Gemma Jane

Hi John,    Porky II was a sort of test bed model, lots or experimenting with finishing foam, twin engines etc. It does still fly but looks pretty awful now! Truth of it is, the model is too heavy. I knew it was getting that way during the build and it just proved that heavy models crash more. It is fast though!   I'm interested in your build as I've also got a silver GWS 38 kit. I'll be following your example to make one that is just fun to fly.

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      

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.  

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

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....

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 

Beat me to it John! Remember Gemma... refresh the page before posting!

Spadeadam was associated with Blue Streak, I don't know of any connection with the TSR2.    This link might help   http://www.visitcumbria.com/car/spadeadam.htm 

This one Stephen?       I caught a programme on Sky+ yesterday about the end of Concorde, grown men literally weeping as they recounted how the fleet was lost... I couldn't help joining them as I watched.   I know XR222 was just an empty hull when it arrived at Duxford, it appears she survived because the College of Aeronautics at Cranfield requested a TSR2 airframe after the cancellation - write up here

I'm not sure I would recommend the 18T bell motors I put in my 'beast' GWS 38! They develop four times the power recommended lol.   It is fun, but the trouble is if it's a belly lander it's really easy to bend the motor spindles as they are not properly hardened on these motors. Not too bad to replace as spares are available but it's a bit annoying as spindle replacement is a workshop job.    For reference the RC groups build thread which has the piccies intact is at:   http://www.rcgroups.com/forums/showthread.php?t=1083990&highlight=gws+38    I can't help posting a piccy of Porky II at her best     I'm sure Tim will be along soon with a motor suggestion for you and I'll be looking forward to your build thread 

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. 

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??

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!  

I lost several LiPos after they were left in a car in the sun by mistake last year.   They swelled at the time and seemed to recover so I kept using them. Over the winter whilst stored though they all puffed and had to be disposed of.   Pretty expensive lesson that one. I think that is a case of previously abused LiPos getting very sick when in storage.  

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!

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.    

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