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Rob Thomson

Spoilers down - aileron up - thoughts?

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Rob Thomson

Right.  I have a project on the table that introduces an interesting choice.

 

What I have is a three meter glider with a built up wing.   It is a relatively heavy glider at around 2.5kg flying weight - and in effect has a lot of inertia  making landing a bit challenging.  Essentially you have to contend with a model that does not slow down, and floats for ever and ever.

 

The simple solution for me is to simply install some scissor type spoilers on the top of the wing.  This can be achieved with minimal surgery and will be effective at helping to bring the model down.  

 

What I am curious about is a slightly different aerodynamic approach that will help to slow the model down.

 

To my understanding, a spoiler on the bottom the wing is more effective at slowing the model down.  But less effective at reducing lift.  This leads me to wonder if anyone has ever tried:

 

Spoilers on the bottom of the wing, and ailerons up to act a little bit like 'crow'.

 

Theoretically the ailerons up would reduce the lift and help stop tip stalls.. and the spoilers down would help slow you down.

 

In practice.. it may all go wrong!

 

Any ideas?

 

My other option is to retro fit flaps - but I really don't fancy recovering the model!

 

Rob

 

 

 

 

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pete beadle

Hi Rob

 

It seems that what you are considering is a setup that most of us were considering back in the '70's and '80's

One more consideration that had to be put into the mix was/is whether you intented operating this model, with the built-up wing, off the slope or off the flat (or both)

Our major considerations then, were whether the brakes/spoilers were being fitted to a scale model or a sport one, and/or being retro-fitted with minimum work as distinct from maximum efficiency (theoretical or otherwise)

So, we thought then of fitting downgoing flaps as spoilers, but didn't, as they were often ripped off on landing at speed into long grass and the like

or we retro-fitted blade or letterbox spoilers just behind the mainspar so as not to compromise the strength and integrity of the (usually wooden) mainspar rather than where they worked most efficiently. i.e. on/in the spar

Also, our servos, operating the brakes were usually fitted in the fuselage and operated by wires and, when we landed in/on long grass the wing halves moved out and up popped the brakes

So our considerations were different and, for example we just didn't have servoless, unit airbrakes that could be retrofitted with a minimum of work, we didn't have micro servos that  could be fitted into the thin wings and we didn't have options on our Tx's that were capable of being mixed, reversed, slowed, made sequential or even more accurately so, I think you'll have to look at your priority list and re-assess your requirements in the light of what is now available and then see whether one particular requirement - eg price, or ease of fitting or relative efficiency makes the choice for you and is actually what you want

Why not consider retrofitting brakes, for example, rather than flaps, so you'd not need to re-cover, not if they were more (or less) efficient on your criteria list......

Good luck

Regards

 

Pete

BARCS1702  

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Rob Thomson

Hello,

 

Thats exactly my thoughts.

 

I am thinking servo-less brakes.   Just trying to decide if better to put them on the top or bottom!  

 

Would it work well on the bottom, with ailerons up to give sue-do crow, or just plug them on the top as spoilers only?

 

Rob

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f3fman

Retro fit flaps, gives you more options, full span flap +ve for extra lift, -ve for extra speed. Plus extra aileron authority, and crow brakes.

You know it makes sense !

 

(Easy for me to say, I'm not the one doing it ! ) :)

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Rob Thomson

I am erring to that option.

 

Just such a pita.

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Phil.Taylor

To my understanding, a spoiler on the bottom the wing is more effective at slowing the model down.  But less effective at reducing lift.  This leads me to wonder if anyone has ever tried:

 

Spoilers on the bottom of the wing, and ailerons up to act a little bit like 'crow'.

 

Rob

 

One more time:

- spoilers/brakes dont "slow the model down" - it needs the airspeed to keep flying

- you control the airspeed with the elevator

- you control the descent rate by killing the lift & adding drag - spoilers (on top), flaps, CROW, spoilerons, whatever

 

of course, spoilers under the wing snagging on the grass might slow it down...

 

Phil.

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Rob Thomson

I disagree.

 

A spoiler on the top of the wing does not slow the model down.  It interferes with the lift - with no speed reduction. This is largely because it is operating in the low pressure zone.

 

A spoiler on the bottom of the wing will act much like a flap being down at greater than 70 degrees.   It is in the high presure zone and will act as a brake first - and a lift reduction second.

 

This is the crux of why I have suggested doing a spoiler on the bottom as a brake, and ailerons up.

 

A pseudo crow.

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Phil.Taylor

disagree as much as you like - you are confusing "adding drag" with "adding a braking effect"

add drag/brakes to a car & it slows down

add drag/brakes to a glider - its different - you actually steepen the glide angle - it still needs the forward speed to produce lift - it doesnt slow down like the car - like this:

http://www.grc.nasa.gov/WWW/k-12/airplane/glidvec.html

 

what you are actually asking is - what is the best way of adding drag and reducing the lift?

 

btw - the local airspeed over the top of the wing is greater than that on the bottom - its why your plane flies...

 

Phil.

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Rob Thomson

i am well aware of that. i think confusion with terms.

as i said earlier. a pseudo crow.

what i simply want to know is has anybody done similar, and what is the end effect.

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Rob Thomson

Out of interest.... and to be pedantic.

The air is moving faster on the too, but is less dense as it is a little a lower presure. That is what gives t he associated lift.

The high presure below is more dense, making the brake more effective .

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Phil.Taylor

trouble with theory is you need to understand all of it

I like practical stuff too - so:

suggestion - go try a practical experiment before you hack the wings

ingredients: - sticky tape & cardboard + 1x floater glider

a  tape "spoilers" on top of the wing - how does it fly?

b  tape "spoilers" underneath the wing...

c  tape on some nice big flaps...

d  just the ailerons up ...

c  report back

 

what I can tell you from practical experience is that top spoilers on an old floater of mine worked just fine - also an old original flapless Alpina (and DG500) - nothing like as effective as CROW with big flaps - but it got the plane down - Alpina also had some up-aileron too (Alpina & DG500 video somewhere)

 

Phil.

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wookman

Full size gliders have settled for brakes on top, I don't know why but there has got to be a clue there somewhere.

Blade brakes on the bottom of a model wing are gonna get broken, the proximity of the ground will see to that sooner or later.

The lift "contribution" is likely to minimal because the brake will be too far forward on the wing. The difference in drag is likely to be insignificant top or bottom. Flaps is the way to go, failing that brakes on top. Tearing the bottom brake out of the wing will make fitting flaps look easy!

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Phil.Taylor

Out of interest.... and to be pedantic.

The air is moving faster on the too, but is less dense as it is a little a lower presure. That is what gives t he associated lift.

The high presure below is more dense, making the brake more effective .

 

pedantic? - no, just totally wrong

 

the air density doesnt change around the wing until you are transonic - near Mach1 - our gliders live in an incompressible flow world

 

like I said - for theory, you need all the theory

personally, I learned a lot from sticky tape & cardboard

 

Phil.

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Rob Thomson

Agreed. Nothing beats crow and practical tests.

Beauty of forums is chances are someone has done them before, and can save you the trouble ;)

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f3fman

It's all a load of Bernoulli's

 

B) 

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Phil.Taylor

It's all a load of Bernoulli's

 

B) 

 

now theres a man who knows his theory

(I've heard he's good with sticky tape too)

 

Phil.

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Rob Thomson

So the simple answer to my question was no, it will not work as well as you think, and this is why. ..

Sometimes getting positive advice from people who have the experience - can be really hard :)

I will probably go for full flaps . Just a pita to install!

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Jef Ott

Interesting discussion.

 

Refusing to get drawn into the theorising of it, but Crow is best for models that can take the load of a nose first landing and IMO spoilers are more controllable, as far as just changing the glide angle is concerned.

 

Personally, I would try the addition of spoilers first. If that doesn't cut the mustard for you, bite the bullet and go for crow. 

 

If you are only flying at Thurnham, spoilers on the weight and size of model you describe will be fine. If trying to land the same model on The Wrecker... then expect to be doing some repairs, unless you have an injection of the luck virus handy.  

 

Hope this helps,

Jef 

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oipigface

 If trying to land the same model on The Wrecker... then expect to be doing some repairs, unless you have an injection of the luck virus handy.  

 

 

It doesn't matter what you've got to slow your plane down with, landing on the Wrecker will break it (except in very, very light winds).

Cross over the road to the Back of the Wrecker.

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