The "p" factor

[Gemma Jane]

Old Sarum is on my list of places to go David so for sure I'll drop by for a coffee. Many years ago I was on holiday in the area and spent an afternoon watching people doing circuits there thinking 'one day'. One day came evenutally.

 

For a long time I've had a bit of a mad idea to buy a Chippie and perhaps following through to be able to instruct on it  one day, so perhaps we should have a good natter!

 

I did think though that real flying was done on 2.4Ghz??

[sparks59]

Posted by Chris Bott on 26/07/2010 16:24:59:

2 things:-

 

...

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"  

 

I couldn't agree more Chris, whilst it is interesting to debate these things, and maybe learn something or get a few conflicting views, the bottom line is, use your experience, or get advice from others and do what needs to be done to get it flying how you want it to fly.

 

Sorry Gemma, haven't yet had chance to research any better video of the helix effect, single or double, i do like the party popper experiment though. Just set something up and see what you get, a man after my own heart!

sparks

 

[Gemma Jane]

I liked the experiment too, but I know what I'm seeing and it doesn't change my view on helical prop wash!

 

I was considering two flat plates on either end of rod, sliding through a tube through the fin. It should move to one side.. been too busy today though with the Marauder build to try it. 

[001]

I have no wish to be argumentative or pedantic, but I just do not understand this helical propwash thing. I also respect the opinions of the experts on the subject even if I do not entirely agree with all of them.

I should say that I have not discussed, posted or referred to any other forum, model or full size on the subject.

I look forward to seeing some figures, measurements or photos that illustrate the sideways air pressure on the fin.

In the meantime, enjoy the photo.

[Former Member]

[This posting has been removed]

[Gemma Jane]

Hi Richard I was not suggesting it was your posts on other forums, rather how this thread began, with the statement that 'P-Factor' is the cause of left yaw rather than helical flow. This myth appears to have been started on the internet by an individual who perhaps enjoys the attention, or the arguments it causes. I caught up with this after debunking the idea in this thread with my first post.

 

The picture shows exactly what you would expect to see, just as your experiment did that the net airflow is backwards. It would be, hence why planes fly forwards (mostly). Martin's suggested experiment would show exactly the same thing, the threads will just blow backwards in the slipstream and bob about a bit in the turbulent flow leaving the fin.

 

What you need to measure is whether or not there is a differential tangential force on the fin.

 

I decided to take a look at this with a model also. Anyone can try it who has an RC model.

 

Test 1) I placed my hand flat with the my palm out on the left of the fin, then the right. The flow was tangibly greater on the left side with a clockwise spinning prop.

 

Test 2) Here I was going to devise the plates as I've suggested. What we want to know is,  is there a differential tangential force on the tailplane, not the obvious that the net flow is backwards! I didn't bother though, because with the test model I had, a PZ BF109, the entire rear fuselage visibly flexed to the right under the tangential force of the prop wash! Hence why it has so much right side thrust built in.

 

The trouble with the argument is Richard, from my point of view, is why would anyone think there isn't a helical flow? Why canter engines then? Who made it up? Why on earth didi I study aeronautics for 8 years to argue this stuff on forums

 

One of the things that continues this 'debate' is that apparently helical flow isn't quantified in text books.

 

Two points:

 

a) well actually it is if  you know which ones to look in

 

b) engineers are capable of their own analysis when required, it doesn't have to be in a text book to be a fact

 

c) In the days when props ruled the world, we still built prototype aeroplanes. If they pulled to one side in level flight, we altered the engine mounts or fin until they didn't or provided a rudder trim tab.

 

The fact that engine canter and offset fins can be observed on both full-size and models might suggest that something is causing them to yaw in the air. As it certainly isn't P-Factor it only leaves one answer, so your view ends with the need to put forward that answer or accept what is established fact, even, and I would agree, if it isn't always the easiest thing to visualise.

 

Having said that, I discussed with my old PPL instructor the issues today, his response, just fly the aeroplane 

[Gemma Jane]

Eric, your picture or one like it is on Airliners.net, it clearly shows all four prop tip vortices being sucked down to the upper surface of the wing and apparently disappearing.

 

I know though that the Herc doesn't have canted engines, so it serves as more evidence that those that do are experiencing the helical prop wash hitting the vertical tail.

 

Just stand by a herc then a light plane or single engined model... it's pretty obviously not the same. If you put a single huge prop on the nose of the herc though....

[Gemma Jane]

double post

Edited By Gemma Jane on 27/07/2010 19:54:12

[001]

Gemma, thank you for spending so much time on replies.

 

I do not think that there is any spiral flow around the fuselage. I do not believe that there is a cylinder or tube of rotating air around the fuselage.

 

I do not think that there is a mysterious force that by-passes the smoke seen at hundreds of air displays.

I do not like mysteries.

 

I have been to many airshows where smoke is being used and whilst the aircraft is flying straight, with wings level,  there has been no evidence of spiral flow around the fuselage.

 

In my opinion, what might be happening is that whilst on the ground, the wash from the propeller is being reflected from the surface below the aircraft, causing a force on the fin and rudder which has to be compensated.

 

There are also cases where errors in standard textbooks have been repeated over and over again by later authors.

 

Why should a propeller be any different from an office fan?

Why are aircraft generally not fitted with devices behind the propeller to straighten out the airflow, if it is such a problem?

But, then, I am not young enough to know everything.

 

Also, I think that Occam's razor applies.

I am leaving it at that.

 

 

 

 

 

 

 

 

 

[Dusty]

@ Eric, something for fun!

 

[Chris Bott - Moderator]

I recon a lot of it has to do with torque. The Prop goes one way so there must be a reaction in the other direction so we can keep pushing it round.

 

I observed an interesting take on this when there was snow on the ground. Flying a ski equipped model, the left ski would dig much further into the snow than the right when under power.

 

This photo doesn't show it very well, but I think it does show it.

Since this session, I've been able to cope much better with models that roll left just after takeoff. That's because the left wheel is doing most of the support as they come off the ground. 

 

[Myron Beaumont]

Richard

Well put ! I didn't know exactly what to say on this subject -But you've more or less said it all .All I know is that a few years ago I was involved in the build of a small biplane full-size (human in it ) and there were about 5 degrees fin/rudder offset  built in to the design . Can't remember the name of it ,but we built it in Pendeen / west cornwall on the small industrial estate there .

PS  Those were the days 

Edited By Myron Beaumont on 27/07/2010 21:33:32

[Gemma Jane]

I think perhaps Richard what really applies is you should have some grasp of a subject before attempting to form opinions.

 

I don't believe the considerable amount of time I've spent in trying to explain very simple concepts in response to your questions has been worth it all.

 

 

On one of the other threads where this subject came up an American described the argument as like 'wrestling with a hog in mud'

 

It's very hard work and sooner or later one realises the hog is enjoying the battle.

 

You don't really think someone who studied aeronautic for 8 years gets their information from a few textbooks? No doubt you believe the rubbish that helical flow first appeared in Stick & Rudder and was then passed along? Total nonsense.

 

I could carry out an analysis of a single particle hitting the fin from the start of it's journey... I could look at the entire flow field modeled on a computer. I could post 3 equations that would save me typing so much...Not much use to you though as no doubt you won't 'believe' or understand them.

 

You can't figure the office fan? Not much hope really.  I think I'll leave it at that.

 

[Myron Beaumont]

Gemma

Wouldn.t a simple vector diagram show actions and reactions , I'm in admiration of your knowledge/know how and experience, If only we could ask birds how they fly ?

Your friend / engineer

Myron 

[Tim Mackey]

Please dont start getting rude about this subject people - Richard did after all stress that he was not looking to be argumentative or pedantic - merely not understanding something does not warrant insulting remarks.

I had a horrible feeling that this overly complex subject would end in tears, and I fear I may be right.

Thread will be closed if friendliness and patience is not restored.  Please be nice

[001]

Well, I didn't make a personal attack on you Gemma. I wasn't arguing but expressing an honest opinion based on what I have observed. No contempt needed.

The trail of smoke in the Harvard picture goes nowhere near the fin. Why not?

According to the theory, it should not be straight. I do understand that much. A simple question. With no irony or sarcasm intended.

 

And, 50 years ago I did study Mathematics, Geometry, Calculus and Algebra (as it was called then) to a pretty high standard.

 

[Gemma Jane]

What theory says it should not be straight Richard? There is absolutely no conflict behind helical flow being produced by the prop and the smoke trail.

 

Why does the smoke trail go nowhere near the fin in the Harvard... because the Harvard is flying away from the smoke trail as it's formed, probably at around 250 KTS

 

Imagine an old steam ship going across the ocean, the smoke trail will look like it is trailing behind it as the ship is moving away from it all the time. All it tells us is the ship is moving, in the same way as the smoke trail from the Harvard simply tells us the aircraft is moving through the air mass.

 

I get  your argument but it's a bit like saying the smoke should be wrapped around the wing vortices.. I can't see any effect from them in the smoke stream either but I know they are there  

 

 

[Gemma Jane]

Richard,

 

I would like to openly apologise for my tone in previous posts.

 

Unfortunately it can be too easy for me to get too involved in the 'argument' and forget it is another person that I'm addressing.

 

I don't like it when it happens to me on forums, so I've no excuse for doing it myself.

 

As is often the way the heat of the moment can lead to a press of the 'Add Posting' button when one should really have perhaps considered if the post was appropriate. Certainly reading back through a previous post I clearly went well over the mark.

 

Again I'm sorry for that, trust me the subject of aerodynamics has caused me untold grief on forums in the past. Perhaps I should consider it like religion and politics and just let people discuss their opinions on the subject whilst keeping mine to myself. 

 

 

  

[001]

Accepted.

[David Ashby - Moderator]

Hey, there are two subjects people will never agree on  - aerodynamics and electronics....oh and economics too now I come to think.  

Edited By David Ashby - RCME Administrator on 29/07/2010 08:09:10

[Martin Harris - Moderator]

I'm feeling a bit guilty as the first person to mention the "helix" word in this thread!

 

I must say that this thread has got me thinking in my limited fashion as the helix effect is something that I've just blindly accepted. 

 

Gemma, would it be a closer description to state that the airflow is slightly deflected in a lazy helix from the aircraft's centre line, rather than flowing round in a tight twist as depicted in most diagrams I've seen, and gives the fin a small lateral angle of attack in relation to the direction the aircraft is travelling?

 

This would fit in with Richard's pictures and explain the apparent lack of disturbance to the smoke trail from the Harvard - I suspect that a view from above would reveal a very small offset of the trail to starboard?

Edited By Martin Harris on 29/07/2010 10:45:07

[Martin Harris - Moderator]

Scrub the last remark - having looked at Richard's picture again the smoke is all below the tailplane so possibly not influenced greatly by the flow over the fin?  Just to complicate things again, note the smoke trail deflection on Dusty's second picture but there would be all sorts of complicated effects beyond my comprehension going on here due to ground (water?) effect etc.

[Simon Chaddock]

Wow!

I feel guilty for even starting this thread!

[Gemma Jane]

You shouldn't feel guilty Simon,  forums, good for having a chat, good for getting some quick advice and sharing experiences, good for showing your latest build, good for getting some help when considering purchasing, good for making contacts..

 

..very bad for discussing the finer points of 3 dimensional turbulent flow fields!

 

Aerodynamics is a subject best understood by the maths, it's not perfect maths but it beats not having anything to go on as the Wright brothers found. Equations also take up a lot less space and convey far more information than hundreds of words trying to explain topics which can very easily get very complicated.

 

Unfortunately the whole P-Factor vs helical flow thing was started by a pilot. People naturally assume pilots understand aerodynamics. Afraid not, aerodynamicists understand aerodynamics, well most of it any way. Not sure airline pilots get much time for studying the finer points of fluid dynamics when at 30,000', but I've certainly heard some pretty big misconceptions from professional pilots regarding the subject. That's not to say they are any lesser a good pilot though nor that some pilots didn't do their aeronautics degree and have a much better grasp of the topics. The fact is you don't need to understand it to fly models or full size. Though for me understanding how things fly has always been just another pleasure I get from aeroplanes.

 

Attempting to put these subjects into words, very long posts, very bad tempers and very little of the feel of the subject actually gets expressed. I suspect most people switch off by the second line of most of the posts and who can blame them!

 

It might just be with this topic the 'sticking' point is the pictures illustrating helical flow suggest  a laminar flow field from the prop. A prop blade may be seen as a rotating wing, the flow is turbulent, smoke can't follow the flow field or reveal the momentum interaction between the particles.. it will just look like smoke! It's a shame we can't use smoke to reveal the turbulent flow fields around a plane, if we could half the subject would be done and dusted and my fingers wouldn't get so tired of typing.

 

Smoke is only any good for studying perfectly laminar flow guys, so I hope you keep on discussing this topic, but I don't think a picture of an aircraft trailing smoke is going to help much.

 

What you need to start looking into is the Navier-Stokes equations and evaluation of momentum in 3 dimensional flow fields.. the answers to this one are there. 

 

 

 

 

 

 

[Myron Beaumont]

Hmmm ? 

Just thinking about what Martin said. ie The fin giving a small lateral angle of attack . Maybe that is why there are no birds that have a vertical tail surface ? (since they don't use a helical form of propulsion ) . Food for thought  Eh ?