SEN issue 1161

Joined: 27 Jul 2006 Posts: 42

The convention on most models is to have washin on the inboard wing centre panel in order to provide thermal seeking. Why not use washout on the outer centre panel ? Has anyone tried it ? Is there any difference between the two and if so what, why and by how much ?

Obviously this question relates to models outside of FAI as well but I didn't want to double post.

CHE

Joined: 21 Sep 2005 Posts: 142

Good questions....

Many people do many things with wash. I know that some people have washed out a tip to do the same. However, I think that the feeling is that it takes more wash out (Leading edge down) to accomplish the same as a washed in panel- If a panel is leading edge down (wash out) there is less drag or change with the same amount put in than if you washed in the panel. As aspect ratios increased, there is less wash needed to produce the same result, because of increased moments.

One thing to think about is that a washed in panel not only increases lift but also increases the drag of that panel. So, the wing may not only lift but also yaw towards that panel. This is where it can get interesting when watching a model in power. It may look like it is turning right when in fact it is rolling left. In this case the model actually wants less incidence in the right panel. This is very common with FAI style models. We ran into this with our J models and kept adding rudder when what it actually wanted was a wash change. Look very carefully at what it is doing and then think about what is actually needed...

Hopefully that helps a small amount...

Joined: 27 Jul 2006 Posts: 42

John,

Thanks for you comments. Your note about the extra drag coming from washin on the inner panel is of course correct but if you had washout on the outer panel instead then the drag there would be less and you'd have the same effect wouldn't you ?

I take your point about power models. To be honest I was only really interested in the glide and thermalling effects so perhaps I should clarify that point.

Any other thoughts ?

Joined: 21 Sep 2005 Posts: 142

The answer to the question on the outer panel would probably be yes. But, I think it "may" take more to produce the same effect. Typical tapered tips would force more but then again because of the increased moment....Heck, what do I know....

The effects of wash in glide are, in my opinion, not as dramatic at normal speeds. However, the latest FAI models do use wash to enhance the thermalling capabilities. If I recall when talking to Eugene Verbitsky he stated that one reason he used wash out on the right center panel was to allow the model to tighten up from the model's normally large circle (we glide about 1 minute plus glide circles- in fact, one max that John had at the world champs was about 7/8 circle in 3 minutes). However, after we broke one model, Eugene said that it "can" happen because of the way that wing was washed. Here we are talking about .04-.06" wash total. Not much in light of the size of the model.

I know that I always used to wash out both tips a fair amount for stability in glide. The idea always was to make the tips stall after the center section. I have never been able to dismiss this concept as it seems to always work and many poeple do it.

Maybe this helps a little bit...

Joined: 21 Sep 2005 Posts: 142

The answer to the question on the outer panel would probably be yes. But, I think it "may" take more to produce the same effect. Typical tapered tips would force more but then again because of the increased moment....Heck, what do I know....

The effects of wash in glide are, in my opinion, not as dramatic at normal speeds. However, the latest FAI models do use wash to enhance the thermalling capabilities. If I recall when talking to Eugene Verbitsky he stated that one reason he used wash out on the right center panel was to allow the model to tighten up from the model's normally large circle (we glide about 1 minute plus glide circles- in fact, one max that John had at the world champs was about 7/8 circle in 3 minutes). However, after we broke one model, Eugene said that it " can" happen because of the way that wing was washed. Here we are talking about .04-.06" wash total. Not much in light of the size of the model.

I know that I always used to wash out both tips a fair amount for stability in glide. The idea always was to make the tips stall after the center section. I have never been able to dismiss this concept as it seems to always work and many poeple do it.

Maybe this helps a little bit...

Joined: 16 May 2006 Posts: 17 Location: O.C., CA

CHE,

The answer in milliseconds is: MOX NIX!

IMHO, based on 50+ years of struggle with similar issues, whether the
wing washin is plus inboard or minus outboard is not important.
The AIRPLANE will seek trim equilibrium no matter which method is applied.
I have seen and had gliders with each setup work equally well as to thermalling ability.
Conversely, several of my F1A compatriats and I have tried flat
mains with ONLY tip differential and found them generally to
be dogs in thermal condition.
Therein lies one reason to have picked the right glider for any given flight.

DIFFERENTIAL is the key, i.e., having just the amount of positive incidence on the inboard wing to do the job you desire.
As John L stated on his answer to your post: 'As aspect ratios increased, there is less wash needed to produce the same
result, because of increased moments.'
Hence, I have found that a 2 meter glider can and must use more differential than a 2.5 meter glider.
Examples from my fleet: 2.1m Wishbone has about 0.4 deg washin, my 2.25m Li'l ALs have 0.2-.3 deg, my newest,
2.4m EOS has ~0.1 deg, while my 2.5m Buntero is about the same.
All the above is with flat main panels that have been trimmed using
wing wigglers, not panel twists.
Mind you, the ears have washout. Usually the left[I glide to right]ear
has a bit more than the right, or inboard ear.

This goes back to my comment above related to the job the glider is intended for.
Is it a thermal pig, where a bit more drag won't matter, or is it your ultimate FO bird, where any added drag
may mean enuf more sink to be an all-so-ran?

Ciao,
Leeper

Joined: 16 May 2006 Posts: 17 Location: O.C., CA

Big John,
A small point of order, related to your " latest FAI models do use wash
to enhance the thermalling capabilities. If I recall when talking to Eugene Verbitsky...".
I believe you are specifically speaking of F1C models here, as
most F1As do use washin of inboard wing to enhance thermalling, with
less fear of spinning in from violent air.
I have been studying the use of washin on outboard wings, as used by
Andriukov-inspired F1B fliers. Now you indicate Verbitsky-inspired F1C
types are doing similar trim setup.

Hmmm...here is where I stick my neck out...my unsubstantiated idea is:

This is somehow caused by the extremely long tail moment positions
of the current B & C design. As a percent of wing span, B & C booms
are much longer than F1As, in general these day.
This leads me to believe there is a newish paradigm of forces which
create this turning lift from ourboard wing, coupled with the rudder
turning forces, which causes turn tightening from wide to speeding up
and turning into the lift.
The great 2-time A2 glider champion, Rudi Lindner is said to have used
such a trim in the 50s, but I saw some attempts to use the
so called 'Lindner trim' meet with less than satisfactory results, shall
we say...
Which leads me back to my synopsis that long moment arm and small
fin area somehow are combining with the wide circling of these ultra
clean[read 'low drag']design types, which I don't see it being a viable
trim direction for F1As, in current planform arrangements.

There, I have said it, so blast away at it, if you want...

Ciao,
Leeper

Joined: 21 Sep 2005 Posts: 142

Lee Buddy, et al....

Yup, I always look at it from what I have some experience in, which is F1J and F1C. We had one of our best models climb beautifully, bunt very well, and the proceed to do a slow right spiral into the ground- at the WC, of course- and, of course, it hit the road...Destroyed the aluminum fron end, broke the tail boom, bent up a wing and dinked the stab...Talked with Eugene about this and that is where I got the info, as this was one of his models and we flew it like it was set up. He said that the warps were in it primarily to help the thermal seeking capabilities but what happened to us "sometimes happens".

I do agree that there must be something to do with our somewhat largish rudders (fins), long tail booms, high aspect ratios. I would hate to take one of these models and really crank in some glide turn. We run, of course a minute or so circle and this helps keep the wings much more level than in a tighter turn. I have seen it too many times when these things tighten up and it looks like they are in the spiral/crash mode.

I also know that some folks have tried (somewhat ssuccessfully) offseting the dihedral break (slanting it so that the break is not parallel to the fuse centerline) to provide the wash in the tips. Dick Swenson did this in the 80's on his F1C models.

Everyone has his own deal, but I think we can pretty much state that we need the tips washed out some and possibly the inboard section could have some wash in (or out, depending on which side of the pond you are on). With many AMA gas model, low thrust models will have the tips washed out and the right center section washed in (to keep it up in power).

Hell, what do I know, it is the kid that does all the good flying!!

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SEN issue 1161 - 4 December 2007