Wake turbulance

[Paul Allen]

Could some one explain what wake turbulance is,I know the affect it just happen to me
on the last corner of the triangle, when the plane pancaked in.
Thanks
Paul

[pmackenzie]

Any time a wing makes lift there will be a vortex trailing behind it.
More lift = stronger vortex, so in a corner the resulting vortex is large.
Vortices take a while to dissipate, so if you fly through the same spot soon enough you will hit the vortex.

If there is enough wind the vortex gets blown away before you get to it.
If the wind is light then taking a step back each time is usually enough to get the model out of the previous vortex.
If the wind shifts and it is in your face you need to step forward, which can feel very strange!

Lots of pictures of them here

https://www.google.ca/search?q=tip+vortex&biw=1728&bih=896&source=lnms&tbm=isch&sa=X&sqi=2&ved=0ahUKEwjuxdLX-LjNAhUBV1IKHdcKDr8Q_AUIBigB

[Dallas Hanna]

Could some one explain what wake turbulance is,I know the affect it just happen to me
on the last corner of the triangle, when the plane pancaked in.
Thanks
Paul

So it's been the same conditions in the Hunter as well Paul!  We had the same conditions at Whalan Reserve a couple of weeks back with thermal activity making it a coffee drinking morning instead.

HH

And a PS here to Pat Mackenzie!  Hope you and Peter had a good time in Perth for the W/Champs and a safe trip home.  Come again to OZ.  Nth Richmond is closer to you and Paul is only 2 hrs north of me!  Always welcome.

[Paul Allen]

Thanks Pat for the info.
Herb,I got caught out,never too many clam days in the Hunter
and no warning!
Paul

[Dan McEntee]

   It's worse for larger models with flaps. Your are flying along and deflect the controls and the flaps and elevators disturb all kinds of air since they are perpendicular to the direction of flight. Put you hand in a pool and move it back and forth at different angles of attack and see what it does to the water. Then add in prop wash from a large prop at high RPM and the air gets pretty busy!
   Type at you later,
   Dan McEntee

[Paul Allen]

Ty
    That's what happened , could not have described it better.
Paul
In OZ

[Steve Helmick]

FWIW, PW spoke about an experiment he did regarding "Wake Turbulence".  He said he tried a much smaller prop on his .40VF...I think he said a 9-4. NO wake turbulence, but also NO line tension above 45 degrees.

Hmmmm. Well, maybe there is a happy medium at around...11.3" diameter?  Steve

[Keith Renecle]

Could some one explain what wake turbulance is,I know the affect it just happen to me
on the last corner of the triangle, when the plane pancaked in.
Thanks
Paul

Hi Paul,
Why don't you just ask Loren Nell? After writing off two models in OZ, he should have some understanding of the effect!  The first time was during the Nats last year and I was with him at the world champs this time. It was during a practice session early in the morning before his first official flight. The air was more or less dead calm and he had done the inside loops with zero sign of wake turbulence. He got through the first outside loop no problem and at the bottom of outside loop #2, the plane wobbled and hit the deck. Two good flying Max Bee's later, he will be more wary of wake turbulence!

It's always difficult to avoid because in dead air sometimes there is a small amount of air movement so you get fooled into thinking that you don't need to walk backwards. You also get the instance where you can get the small breeze moving towards you exactly from where you need to perform the stunts in a competition. In this case walking backwards only makes it worse, and of course walking forwards will give less line tension, so what do you do?? In a competition you are pushing your luck and most likely you will hit the deck in these conditions if you are trying to get those low pull-outs done. It's then better to fly a little higher and hope that this is your throw-away round IMHO.

Keith R

Keith R

[Peter Germann]

Somewhere here, Paul Walker recently mentioned he found the force of (wake-?) turbulence generated significantly depending from prop diameter. It would definitely be very interesting to look into this in detail...

[Paul Walker]

I have found that there are two components to the problem in this thread. One is the vorticies that are a byproduct of list, and the second is the disturbed air from the propeller.

Both can be minimized, but not eliminated. A very light wing loading and a small prop diameter will minimize the problem. As I mentioned in a SN article, i used a 9*4 prop on an OS 45 FSR in a Bad News that had 725 in^2 wing area that weighed 43 ounces. With that combination I could just stand still with no issues. This was verified by dozens of flights in that configuration.

As soon as I changed back to an 11.3*4 prop, I clearly had to back up in the dead air. So, for that plane the prop diameter was the driving factor.

Note:  The problem with the 9*4 was flying in the wind. Did great hourglasses but couldn't do an overhead eight,

For more "normal" wing loadings the lift component becoms more significant than the prop diameter. However, the prop diameter is still a factor. More prop diameter makes it feel more secure as there is more line tension, right up to the point of finding your "wake" and pancaking the plane. I find that the smaller diameter props give me small warnings that things aren't right, saying "please back up"!

With electrics, the Igor power control system can be adjusted to help with the line tension in the dead air and use a smaller prop.

In summary, to mimimize the problem, keep the weight down and prop diameter as small as possible.

[Igor Burger]

A very light wing loading and a small prop diameter will minimize the problem.

Right, however I think that wing load is more important. The amount of tip vortices is function of lift coefficient and aspect ratio. So low wing load (even at the same weight) leads to low lift coefficient and thus less turbulence. The good example are our indoor models with span 36" WITHOUT FLAPS and prop 10" (so really large props to the size of model) which have absolutely no problems in calm gymn. That is reason why we do not use flaps on smaller wing - flaps make lift coefficient very high and thus lot of tubulence.

That large prop probably does not make too much problems on indoors bacuse it has very low slippage as the model has very low drag - unlike flapped model with thick airfoil. So that is probably reason that larger prop makes it worse - larger prop simply pulls harder in maneuvers and recovers energy of model lost in vortices back to model, so it can make vortices further - means again function of wing load. - just theory, but I think it will be something like that.