot to use right rudder to counter it. When he does it, the ball centers, there is no sideslip, and the wings are level.
Let's assume the aircraft in this post have all CW-rotating props.
Now, the books, and many texts for aviation name the torque effects as causing a rolling moment ( to the left ). The simulators I have used translate mostly into roll the sum of the 4 above mentioned prop effects, and you really have to use aileron, or aileron trim, because if you use only rudder, there will be a lot of it required to level the wings, and when they're level, "your ball" will be almost fully to the left (???) I always found this representation of the prop effects in most flight simulators not precise.
When I talk to fellow pilots and ask them about what they feel on their Cessnas, Pipers, Robins, etc... they all say that it's mostly yaw.
When I try to refute the arguments of flightsim designers they, most of the time, come up with a justification based on the fact that the pilot almost unconsciously adds yoke / manche inputs to counter the roll from torque, and levels the wings, but RL pilots tell me this is not the case, and that they can even let the hands off their yokes while they use pure rudder to counter the prop effects at high power / high AoA, keeping the ball centered and the wings level, without an hint of aileron or aileron trim use.
Also, most GA aircraft do not even have trimable rudder or ailerons and some have only a rudder trim. Yes I am aware that they have fixed trim tabs, some on the rudder others on the ailerons too.
Now, can you help me understanding why:
- In RL apparently the prop effects translate mostly into YAW, and of course roll will naturally be induced by if you do not take action to counter the yaw and stop the sideslip;
- In Theory, and many aviation syllabus the roll due to prop/engine torque effects is mentioned as existing and playing it's role;
Thanks in advance for any help/suggestions :-)
Last edited by jcomm; 7th Mar 2014 at 09:21.
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Well, here's my thoughts on the matter:
First, gyroscopic precession.
There is no gyroscopic precession (acting in yaw) when there is no pitching motion. That's not to say that it doesn't exist; it certainly does when the tail comes up on a high powered taildragger during the takeoff roll. But in a steady state climb in a stable constant pitch attitude, there is no precession going on. So if we're talking about the forces in a constant climb, we can ignore precession.
Next, reactive torque. Sure there is some, and it does act around the longitudinal axis of the plane, but how big is it really?
well lets consider the C-172 engine which develops 150 hp at 2700 rpm. That equated to 291 ft-lbs of torque at full power and rpm. So how much is that in real terms? It's about the same as having a 200 lb passenger sitting in a seat 1.45 ft to one side of the aircraft center line. In other words if the rolling moment from engine torque was unopposed (more on that in a moment) it would be about the same force as the difference between having an adult male in the passenger's seat, and not having one. It's not non-existent, but it's also not very significant.
The thing is, it *isn't* unopposed, which brings us to spiraling slipstream. Personally, I suspect that the propeller slipstream doesn't "spiral" quite as much as it's made out to in the primary flying texts. But, no doubt it does some, and whatever spiraling it does will tend to roll the airplane as it impinges on the wings, fuselage and empennage. But it tends to roll the airplane *opposite* the reactive torque from the engine. So, it tends to reduce the already, "not very large" rolling moment from the engine torque.
So that leaves us with P factor, or asymmetrical propeller disc loading, which in a low airspeed, high power climb, is the predominant force and acts only in Yaw (or very nearly so)
so to summarize:
Precession: Absent in a constant climb
Engine torque: Produces relatively small rolling moment in a typical GA aircraft.
Spiraling slipstream: Serves to counteract rolling moment from engine torque.
P-factor: No significant rolling moment.
Now, I fly single engine GA aircraft (although regrettably not as much as I'd like to) and like the others you've talked to, I'm not aware of having to hold significant aileron input on a climb, but I am aware of the need to hold rudder. No doubt I do, subconsciously, hold a very small amount of aileron.
Now, I don't fly single engine flight simulators, but if the required aileron input in a *real* airplane is well below the consciously discernible level (and it is) , and the required aileron input in the simulator is well above it, (like you say) then it would seem to me that the simulator is not programmed to be an accurate representation of the real thing.
I'm not sure how you'd conclude anything else.
First, gyroscopic precession.
There is no gyroscopic precession (acting in yaw) when there is no pitching motion. That's not to say that it doesn't exist; it certainly does when the tail comes up on a high powered taildragger during the takeoff roll. But in a steady state climb in a stable constant pitch attitude, there is no precession going on. So if we're talking about the forces in a constant climb, we can ignore precession.
Next, reactive torque. Sure there is some, and it does act around the longitudinal axis of the plane, but how big is it really?
well lets consider the C-172 engine which develops 150 hp at 2700 rpm. That equated to 291 ft-lbs of torque at full power and rpm. So how much is that in real terms? It's about the same as having a 200 lb passenger sitting in a seat 1.45 ft to one side of the aircraft center line. In other words if the rolling moment from engine torque was unopposed (more on that in a moment) it would be about the same force as the difference between having an adult male in the passenger's seat, and not having one. It's not non-existent, but it's also not very significant.
The thing is, it *isn't* unopposed, which brings us to spiraling slipstream. Personally, I suspect that the propeller slipstream doesn't "spiral" quite as much as it's made out to in the primary flying texts. But, no doubt it does some, and whatever spiraling it does will tend to roll the airplane as it impinges on the wings, fuselage and empennage. But it tends to roll the airplane *opposite* the reactive torque from the engine. So, it tends to reduce the already, "not very large" rolling moment from the engine torque.
So that leaves us with P factor, or asymmetrical propeller disc loading, which in a low airspeed, high power climb, is the predominant force and acts only in Yaw (or very nearly so)
so to summarize:
Precession: Absent in a constant climb
Engine torque: Produces relatively small rolling moment in a typical GA aircraft.
Spiraling slipstream: Serves to counteract rolling moment from engine torque.
P-factor: No significant rolling moment.
Now, I fly single engine GA aircraft (although regrettably not as much as I'd like to) and like the others you've talked to, I'm not aware of having to hold significant aileron input on a climb, but I am aware of the need to hold rudder. No doubt I do, subconsciously, hold a very small amount of aileron.
Now, I don't fly single engine flight simulators, but if the required aileron input in a *real* airplane is well below the consciously discernible level (and it is) , and the required aileron input in the simulator is well above it, (like you say) then it would seem to me that the simulator is not programmed to be an accurate representation of the real thing.
I'm not sure how you'd conclude anything else.
Last edited by A Squared; 3rd Mar 2014 at 08:27.
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A Squared,
thank you so much for having so clearly explained your oppinion on this subject.
I agree with every sentence you have written, and indeed I also use the same arguments next to those who keep finding that the excessive roll due to torque is correctly modeled on some flight simulators.
The spiraling slipstream hitting the vertical fin / rudder, which is usually above the CoG of the aircraft, as well as the down sides of left wing and left side of the horizontal stab also accounts for a moment that is opposed to that of reactive torque. Add to it the mild force that is actually produced by the engine/prop torque on your C172 example, and we have excellent reasons to understand why roll is far from being the main effect...
All summed up together will certainly account for very mild roll effects as opposed to yaw - which indeed should / is the prevalent effect.
thank you so much for having so clearly explained your oppinion on this subject.
I agree with every sentence you have written, and indeed I also use the same arguments next to those who keep finding that the excessive roll due to torque is correctly modeled on some flight simulators.
The spiraling slipstream hitting the vertical fin / rudder, which is usually above the CoG of the aircraft, as well as the down sides of left wing and left side of the horizontal stab also accounts for a moment that is opposed to that of reactive torque. Add to it the mild force that is actually produced by the engine/prop torque on your C172 example, and we have excellent reasons to understand why roll is far from being the main effect...
All summed up together will certainly account for very mild roll effects as opposed to yaw - which indeed should / is the prevalent effect.
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Hi, the problem is that all these unbalancing forces can be cancelled out by trim tabs and/or uniform loading (e.g. having two people in a twin seater.) However the balance only occurs for one set of conditions... Alter anything and the plane will no longer fly hands-off. The two examples I can quote are as follows...
During my QXC in a Tomahawk I was obviously flying solo and the weight of just me in the LH seat made the plane pull to the left quite a lot. After the first landing I moved my heavy pilots case from the back to the RH seat, and that made some improvement.
The second example is a C172, this is rigged to climb at full power without hardly any rudder requirement when two up. However in the cruise at the lower power setting, the rudder trim needs setting two notches to the left to centralise the ball... This in turn requires a slight amount of right aileron to stop the resulting turn.
One further factor is that fuel can migrate from one tank to the other if the fuel selector is set at 'Both'. If the wings are not level, the condition can get progressively worse, as fuel transfers to the lower tank, and it also does not help if you park with one wheel in a hollow. Also some aircraft without a 'Both Selector' only use fuel from just one tank at a time.
During my QXC in a Tomahawk I was obviously flying solo and the weight of just me in the LH seat made the plane pull to the left quite a lot. After the first landing I moved my heavy pilots case from the back to the RH seat, and that made some improvement.
The second example is a C172, this is rigged to climb at full power without hardly any rudder requirement when two up. However in the cruise at the lower power setting, the rudder trim needs setting two notches to the left to centralise the ball... This in turn requires a slight amount of right aileron to stop the resulting turn.
One further factor is that fuel can migrate from one tank to the other if the fuel selector is set at 'Both'. If the wings are not level, the condition can get progressively worse, as fuel transfers to the lower tank, and it also does not help if you park with one wheel in a hollow. Also some aircraft without a 'Both Selector' only use fuel from just one tank at a time.
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Yes, there would be a yaw effect, I neglected to mention that as I was focusing on what did or did not create a rolling effect. But you're right that the spiralling slipstream would create both a roll (counter to engine torque) and a yaw. Whether it can be called *significant* or not, I don't know. I believe that it is substantially less that the P-factor yaw (but in the same direction.)
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the spiralling slipstream is largely a nonsense.
what happens is that the air flows straight back over the fuselage.
as the propellor blade scythes through the air there is a small sheet of vortices coming off the trailing edge. these are quite thin. these then move back in the slipstream and appear to spiral down the fuselage.
you can prove this for yourself with a piece of tape in the oil streaks on the underside of the fuselage. if you accidentally get it right you get a 2 per rev flap flap flap sound as the vortex sheet passes which is totally dependent on rpm.
look in the oil streaks after the flight and it will be apparent what is occurring.
a squared has given a pretty good accounting btw.
what happens is that the air flows straight back over the fuselage.
as the propellor blade scythes through the air there is a small sheet of vortices coming off the trailing edge. these are quite thin. these then move back in the slipstream and appear to spiral down the fuselage.
you can prove this for yourself with a piece of tape in the oil streaks on the underside of the fuselage. if you accidentally get it right you get a 2 per rev flap flap flap sound as the vortex sheet passes which is totally dependent on rpm.
look in the oil streaks after the flight and it will be apparent what is occurring.
a squared has given a pretty good accounting btw.
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Thanks. In an earlier post I said I was skeptical that there was much spiraling going on. My reason for questioning that like you describe. I used to fly the DC-6. The engine nacelles, especially the longer inboard ones were a fairly regular cylindrical shape until they met the wing. There was always a plentiful supply of engine oil running everywhere, and there were plenty of streamline streaks of dirty oil on the nacelles which pretty clearly indicated where the air was flowing (you could see how it flowed around the intake and oil cooler scoops, for example) but one thing that you couldn't see (and I studied it pretty closely) was *any* indication that the air flowing down the nacelle was spiraling. That's where I started to develop my skepticism about the "spiraling slipstream" theory.
Doubleyew eight: regarding your post, I have been flying a tail wheel single off rather muddy private strips and it is very obvious that more mud splats appear on the port side of the fin and tail plane. Why would that occur with symmetrical flow from the prop?
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@dubbleyew eight:
interesting your thoughts regarding the true importance of the slisptream itself, but then a question rises in my mind... Do you think that the force exerted by that spiraling flow, and the way it hits some aircraft surfaces asymmetrically, like the low section of the left wing, the tail fin and rudder, etc... should not be considered as opposing the rolling moment due to torque only ( this because there will always be yaw-induced roll if the pilot does not take action to center the ball and eliminate the sideslip... ) ?
Since I thought that that slipstream hitting the lower section of the left wing and also the vertical fin, above the CoG, would account for a right rolling moment, and thus oppose the torque-based left rolling moment, that would "kill" my theory :-/
interesting your thoughts regarding the true importance of the slisptream itself, but then a question rises in my mind... Do you think that the force exerted by that spiraling flow, and the way it hits some aircraft surfaces asymmetrically, like the low section of the left wing, the tail fin and rudder, etc... should not be considered as opposing the rolling moment due to torque only ( this because there will always be yaw-induced roll if the pilot does not take action to center the ball and eliminate the sideslip... ) ?
Since I thought that that slipstream hitting the lower section of the left wing and also the vertical fin, above the CoG, would account for a right rolling moment, and thus oppose the torque-based left rolling moment, that would "kill" my theory :-/
Last edited by jcomm; 3rd Mar 2014 at 13:44.
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Importance of spiralling slipstream
Here is a relevant point-- take an engine with clockclockwise-rotating prop (as viewed from rear) and mount it high up at the mid-fuselage like this http://blog.aopa.org/letsgoflying/wp...New-Talon1.jpg, and what happens? Now you need to hold LEFT rudder, not right rudder, in a full-power climb at high angle-of-attack / low airspeed. Even at cruise power/ cruise airspeed, the aircraft will tend to need some left rudder, if there is no compensating trim tab.
Why? Surely because with the high engine/ prop location, the fin is primarily feeling the BOTTOM half of the spiralling slipstream, not the top half. So the aircraft tends to yaw right, not left.
Many, many ultralights and lightsport aircraft share this configuration and they all experience this effect. I have some firsthand experience with such aircraft. Take a look at the trim tabs on the rudders of such aircraft-- they are always set to cause the rudder to deflect to the left to help compensate for this effect. The OPPOSITE of what we typically see when we look at the trim tab on the rudder of a more "conventional" GA plane, with the SAME direction of prop rotation.
You CAN'T explain this with P-factor, engine torque, etc. It's got to be due to the yaw effect of the spiralling slipstream striking the vertical fin.
You can't tell me that the spiralling slipstream is not important! It certainly affects the balance of YAW torques. And through slip/ dihedral coupling, this has an influence on roll.
The balance of ROLL torques is another story. In the case of a clockwise-rotating prop, as the prop spins the air clockwise, the aircraft will tend to roll counterclockwise (left). Any ROLL torque directly created by the impact of the spiralling slipstream against the wings, fuselage, etc will take some of the "spin" out of the propwash, which will REDUCE BUT NOT ELIMINATE the net left roll torque created by the prop. It's an issue of conservation of rotational momentum. It is well explained here: section 9.5: *9**Roll-Wise Torque Budget
PS near the end of section 8.5.2 of the same source, we read that the yaw effect of P-factor is SMALL compared to the yaw effect from the spiralling slipstream striking the vertical fin:
*8**Yaw-Wise Torque Budget
We can deduce the same by noting that even with a clockwise-rotating prop, these aircraft with high-mounted engines tend to yaw right at high power, not left. P-factor would induce left yaw, just as with any more "conventional" aircraft with the same direction of spin of the prop.
We do all understand slip-roll coupling due to dihedral, right? If the ball is significantly off-center (say to the left), that generally means the aircraft is flying a bit sideways through the air (in this case a "yaw string" would blow toward the right). That sideways airflow interacts with dihedral to make a "downwind" roll torque (toward the right in this case.) To look at any roll effects from the prop that are NOT due to yaw/ sideslip, you'll need to first start by applying whatever rudder pressure is needed to center the ball, or more precisely, to achieve zero sideslip. (Yes there is a difference between these two things, as becomes apparent when we are dealing with very large rudder deflections-- eg single-engine flight in a twin-engined aircraft-- but that's a rather fine point in the context of a single-engine aircraft. Zero sideslip is achieved with slightly less rudder deflection than would be needed to fully center the ball.)
Re the original question-- let's say we have a prop effect-- which again I contend is mainly due to the spiralling slipstream-- that makes a left yaw torque. The nose will yaw some degrees to the left of the flight path, displacing the ball to the right and then the net yaw torque will be zero. Now the aircraft will fly along at a constant yaw/slip angle-- the ball is deflected to the right. The sideways flow over the aircraft will have a SMALL tendency to drive an unbanked left turn. But the sideways flow over the aircraft will also interact with dihedral to make the aircraft roll left. As the bank angle increases, this will create a much stronger left turn than would ever be created by the yawing/slipping condition when the wings were level. Sure, if you try to fix the problem by holding right aileron, the adverse yaw from the deflected ailerons will shift the ball further to the right. You can fly in a straight line this way, but not quite wings-level, and it's certainly not efficient-- the wind/ airflow is slamming into the right side of the fuselage. If you had fixed the problem by applying right rudder to center the ball, you would never have needed to apply the right aileron input. As a glider pilot you understand the yaw string. The ball is essentially the same, it just moves the opposite way. Center the ball with the rudder before you figure out what aileron input is needed. You might be pleasantly surprised. You are interpretting the fact that you are holding lots of aileron, as an indication that the prop is mainly making the aircraft roll, not yaw/slip . That's not a valid conclusion. The prop is making the aircraft yaw/slip , and because you aren't fixing that with the rudder, dihedral is then making the aircraft roll into a banked turn, unless you hold some aileron input.
PS Sorry-- I just re-read your question-- you are stating that on these sims, holding rudder to center the ball does not leave the aircraft balanced in roll, and holding rudder to stop the aircraft from turning does not leave the ball centered. It sounds like the sims are modelling a strong left roll torque from power, even with the ball centered or nearly so. In theory there ought to be SOME left roll torque from power, that should need some left aileron to counteract (w/ ball centered), but it sure sounds like these sims may be over-doing it. But maybe that is indeed realistic for prop-driven aircraft with very powerful engines-- that's outside my area of experience.
PPS re oil streaks on the fuse etc-- it really doesn't take that much spiral in the slipstream to have a strong effect on the vertical fin. A few degrees of change of angle-of-attack of the fin will surely have a significant yaw effect.
Why? Surely because with the high engine/ prop location, the fin is primarily feeling the BOTTOM half of the spiralling slipstream, not the top half. So the aircraft tends to yaw right, not left.
Many, many ultralights and lightsport aircraft share this configuration and they all experience this effect. I have some firsthand experience with such aircraft. Take a look at the trim tabs on the rudders of such aircraft-- they are always set to cause the rudder to deflect to the left to help compensate for this effect. The OPPOSITE of what we typically see when we look at the trim tab on the rudder of a more "conventional" GA plane, with the SAME direction of prop rotation.
You CAN'T explain this with P-factor, engine torque, etc. It's got to be due to the yaw effect of the spiralling slipstream striking the vertical fin.
You can't tell me that the spiralling slipstream is not important! It certainly affects the balance of YAW torques. And through slip/ dihedral coupling, this has an influence on roll.
The balance of ROLL torques is another story. In the case of a clockwise-rotating prop, as the prop spins the air clockwise, the aircraft will tend to roll counterclockwise (left). Any ROLL torque directly created by the impact of the spiralling slipstream against the wings, fuselage, etc will take some of the "spin" out of the propwash, which will REDUCE BUT NOT ELIMINATE the net left roll torque created by the prop. It's an issue of conservation of rotational momentum. It is well explained here: section 9.5: *9**Roll-Wise Torque Budget
PS near the end of section 8.5.2 of the same source, we read that the yaw effect of P-factor is SMALL compared to the yaw effect from the spiralling slipstream striking the vertical fin:
*8**Yaw-Wise Torque Budget
We can deduce the same by noting that even with a clockwise-rotating prop, these aircraft with high-mounted engines tend to yaw right at high power, not left. P-factor would induce left yaw, just as with any more "conventional" aircraft with the same direction of spin of the prop.
We do all understand slip-roll coupling due to dihedral, right? If the ball is significantly off-center (say to the left), that generally means the aircraft is flying a bit sideways through the air (in this case a "yaw string" would blow toward the right). That sideways airflow interacts with dihedral to make a "downwind" roll torque (toward the right in this case.) To look at any roll effects from the prop that are NOT due to yaw/ sideslip, you'll need to first start by applying whatever rudder pressure is needed to center the ball, or more precisely, to achieve zero sideslip. (Yes there is a difference between these two things, as becomes apparent when we are dealing with very large rudder deflections-- eg single-engine flight in a twin-engined aircraft-- but that's a rather fine point in the context of a single-engine aircraft. Zero sideslip is achieved with slightly less rudder deflection than would be needed to fully center the ball.)
Re the original question-- let's say we have a prop effect-- which again I contend is mainly due to the spiralling slipstream-- that makes a left yaw torque. The nose will yaw some degrees to the left of the flight path, displacing the ball to the right and then the net yaw torque will be zero. Now the aircraft will fly along at a constant yaw/slip angle-- the ball is deflected to the right. The sideways flow over the aircraft will have a SMALL tendency to drive an unbanked left turn. But the sideways flow over the aircraft will also interact with dihedral to make the aircraft roll left. As the bank angle increases, this will create a much stronger left turn than would ever be created by the yawing/slipping condition when the wings were level. Sure, if you try to fix the problem by holding right aileron, the adverse yaw from the deflected ailerons will shift the ball further to the right. You can fly in a straight line this way, but not quite wings-level, and it's certainly not efficient-- the wind/ airflow is slamming into the right side of the fuselage. If you had fixed the problem by applying right rudder to center the ball, you would never have needed to apply the right aileron input. As a glider pilot you understand the yaw string. The ball is essentially the same, it just moves the opposite way. Center the ball with the rudder before you figure out what aileron input is needed. You might be pleasantly surprised. You are interpretting the fact that you are holding lots of aileron, as an indication that the prop is mainly making the aircraft roll, not yaw/slip . That's not a valid conclusion. The prop is making the aircraft yaw/slip , and because you aren't fixing that with the rudder, dihedral is then making the aircraft roll into a banked turn, unless you hold some aileron input.
PS Sorry-- I just re-read your question-- you are stating that on these sims, holding rudder to center the ball does not leave the aircraft balanced in roll, and holding rudder to stop the aircraft from turning does not leave the ball centered. It sounds like the sims are modelling a strong left roll torque from power, even with the ball centered or nearly so. In theory there ought to be SOME left roll torque from power, that should need some left aileron to counteract (w/ ball centered), but it sure sounds like these sims may be over-doing it. But maybe that is indeed realistic for prop-driven aircraft with very powerful engines-- that's outside my area of experience.
PPS re oil streaks on the fuse etc-- it really doesn't take that much spiral in the slipstream to have a strong effect on the vertical fin. A few degrees of change of angle-of-attack of the fin will surely have a significant yaw effect.
Last edited by flyer101flyer; 5th Mar 2014 at 17:52.
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Flyer101Flyer,
excellent explanation to add to the above contributions!
It's all been very important to try to organize my ideas on this tricky ( in the world of flight simulation... ) subject!
Thank you.
excellent explanation to add to the above contributions!
It's all been very important to try to organize my ideas on this tricky ( in the world of flight simulation... ) subject!
Thank you.
Last edited by jcomm; 6th Mar 2014 at 20:05.
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phiggsbroadband Does anyone know what angles are used to set the engine crankshaft in shall we say a Cessna 172. What Side and Down Thrust is engineered into the mounting cradle?
The closest I've got is this image from the manual where I added "sight" lines to guesstimate the angle of the prop.
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Hi chksix, just went for a flight in the C172 today, and took a plum-bob.
It showed that the lower tip of the prop is a good 4 inches in advance of the top blade, making an angle of about +4 degrees wrt the vertical with the nose-wheel suspension at its usual 3 inch extension.
I could not detect any significant side thrust, as the canopy is not too solid, and my measuring string was a bit elasticy.
It showed that the lower tip of the prop is a good 4 inches in advance of the top blade, making an angle of about +4 degrees wrt the vertical with the nose-wheel suspension at its usual 3 inch extension.
I could not detect any significant side thrust, as the canopy is not too solid, and my measuring string was a bit elasticy.
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Originally Posted by From the article
But inverted flight required left rudder rather than right. The overriding force, therefore, was P-factor; slipstream rotation had no noticeable influence... Crow found the same in his Extra: right rudder upright, left rudder inverted and no apparent contribution from the slipstream.
One factor that no one here has mentioned (and Pete Garrison mentions only in passing in that Flying article) is that - if there is torque roll imparted to the aircraft while on the ground, counter to the (clockwise) prop rotation, it will tend to push the left tire down harder and increase tire drag, adding to whatever left-yaw tendency exists from other sources (p-factor, etc).
After take-off, of course, that extra yaw impulse disappears.
And, yes, I'd say desktop sims tend to overcook the apparent effects (along with other things, such as X-winds). They only have one dimension in which to replicate and "teach" the complexities of flight control (no 3D environment, no kinesthetic "seat of the pants" motion), so they tend to overdo what the computer can show, in order to make the sim more challenging.
After take-off, of course, that extra yaw impulse disappears.
And, yes, I'd say desktop sims tend to overcook the apparent effects (along with other things, such as X-winds). They only have one dimension in which to replicate and "teach" the complexities of flight control (no 3D environment, no kinesthetic "seat of the pants" motion), so they tend to overdo what the computer can show, in order to make the sim more challenging.