RE: Park Bench Ailerons

RE: Park Bench Ailerons
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RE: Park Bench Ailerons - 2/4/2005 7:07:54 PM

BMatthews

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Adam, I think you may be on thin ice.. or thin boundry layer here...

Two or so degrees of deadband is quite a bit of motion. I would think we'd see more reference to a soft neutral with full sized aircraft if this was the case. Certainly I do not find any sort of soft response when adding small trim amounts with my more sensitive models. The trim or very small control inputs compared to larger inputs all seem to respond in a linear manner for normal airspeeds.

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RE: Park Bench Ailerons - 2/4/2005 7:31:58 PM

allanflowers

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"The air within the boundary layer has almost the same speed as the wing itself so its impact on the control surface is much less compared with the free stream." - adam_one

This assumption is true for one aspect of aerodynamics, that is FRICTIONAL DRAG - where the air passing along the surface creates drag.
However, the major issue here is PRESSURE on the wing/aileron surfaces, which is always NORMAL (at 90 degrees) to the surface and is transmitted through the boundary layers regardless of their velocity relative to the wing. So, in that regard, your assumption does't hold true.
Allan

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RE: Park Bench Ailerons - 2/4/2005 8:12:21 PM

adam_one

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Let's try again:
Since the different pressures that occur around the wing (and on its boundary layer) depend on the "outside" free air stream's speed and direction, small deflections of the control surfaces within this slow air layer hardly affect the outside free stream's speed and direction, thereby producing little or no pressure changes on the wing…

In other words: a small aileron deflection only narrows the thicker boundary layer on it, having little or no effect on the free stream outside.
The boundary layer at the TE is thicker than at LD and the thickness increases as the airspeed decreases, that's why the lack of aileron effectiveness is only evident at low speed.

< Message edited by adam_one -- 2/4/2005 8:52:25 PM >

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RE: Park Bench Ailerons - 2/4/2005 9:34:44 PM

Johng

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quote:

ORIGINAL: adam_one

Let's try again:
Since the different pressures that occur around the wing (and on its boundary layer) depend on the "outside" free air stream's speed and direction, small deflections of the control surfaces within this slow air layer hardly affect the outside free stream's speed and direction, thereby producing little or no pressure changes on the wing…

NO. The problem is, the boundary layer moves with the aileron, which then affects the freestream which created the pressure change needed for rolling. The boundary layer doesn't affect that. This symptom does exist, no doubt. But you have identified the wrong disease. This type of thing occurs when there is air separation over the control surface, which happens quite frequently. Air separation creates a deadband because the surface is in dead air.

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RE: Park Bench Ailerons - 2/4/2005 9:44:50 PM

dicknadine

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OK. the subject has pretty well been regurgatrated long enough. what application does it, pro or con, have to do with Model Aviation ? we are Not theroist(?) and all the dicussions has done nothing, except thump a few chests. come guys, shave more balsa. dick

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RE: Park Bench Ailerons - 2/4/2005 10:20:59 PM

dick Hanson

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Along that line of thought ----
In that the lightweight 3D type model is a FACT--
Anyone care to venture just how much aileron deflection (measured in degrees ) can actually be used to produce a useable controlled response?
Also elevator and rudder.
If available, textbook theory or actual tests which support your reply.

< Message edited by dick Hanson -- 2/4/2005 10:23:35 PM >

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RE: Park Bench Ailerons - 2/4/2005 10:23:42 PM

adam_one

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John wrote:

quote:

NO. The problem is, the boundary layer moves with the aileron…

How about air compressibility within the boundary layer?
Won't the slower and often turbulent boundary layer at TE be compressed by a small aileron deflection against the free stream outside and become thinner as it is at the wing's leading edge?

< Message edited by adam_one -- 2/4/2005 10:25:41 PM >

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RE: Park Bench Ailerons - 2/4/2005 10:30:08 PM

Johng

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Look at it this way; If something is pushing on the boundary layer to make it thinner, it's pushing back on the freestream, and it's pushing on the aileron. If you are thinking of it like a spring, the spring pushes on both ends. A spring on the table will be compressed when you push down on it. And you will fell the force in your hand. And the table will feel the force as well. In this case, the table will be the wing, and it will feel the force. And it will roll.

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RE: Park Bench Ailerons - 2/4/2005 10:50:25 PM

adam_one

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quote:

Look at it this way; If something is pushing on the boundary layer to make it thinner, it's pushing back on the freestream, and it's pushing on the aileron.

Hmm, that's assuming the boundary layer has the same compressibility as the freestream…

quote:

If you are thinking of it like a spring, the spring pushes on both ends. A spring on the table will be compressed when you push down on it. And you will fell the force in your hand. And the table will feel the force as well. In this case, the table will be the wing, and it will feel the force. And it will roll.

Yes, I just think the boundary layer as a spring that damps the aileron force and only gives the full force to the freestream when it becomes totally compressed…

< Message edited by adam_one -- 2/4/2005 10:51:09 PM >

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RE: Park Bench Ailerons - 2/4/2005 11:04:13 PM

Johng

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That does not make sense, to be blunt. You have to obey newtonian physics. An action will have an equal and opposite reaction. You can't apply a force to the boundary layer that won't be reacted. It's a simple statics problem. A spring does not react only when it is "totally compressed"

I'm done here. This has been a full explanation. If you don't want to believe it here, read a book about it.

Plus, your use of the word "compressed" is not accurate, as that refers to the air changing density, which it does not at model speeds. it only changes pressure and velocity.

< Message edited by Johng -- 2/4/2005 11:11:34 PM >

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RE: Park Bench Ailerons - 2/4/2005 11:39:02 PM

adam_one

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Of course the air that is under pressure becomes compressed to a certain extent…
I guess that the air within the boundary layer at LE is under much more pressure and gets a higher local compression than the air at TE, where it often becomes turbulent at low speed.
By the way, you used the term "dead air" when referring to air separation, what you mean with "dead air"? could you develop?

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RE: Park Bench Ailerons - 2/5/2005 3:09:28 AM

BMatthews

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quote:

ORIGINAL: dicknadine

OK. the subject has pretty well been regurgatrated long enough. what application does it, pro or con, have to do with Model Aviation ? we are Not theroist(?) and all the dicussions has done nothing, except thump a few chests. come guys, shave more balsa. dick

Sorry Dick, in this case this IS the forum to thump our chests and freely discuss how may angels can dance on the head of a pin THIS IS AERODYNAMICS WHERE NO TOPIC IS TOO SMALL OR MUNDANE FOR US! ! ! !.....

Adam, getting back to practical considerations we still have the fact that conventional controls on full sized aircraft do not seem to suffer from your deadband. Small movements of less than a degree still have a positive effect on the flight accoding to all I've spoken too and from some very limited stick time of my own. Similarly I've had models that also happily respond to surface deflections of less than a degree and near as I can tell do it with a nicely linear response.

Also if the boundry layer is so thick how can you explain how wool tuft testing can show smooth flows, or at least relatively smooth flows, of air that is only within 1/4 to 1/2 an inch to the skin? I've not seen any turbulent flow figures for full size aircraft but from all I've read laminar flow is more common for full sized smooth skinned aircraft than for our models just due to the higher Re's.

And in the end there is no denying that the drag of the Junkers system is going to be much higher than the conventional inset style..... although I can't help but wonder what the drag of the HUGE fowler flap track fairings seen on many planes is doing to the airflow....

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RE: Park Bench Ailerons - 2/5/2005 5:34:51 AM

dick Hanson

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I will go with Nadine on this one --
hypothetical nuances of this type may entertain some - that is a given
but where was the practical application for models?
I have wondered how elevator deflections of 45 degrees in each direction could possibly be of benifit .
In actual practice -it becomes obvious .
Not all of what we do (as modelers) follows what I once was given as usuable limits for deflection.
just a thought-----

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RE: Park Bench Ailerons - 2/5/2005 10:48:19 AM

adam_one

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BMatthews wrote:

quote:

Adam, getting back to practical considerations we still have the fact that conventional controls on full sized aircraft do not seem to suffer from your deadband.

Note that the possible deadband is only evident at low speeds and low Re.
And it's true that this sort of aileron (Junkers) causes higher drag as the airspeed increases.
It is mainly used with slow and relatively small aircraft designed for STOL.

John used the expression "dead air" when referring to air separation, and it would be interesting to know what that means in terms of local air pressure, airspeed, momentum and energy.

Anyway:
As the air moves past an object, the air molecules right next to the object's surface stick to the surface. The molecules just above the surface are slowed down in their collisions with the molecules sticking to the surface. These molecules in turn slow down the flow just above them. The farther one moves away from the surface, the fewer the collisions affected by the object surface. This creates a thin layer of air near the surface in which the velocity changes from zero at the surface to the free stream value away from the surface. Engineers call this layer the boundary layer.

The details of the flow within the boundary layer are very important for many problems in aerodynamics, including the development of a wing stall, and the skin friction drag of an object.
Boundary layers may be either laminar (layered), or turbulent (disordered) depending on the value of the Reynolds number Re.
For a fluid in motion, the velocity and Re will have different values at different locations around the object.
To correctly determine the velocity distribution, we have to solve equations expressing a conservation of mass, momentum, and energy for the fluid passing the object.
The flow in the boundary has very low energy (relative to the free stream) and is more easily driven by changes in pressure...
That's why an aileron deflection will make its boundary thinner before it influences the high energy of the free stream, since the boundary "cushions" the movement of the aileron towards the free stream.

< Message edited by adam_one -- 2/5/2005 12:14:17 PM >

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RE: Park Bench Ailerons - 2/5/2005 6:21:05 PM

Siefring

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At model speeds compression is not an issue. There is a transient compression when a surface is moved but the air flows at the speed of sound to neutralize it. If you move a 1 inch aileron it takes about 80 microseconds for the flow pattern to be established (dimension/speed of sound=.025m/340m/s). The entire flow field around a 2m plane is established in about 6 millisecs.

For planes moving at less then 400 or 500 mph compression has almost nothing to do with how they fly.

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