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Rob Thomson

Spoilers down - aileron up - thoughts?

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Rob Thomson

Main reason against is damage on landing.

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Ribs

In the words of the great Ben Goldacre, (and this goes to all of you ;-)), it's a bit more complicated than that.

 

Personally I'm not going to add too much, save to recommend this amazing book - it takes pride of place on my shelf...

 

http://www.amazon.co.uk/Fundamentals-Sailplane-Design-Fred-Thomas/dp/0966955307

 

F=d(m(t)v(t))dt

 

end of!

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Rob Thomson

Ordered !

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chiloschista

See from 3:00 onwards to see the double spoilers.  Seem to work real well.

 

Thanks for that one and sorry if I did not read carefully all the thread long. This recall me that I may better devote my time finishing my 3.88m Habicht (which only has top spoilers, but will be equipped with double ailerons, so crows)

 

In the words of the great Ben Goldacre, (and this goes to all of you ;-)), it's a bit more complicated than that.

 

Personally I'm not going to add too much, save to recommend this amazing book - it takes pride of place on my shelf...

 

http://www.amazon.co.uk/Fundamentals-Sailplane-Design-Fred-Thomas/dp/0966955307

 

F=d(m(t)v(t))dt

 

end of!

And thanks for that one too. I will probably order it. It will find a place between the severals I already have.

As I'm here more to learn than to teach and trying to have reason at any cost, I'm heading right now to continue the couple of Black(Le)Fish I'm preparing for travel to Ligury :P .

Ric

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Rob Thomson

My book : Fundamental of Saleplane design has arrived.

clearly someone has stuck the label on the wronflg book!

Ho hum...

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Tilman Baumann

Lol

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Ribs

Oh no!

 

I think a few people may have suffered the same fate, based on the reviews.

 

Does this alternative offer any guidance on the subject!?

 

btw, also excellent are:

 

http://www.amazon.com/Mechanics-Flight-11th-Edition-Barnard/dp/1405823593

(very readable book that has stood the test of time, think J.E.Gordon, but for aeroplanes)

 

and

 

http://www.amazon.com/Aerodynamics-Aeronautics-Flight-Mechanics-McCormick/dp/0471575062

 

I never read them once when I should have, now they are a couple of my favourite reference books!

 

The Fred Thomas Glider title is certainly harder to come by though - but it does exist and I have a copy here. Here's what it says about spoilers, which Fred refers to as 'Dive Brakes':

 

"...Dive brakes are usually placed only on the upper wing surface, especially in the more modern aircraft. Upper surface dive brakes, especially the simple hinged-panel type that swing up and forward, are sometimes referred to as "spoilers". Lower surface dive brakes can cause ground clearance problems, particularly with mid-wing designs. Also, with upper/lower surface dive brakes the dive brake caps must mate perfectly with the wing in order to prevent air flow from the lower to upper surface. Single upper-surface dive brakes tend to be more effective than an upper/lower surface pair of equivalent size because of the higher air velocities present on the upper surface."

 

Not a definitive comment and there is a little more on the subject in the book, but this is interesting nonetheless.

 

It's worth noting that a 'spoiler' is not going to add much lift while it does its primary job of adding drag. A trailing edge flap on the other hand will alter the drag polar completely, proving both added drag and an increase in Cl_max. You may well argue that the change in Cl_max will be less effective around the portion of the wing where an additional spoiler is used (if memory serves, that was the original concept!), but the increase in drag will remain. An increase in Cl_max allows an aircraft to fly slower (at higher angles of attack), with a consequent slower descent (in general) and landing speed - note how spoilers only tend to be deployed on full size aeroplanes, when you are braking on the runway.

 

From what I can gather, the premise of any spoiler is to add drag. The flying weight remaining constant, then the lift must also remain constant (steady state conditions), so the lift to drag ratio reduces and the glide steepens to balance these changes. The increase in drag will also tend to slow the model down, so the angle of attack must increase also, to increase Cl and possibly taking you closer to a stall.

 

An equilibrium will then be found, where there is no change in speed and the change in moment caused by both the drag and movement of CoL, will be balanced by a trim change of the horizontal stab.

 

In summary, if what you want to do is dump lift in a runaway thermal, you need drag (i.e. spoilers), while if you want to land at a walking pace and look cool among F3f modellers, then you want lots of flap - compensated with up aileron (or crow braking as it is known).

 

Very happy to be corrected on any of the above!

 

cheers,

Chris

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Ribs

A slight amendment...

 

 

 

 

 

 

 

It's worth noting that a 'spoiler' is not going to add much lift while it does its primary job of adding drag. A trailing edge flap on the other hand will alter the drag polar completely, proving both added drag and an increase in Cl_max. You may well argue that the change in Cl_max will be less effective around the portion of the wing where an additional spoiler is used (if memory serves, that was the original concept!), but the increase in drag will remain. An increase in Cl_max allows an aircraft to fly slower (at higher angles of attack), with a consequent slower descent (in general) and landing speed - note how spoilers only tend to be deployed on full size aeroplanes, when you are braking on the runway.

 

 

 

 

 

I think I'm wrong in stating that the primary job of a spoiler is to add drag. Although spoilers will generally do this, what they primarily do is create separation, and lose lift on portion of the wing. To maintain equilibrium, the rest of the wing has to work harder and therefore work at a new (higher) Cl, with a higher corresponding drag coefficient. This in turn adds drag. So the drag effect of the spoiler is two-fold, one from the separated air creating a low pressure area behind them and secondly from the increased induced drag along the rest of the wing.

 

Perhaps it is more accurate to say that the primary job of a spoiler is to...er.. spoil the wing!

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isoaritfirst

I think th ebook has it right.

 

It's all witch craft.

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