The "p" factor

[Simon Chaddock]

On another site there has been quite a discussion on why side thrust is used.

 

Like many I originally thought it was to combat the effect of engine torque or perhaps the effect of the swirling prop wash over the airframe.

 

Others more qualified than me explained it was due to the p factor which is the variation between the left and right hand side of the prop disc, making the plane yaw, which caused by the airflow not being perpendicular to the axis of the prop. 

 

The conclusion is that if the airflow is exactly perpendicular there is no p factor, the thrust is exactly down the centre line of the prop and thus no tendency to yaw but the more the airflow angle (slowing down nose high?) the more the tendency to yaw.

 

It all started when the owner of a Parkzone Radian pointed out that it was badly put together as the motor was off set to the right.

[Simon Chaddock]

At last got my scanner working so here is the diagram.

As the prop axis is inclined to the airflow the up going blade see a reduced angle of incidence and thus less thrust with the opposite effect for the down going one, hence a yawing force.

Well that's the "P" factor theory!

[Bob Cotsford]

so why don't you need right sidethrust if your model needs lots of downthrust?  Sorry, i still think it's the spiral airflow hitting the fin.

 

 

[Martin Harris - Moderator]

I thought P factor was mainly effective during the tail down phase of take-off and just one of the factors leading to swing (others being such factors as helix airflow, torque reaction increasing tyre contact pressure and gyroscopic precession as the pitch angle changes. As far as I'm aware the helix airflow is the most significant reason for right thrust or rudder trim compensation in normal flight.

[Delta Whiskey]

Just google it, there a fair bit on it......

[Gemma Jane]

Another vote for the actual reason for building in side thrust being the helical airflow hitting the fin and rudder.

 

P-Factor is transient as Martin has said. As is gyroscopic procession. We wouldn't build in side thrust to combat them as they are not effects that are constantly experienced. They are effects which we compensate for with the controls, if with a model they are even noticeable.

 

Reaction torque.. well I bet we have all seen it, open the throttle too fast and the model flips on it's back as it takes off the ground (DP take note... it's not caused by using the ailerons!) The cure is then of course... don't open the throttle too fast as P-51D pilots knew very well, so lets leave that out of the discussion.

 

One of the interesting things is that by building in some side or down thrust there is a P-Factor being introduced. I'm not about to sit and do the vector analysis for a typical model (too lazy) but I bet it's pretty much negligible in straight and level flight and we would just trim it out on the transmitter without a second thought. When P-Factor becomes an issue  as Martin has said is when a taildragger lifts it's tail... oops the P-Factor disappears as the prop is no longer asymmetrically loaded so one suddenly finds they are overcompensating.... or perhaps not because right at the same time gyroscopic procession will start to have an effect... how much and how long for? Well it depends how fast the tail is lifted.

 

So Simon, those more qualified than yourself are in my opinion leading you down the wrong path. If you can pick up a copy of Harvey S. Ploude's 'The Compleat Taildragger Pilot' he not only explains all the effects in clear detail, he also carries out a mathematical analysis of P-Factor - you'll soon see it is only transient and that the side thrust is there to counteract helical flow... also in reality adding side thrust or down thrust is going to introduce a P-Factor in level flight - so clearly the reason for adding it is not to counteract P-Factor... as if we didn't add the offsets there would be no P-Factor when the model was in level flight....

 

Time for bed me thinks.... 

 

[sparks59]

arggh, thanks for the description Gemma Jane...but now my head hurts, I wish I had remained blissful in my semi-ignorant state of thinking, that its all about countering the effects of torque reaction from a rotating propeller engine.

 

sparks

[Gemma Jane]

Truth is sparks59 practically anyone in the real world would just consider all the effects as 'torque' and get on with it!

 

When I fly full size or models I'm no more worried about the 'net circulation' around the wing than I am P-factor. Most of these things are either of purely academic interest or worth having some grasp of when designing models or full size. When it comes to flying either, well that's an art not a science and the theory is best left where it belongs in the books!! So I would just keep to thinking what you did, I'm sure it's not done your flying any harm at all.

[001]

What I cannot get my head around is this theory of a spiral propwash going all the way down the fuselage to the fin!

 

 

As an example, look at the exhaust stains on the sides of a Douglas Skyraider, the effect is the same on both sides of the fuselage, the mark curves up, then down towards the wing. And that is clearly visible in front of the wing on both sides.

 On several WW2 multi engined bombers, the streaks of soot can be seen going horizontally back.

If there was this spiral, then surely the stains would curve in an entirely different way. (And the cockpit of nearly every single engined fighter would fill with exhaust fumes from one side when the hood was open.)

 

Then, you have the wing. -  Just how does this alleged swirling spiral of propwash get past the wing without straightening out

[Gemma Jane]

It really depends on the type exactly what the prop wash does Richard, but it's real. One of the other ways to counteract apart from offsetting the motor is to offset the fin, if it wasn't there why would that be done!

 

There is an old saying that states if we could see the airflow we wouldn't fly... I tend to believe it.  Oil stains are little to go on, the boundary layer has a totally different flow pattern to air just  millimetres or more away from the aircrafts skin.

 

They say a picture says a thousand words:

 

But at least this one does show how prop wash can bypass the wing and when it strikes the fin cause yaw, fluid dynamics... who needs it eh!

 

PS I filched this picture from a forum thread where the poster was happily describing the above as depicting 'P-Factor'.... the blind leading the blind!

Edited By Gemma Jane on 25/07/2010 19:17:02

[Former Member]

[This posting has been removed]

[Gemma Jane]

I'm pretty sure Eric that P-Factor doesn't come into rotor theory, heli blades are flappy to avoid it, not fixed like the props on most models and small planes. Could be wrong though, never studied rotor theory much but I think for a heli you would actually be looking at  dis-symmetry of lift... 

 

P-Factor is 'real' it is experienced by fixed wing  prop aircraft whenever the prop(s) is(are) inclined to the free stream velocity. However it isn't why motors are offset....... as you say that is because of the the helical flow.

 

We also should be careful about saying 'theory', to me this stuff is like trying to convince people the earth is round.... aerodynamic theory is a lot more involved, P-Factor and helical flow are just basic physics and known to any pilot and hardly rocket science, I gave up talking about aerodynamics theory on the internet a long time ago!

[Simon Chaddock]

Gemma

Nice picture but what happens in a single fin twin?

As the fin would be largely clear of the prop wash would there be any yaw effect?

Just a thought.

[Gemma Jane]

Hi Simon,

 

I can think of many twins where the engines are quite inboard and it would be possible for helical prop wash to hit the fin and rudder, but with a twin the other factors may now be taking a much bigger role too.One way to look at it all is to have a 'yaw budget' that's pretty much how it is all tackled with full size design. We know there are various factors that might cause yaw, some permanent, some transient and some due to design constraints, so we need to examine howto deal with it all in a design and what to leave to the pilot.

 

Denker tackles this concept here:

 

http://www.av8n.com/how/htm/yaw.html

 

It's probably a bit heavy for some, but at least as usual Denker tackles the subject without all the maths and some clear diagrams.

 

I did notice that he considers P-factor as 'extremely import in helicopters' and describes it as asymmetric disk loading. Can't agree it's the same thing, it's the rigid nature of the prop (or at a given setting for a wobbly prop) that allows the mathematical analysis of P-factor. I think what he means is it would be if the blades were fixed... which of course on a heli that works they are not! Don't know if there are any heli people out there who know whether or not P-Factor is discussed in the rotary world in that context or not??

 

Still there is plenty to read through on that link but some of the Denker stuff is regarded as pretty controversial at the best of times so it's best to consider the arguments given and see if anything is useful, rather than take all as fact.

 

He does though state: 

 

 

8.7  Canted Engine

 

Often the engine is mounted in such a way that direction of the thrust vector is a little to one side of the axis of the airplane. This is done in order to compensate for various nonidealities such as helical propwash. It contributes to the yaw-wise torque budget in the obvious way.

 Which is pretty much how I see it that the helical propwash is the main reason for side thrust.

 

Edited By Gemma Jane on 26/07/2010 01:15:08

Edited By Gemma Jane on 26/07/2010 01:18:28

[001]

Most of the experts say that the propeller produces a cylinder or tube of rotating slipstream, with a diameter equal to that of the propeller disc.

 

But.

 

 The diagram above shows a stream of air starting from one point on the disc, where it conveniently swoops around a smooth fuselage and clings to it without hitting an engine cowling, exhaust stub or canopy and not affecting the movement of exhaust gases or soot. Then hits only the fin!

 

I am not saying that the experts are wrong, but nobody provides any measurements of this force on the fin under varying circumstances, such as what angles and pressures are involved, nor discusses the effect that centrifugial force would have as this 'cylinder of air' whirls about.

 Does it resemble a truncated cone as it travels rearwards? What is its effect on the wing? .

[sparks59]

  

I am not saying that the experts are wrong, but nobody provides any measurements of this force on the fin under varying circumstances, such as what angles and pressures are involved, nor discusses the effect that centrifugial force would have as this 'cylinder of air' whirls about.

 Does it resemble a truncated cone as it travels rearwards? What is its effect on the wing? .

 Ok Gemma, over to you for some good wind tunnel footage ......

 

sparks

[Gemma Jane]

I don't think the stream illustrated is meant to be seen as clinging to the fuselage at all. Remove the fuz from the picture and it's still the same thing. A helical stream hitting the fin. 

There is also an opposite helical flow from the other prop tip not illustrated above... which misses the fin as it passes under the fuz not above it.

Hence why some argue that we should have as much fin under the fuz as above it.

 

The easiest way to visualise prop stream is a double helix.. like a DNA molecule.

 

For an analysis of the exact effects... well it's going to be type specific of course so unless you have access to an aircraft manufacturers calculations I think that's going to be a difficult one to discuss.

 

Also bear in mind the forces involved are not constant. At full throttle even with a engine offset the force on the fin is not entirely cancelled, it needs some rudder to keep straight as the aircraft will want to yaw. The designer is most likely to be looking at minimizing the effect for cruise settings.  

 

[Gemma Jane]

Help me out sparks! Was looking for a good picture to show it all before making another long post, found some good ones of boat screws but we need a prop doing it's thing!

[001]

As an experiment I put a very thin silvered plastic streamer (from a party popper), straightened out, on the front of my Acromaster just behind the prop. The first time it was held 25mm away from the surface facing straight into a slight breeze. The plane was held on the ground. RPM approx. 8700.

 

The tape was flapping about slightly and it did tend to curve around to the right  side of the canopy, but a slight change in windspeed/direction flicked it over to the left. (As viewed from above, looking forward. (Picture inserted.)

 

Then, for a second test I mounted the streamer exactly in line with the tip of the propeller and repeated the test. The streamer again had a slight curve to the right, but, in line with the fin, it was fluttering very rapidly and was curved away from the fin, halfway across the tailplane half..

 

At no point and at no propeller speed did the tape try to wrap itself around the fuselage, most of the time it was lying across the canopy.

No doubt some people will condemn the test as inconclusive and unrepresentative of full size, but I now know where the airflow is going on my Acromaster.

[001]

Since the last post I have repeated the test indoors, sans wind.

 

 With the streamer level with the tip of the prop at TDC, there is a slight curve of the streamer just in front of the canopy, then it goes down the centreline of the fuselage and thrashes around, in turbulence at times on the left of the fin and at other times on the right, because it (the streamer) jumps over the fin.

 

re the diagram, If you remove the fuz from the picture, you also remove the wing.

 

I can understand that a vortex comes off the tip of the propeller, but not a great rotating mass of air coming back. Look at an office fan with streamers on a wire guard, they go straight, a bit waggly, but they don't wrap themselves into a helix.

[Martin Harris - Moderator]

How about holding a streamer either side of the fin half an inch or so away from it - perhaps held on thin sticks in the ground - to give an indication of the flow against the fin?

 

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[David perry 1]

 

As an instructor I'm not getting involved in this debate but I will mention one thing: Critical engine.

 

Enjoying the arguments!

 

And Gemma, I'm tickled pink you are loving the t/w flying! Its the biz isnt it??

 

 

David 

[Chris Bott - Moderator]

2 things:-

 

I recently stood behind a Tiger Moth at tickover. I purposefully stood facing forward checking the prop wash. There was "tons" one side of the fin but virtually none the other. I have no idea now what the wind direction was but i can't remember there being much wind.

 

When I set up my models, I keep changing side and down thrust until the model does what I want. i.e. flies straight and true whatever the throttle setting. What i don't do is worry about the "why"  

[Gemma Jane]

It seems that an individual has posted on a number of threads, first on full size forums, where no end of aeronautical engineers put them straight on the topic, and then having failed there, on model forums, with the idea that 'P-factor' is the reason behind left yaw rather than helical flow... I'm joining the other aeronautical engineers and giving up lol.

 

Yep David, tailwheel addiction here, it's what I had missed from my flying all along, scrap the trikes from training and get everyone learning with tailwheel! Flying the Cub again tomorrow and Wednesday, think I might book a slot for Friday too if Steve has one free!

 

[David perry 1]

Posted by Gemma Jane on 26/07/2010 16:40:19:

It seems that an individual has posted on a number of threads,... I'm joining the other aeronautical engineers and giving up lol.

 

 

Hence my not joining the argument!

 

 

Yep David, tailwheel addiction here, scrap the trikes from training and get everyone learning with tailwheel! 

 

 

I agree.  I have long thought there's a market for expensive but very good training on interesting tailwheel aircraft.  I don't mean for the average Joe who want's a PPL at the lowest price, but for those with significant disposable income who want to say " I fly a harvard" or some such.  But at a fundamental level, the tailwheel makes people a better pilot. Unfortunately you can't escape the fact that trying to land one in a stream of A320s in BRS is a lot less friendly than ditto a trike. !

 

 

When you have got the conversion done have a trip down to Old sarum and I'll buy you a coffee...we can talk about real flying (I mean that done remotely with 35MHz!)

 

D