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Rob Thomson

Spoilers down - aileron up - thoughts?

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Woodstock

I don't do all the theory stuff, but what I have observed in full-sized gliders is that the only ones I have ever seen with underside spoilers have had top ones as well.  I've never seen an "underneath only" set... Is there a message there?

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Rob Thomson

Its the risk of damage.

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Woodstock

Its the risk of damage.

With crow, you should tuck flaps away before touch down, as you could rip them off.  Why not the same for bottom spoilers?

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Rob Thomson

Agreed .

My original question has still not been fully addressed. But purely academic now. I will be doing flaps.

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Jef Ott

Agreed .

My original question has still not been fully addressed. But purely academic now. I will be doing flaps.

 

 

 

Just for clarification, Rob, what bit hasn't been addressed?

 

Jef

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Rob Thomson

The one that said...

 

'has anyone tried it like this'  and 'how well did it work'

 

But no relevant now anyway!

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Gary B

I can relate my full-size gliding teaching experience, it may help or it may not!!

 

Spoilers are lift dumpers only until they reach 90 degrees extension, then they become airbrakes (drag producers).

 

The way in which they work is different. As non-lift induced drag (airbrakes) increases with the square of the speed they become more effective at higher speed, often to the point that they become 'Vne limiting' i.e. with full airbrake it should not be possible to exceed an airspeed that would cause structural failure (usually from flutter). Spoilers have little effect on this.

 

Extending spoilers on the approach usually causes a marked nose down trim change where airbrakes cause none, or a very little nose up pitch change.

 

A glider pilot on final approach is trying to achieve two things, control the airspeed (with the elevator, right hand) and control the 'picture' or approach path with the left hand which could be spoilers or airbrakes. If I was teaching a power pilot to fly a glider I would tell them to treat the airbrakes/spoiler lever as the throttle, this is intuitive for them.

 

Speed wise the airspeed always needs to be increased for the approach, full-size glider manufacturer's calculate the minimum recommended approach speed at 1.4 times the stalling speed (40 knot stall/56 knot approach speed). The extra speed gives a safety margin for control near the ground and also counters the wind gradient effect (decrease of wind speed with height near the ground). On windy days this speed is further increased using a fairly simple rule, for training gliders like the ASK 13/21 we would say approach speed is 50 knots plus half the wind speed with a minimum of 55 at all times. So in a 20 knot headwind the approach speed would be 60 knots.

 

Speed control is assisted (or achieved!) by using the elevator trim, the new attitude, airspeed and trim for the approach is set at the 'low key' point in the circuit, if speed control is not good on the approach the glider is either not trimmed properly or the pilot is pulling/pushing on the stick. 'Low key' for us model flyers on the slope would be the slope edge (going downwind).

 

We make the final turn into wind and ask 'can we make the landing area?', if the answer is no we leave the brakes/spoilers alone, if the answer is yes then the brakes are opened to about 2/3 travel. This setting allows adjustment either way i.e. overshooting needs more brake, undershooting needs less. All being well and the landing area not changing aspect in the pilot's view the 2/3 brake setting is held all the way down to the flare, experienced pilots would then open them fully to reduce the float.

 

The main reason for student pilots floating up the field forever is excessive speed on the approach, not the airbrake design or their use (unless they are reducing the setting greatly from 2/3).

 

Putting this into model flying practise is not entirely easy, the speed part is not too difficult, we can either add a couple of clicks of trim or have a 'Land' flight mode set. On my gliders the Land mode has the extra speed trim and also prevents the flaps from working for roll control. Assessing the 'picture' and how much brake is needed is the tricky part. In F3J we are trying to land in a 40 cm diameter area for maximum points, I've not quite cracked this bit yet!! 

 

All good fun!

 

Gary    

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Tilman Baumann

I think for me the idea of spoilers just clicked.

I was always disappointed with them because they just seemed to make the nose drop hard but did very little for speed. Which made it all in all worse. The model just came down hard and worst case it even picked up speed because of the nose down attitude.

I think I understand now. I take the nose up to loose speed. And use the spoiler to keep the plane from shooting up in a nose up attitude.

Makes sense. It's pretty much how landings with reflex (Ailerons up) work.

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chiloschista

Hello,

 

Bernoulli? Who's that? (seems we are gone a little bit further than those theories ...)

Sticking with pure concrete tests, I never tried spoilers on bottom, but I tried spoilers on top of the wing instead and on several gliders (Salto, Lunak etc).

They are really effective.

I have an option with spoilers mixed with ailerons up also, but I did use it very seldom, because it is a lot more effective and usually I don't need it.

Obviously crow are the very best solution.

 

It was mentioned before that you control speed with elevator: and CG. Want a slower glider? Move the CG forward and increase horizontal dihedral (but that's for sissies :P ).

Ric

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chiloschista

...

P.s.: as the braking efficiency could (or could not) be proportional to the air mass, one should consider that quicker moving and less dense air could almost be comparable to slower, more dense air. But, at the end, braking on quicker air could be more effective (how is it? drag increases by square etc).

And most important: the mass of air passing on the top of the wing is more than the mass passing on the bottom (therefore the circulating theory), so probably spoilers on top are more effective.

Is there around a plane using bottom spoilers? Top spoilers seems confirmed on every level of planes, from flying models to big jets. (I seems to recall there were full size gliders with both top and bottom spoilers, but this was abandoned decades ago.)

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Phil.Taylor

Spoilers, CROW, spoilerons, flaperons, clean, hoping & praying...

whatever ! - just get that thing down

(they are all in here)

 

Phil.

 

and - pertinent to the discussion - watch the DG500 spoilers (also ailerons up a bit) & their effect on final approach

(but the fence was a more effective brake  :) )

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Phil.Taylor

And most important: the mass local airspeed of air passing on the top of the wing is more than the mass local airspeed passing on the bottom (therefore the circulating theory), so probably spoilers on top are more effective.

 

there- fixed it for you

 

basic physics - to change the mass you would have to change the density - and that doesnt happen until transonic flow - heading for Mach 1 - DS has only got to 498mph so far - but may have to cope with compressibility effects when it gets much faster still - that DS whoosh with a sonic boom will be truly spectacular

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chiloschista

there- fixed it for you

 

basic physics - to change the mass you would have to change the density - and that doesnt happen until transonic flow - heading for Mach 1 - DS has only got to 498mph so far - but may have to cope with compressibility effects what it gets much faster still...

Thanks for traying to fix, but, sorry, I wrote what I meant: it's not the local speed, it's the amount of air. Amount = mass (grams to be clear).

On top of the wing there is more air passing than the bottom. The top air comes to trailing edge before the bottom air therefore the theoretical model of air circulating around the wing :)

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Phil.Taylor

Thanks for traying to fix, but, sorry, I wrote what I meant: it's not the local speed, it's the amount of air. Amount = mass (grams to be clear).

On top of the wing there is more air passing than the bottom. The top air comes to trailing edge before the bottom air therefore the theoretical model of air circulating around the wing :)

 

Martin Hepperle is always a good place to start - he did some pretty good airfoils too - MH32 etc.

http://www.mh-aerotools.de/airfoils/velocitydistributions.htm

 

what changes around a wing at subsonic speeds is the velocity & pressure - not the density (mass)

its the different velocity profile top & bottom that the circulation theory applies to

 

Phil.

(please dont ask for all the equations in the transonic & supersonic regimes - I threw out my Aero Engineering Degree notes years ago - but yes - then the density/mass changes, and theres thermo stuff too - it gets hot, which is why the SR-71 Blackbird was built from titanium)

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Tilman Baumann

Mass and density are not the same.

If you want to be pedantic, do it right.

What chiloschista means is mass deflected. Which is pretty reasonable if you are thinking of impulse transferred (breaking).

Call it mass flow if you like. Density is not the only variable, there is also time.

Actual misunderstandings are long cleared. Can we go on or rather let it be?

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Phil.Taylor

OMG - my plane fell out the sky - it flew into some heavy air - its mass got deflected - then it transferred its impulse into the ground

 

Phil.

over & out

(but that bottom spoiler on a model will get broken with slope landings)

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Woodstock

... (I seems to recall there were full size gliders with both top and bottom spoilers, but this was abandoned decades ago.)

As I already mentioned.  

 

There are scale models of such gliders with both upper and lower spoilers.

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Woodstock

 

 

There are scale models of such gliders with both upper and lower spoilers.

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

 

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Rob Thomson

Main reason against is damage on landing.

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