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Glider flap hinge options


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I'll soon be trimming the TE, final sanding, and cutting out the flaps on the new center-setion of my DR 420. Because I don't need the flaps to go up, it occurred to me that I could bottom-hinge them rather than top-hinge.

 

 

1612721912_DR420flapoptions.thumb.jpg.b7b0f0132111576b1bc2a58e672b302b.jpg

 

 

216672973_flaphingeoptions.thumb.jpg.07a8f3c1c22d7e90f83910f6730fd6f0.jpg

 

My thinking is:
Top-hinged flaps in normal flight would have the advantage that I could drop them a few degrees to increase camber without interrupting the airflow over the upper surface, or drop them 45 degrees to act a bit like airbrakes. But, the disadvantage would be that the under surface airflow would be disrupted nearly all the time.


Bottom-hinged would have the advantage that both the top and bottom airflows would be uninterrupted in normal fight and dropping them 45 degrees would badly disrupt the upper surface airflow but this probably wouldn't matter and might even make them more effective as 'brakes'. But, the disadvantage would be that dropping the flaps a few degrees to increase the camber would interrupt the top-surface airflow which might more than override any extra lift created.

 

So maybe, If I DO want variable camber top-hinged will be better and, if I DON'T want variable camber, bottom hinged will better because it will give me maximum wing efficiency and better braking effect.

Am I right?

 

Thanks! And Happy New Year,
David

 

 

  

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The airspeed will be low when the flap is in use so the airflow over the wing top surface will probably have detached before it reaches the flap so the gap will have little, if any, effect. OTOH the pressure on the under-surface will have increased and will benefit from the smoother surface.

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Bottom hinge. Put a small gap cover over the top gap that keeps the flow smooth up to , say, 10 degrees down. Past that will be when you are wanting more drag than lift so it doesn't matter. For best effect/drag the flaps bneed to get to 70 plus degrees. I aim for 90.

 

Many full house gliders use/used bottom hinge flaps

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1 hour ago, Martin Hodgson said:

Bottom hinge. Put a small gap cover over the top gap that keeps the flow smooth up to , say, 10 degrees down. Past that will be when you are wanting more drag than lift so it doesn't matter. For best effect/drag the flaps bneed to get to 70 plus degrees. I aim for 90.

 

Many full house gliders use/used bottom hinge flaps

Thanks Martin, that sounds like a very good idea. At 10 degrees down I reckon the top gap will only be 2mm, 3 at the most.
What would you do? Glue a strip of very thin ply to the top of the trailing edge recessed into the top of the flap?

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David, this is a photo of my Patrician design which has bottom hinged ailerons & flaps. The ailerons have a flexible plastic material (can't remember what) used as a gap cover, the flaps top front section is made of thin ply to protrude into the airflow acting as spoilers.

 

1412754717_P1020860(1280x960).thumb.jpg.ee33697d0fecb982939c82fac8491a3b.jpg

 

The flaps are barely visible in this photo but are from the root to about half way towards the aileron.

 

Flaps.jpg.f8c5624224e1a1c09a7c6d338eac8da3.jpg

 

The original intention was only to use the flap in position A for normal flight & C for landing approach. However after a little experimentation I discovered that selecting position B actually improved the glide duration & used this to advantage when any thermal was detected despite the ply front section "spoiler" protruding into the airflow.

 

Bottom line - if the ailerons shrouds were not already in place I wouldn't have bothered with them & if I were you I wouldn't bother with any gap covers - full size gliders are operating at mich higher speeds & Reynolds numbers than our models. 

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It surely depends on what is to be the main function of the flap.

Up to about 10 degrees of down flap the top surface is more important for lift to drag than the bottom, i.e. Circling in a thermal, so top hinged and sealed. 

Above 30 degrees the flap is as much a brake as a lift generator so the top surface airflow is no longer critical. The airflow would separate anyway at a 30 degree break.

 

If you want the best of both worlds then off set pivot points and matching profiles between the wing and flap leading edges is the answer. ?

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1 minute ago, Simon Chaddock said:

It surely depends on what is to be the main function of the flap.

Up to about 10 degrees of down flap the top surface is more important for lift to drag than the bottom, i.e. Circling in a thermal, so top hinged and sealed. 

Above 30 degrees the flap is as much a brake as a lift generator so the top surface airflow is no longer critical. The airflow would separate anyway at a 30 degree break.

 

If you want the best of both worlds then off set pivot points and matching profiles between the wing and flap leading edges is the answer. ?

It's the angle of attack forward of the thickest part of the wing section & airspeed that determines where the airflow over the wing upper surface separates. The airflow cover the top surface can separate before reaching the flap hinge point whether the flap is deployed or not.  

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Thanks Don, Pat, and Simon,

Pat, with your ABC flap/spolier setup, I'm surprised that in position B the model gained more (in lift) than it lost (in drag) but that actually makes me think that with the simple bottom-hinge setup I am thinking of (as per diagram at top of thread) the same would be true (at 10 degrees down I'd gain more than I lost) and so a top-gap-cover is unnecessary. Mind you, I rather like the gap cover on your ailerons. I saw an advert recently for plastic draught excluder tape on a roll which was very thin and self adhesive down one side. Maybe that would work.

Simon, I'm not entirely sure what "off set pivot points and matching profiles between the wing and flap" would look like. Is it weaker and more complicated than a full-length simple film hinge?

Please excuse my ignorance.

 

David 

Edited by David Ramsden
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The model doesn’t look too racy and has quite a generous amount of dihedral so my  thoughts are that it will turn easily in thermals without a need for any flap deflection, which although might contribute to adding a little lift the increase in drag will offset the advantage. Therefore the best purpose for the flaps is to use them for steeps descent and landings for which a variable and large deflection is required. Top hingeing is more aesthetically pleasing when looking at the model on the ground, and in the air you won’t notice the underside gap. If you feel the need to cover the gap then mylar strip with double sided tape is a good option and the draft tape is pretty much that. 

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David

A fully shrouded flap looks something like this.

ShroudedFlap.jpg.9eecab3796ace58a5406fcf6eea8deb6.jpg

Note the hinge line is within the flap itself so it requires extended individual hinge mounts but it does give a smooth top surface all the way from 0 to 90 degrees.

So the answer to your questions is a definite yes and yes!

On a power plane where the prime use of a flap is to reduce the safe flying speed and at higher angles to increase the drag a simple bottom hinge is the obvious choice.    

Edited by Simon Chaddock
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16 minutes ago, Simon Chaddock said:

David

A fully shrouded flap looks something like this.

ShroudedFlap.jpg.9eecab3796ace58a5406fcf6eea8deb6.jpg

Note the hinge line is within the flap itself so it requires extended individual hinge mounts but it does give a smooth top surface all the way from 0 to 90 degrees.

So the answer to your questions is a definite yes and yes!

On a power plane where the prime use of a flap is to reduce the safe flying speed and at higher angles to increase the drag a simple bottom hinge is the obvious choice.    

Thanks Simon,

that certainly looks like the ultimate solution but how would I actually build/install "extended individual hinge mounts" and how many would I need on a flap that's about 400mm long? Please do you have any photos of hinge mounts like that? Thanks.

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This is how I hinged all my ailerons and flaps on high performance  1/4 scale sailplanes. Never had any come loose and over 15 years and so simple to do. This diagram was previously posted on here and saves a lengthy description of the technique. For the tape have a look for White Tessa or a tear proof clear scotch tape.

 

image.jpeg.9cd8f5b51cfacaa47d9999c12ffbe750.jpeg

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David

All my flaps are on power planes where I have used bottom tape hinges in the style shown in eflightray's diagram.

The biggest properly shrouded aileron I have is on my Ballerina. It is 240 mm long.

AileronC.JPG.80da7bcbf556d388057004fc9880c74c.JPG 

The wing section is quite thick which makes the aileron pretty rigid so it gets away with being pivoted only at each end.  The offset axis is thus no problem although once it is all glued in it is 'permanent'.

It does gives a very smooth joint and it is absolutely free moving with a narrow 1 mm gap over its full range of travel. I would say the trouble and effort to set it up is not really worth any minute performance gain unless you ae simply trying to prove a point which in my case I was. It is all done in Depron foam! ? 

A full size Cessna 172 flap manages with just 2 hinge brackets.

Cessna172Flap.thumb.jpg.6fb21c23b89adcbffef83dfcb7be0653.jpg

But of course it is big enough to allow access panels so you can get at the pivot points buried within the flap.

By comparison a bottom 'tape' hinge for a flap is so simple to do.

I hope this helps. 

 

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23 hours ago, Nick Somerville said:

This is how I hinged all my ailerons and flaps on high performance  1/4 scale sailplanes. Never had any come loose and over 15 years and so simple to do. This diagram was previously posted on here and saves a lengthy description of the technique. For the tape have a look for White Tessa or a tear proof clear scotch tape.

 

image.jpeg.9cd8f5b51cfacaa47d9999c12ffbe750.jpeg

Hi Nick,

Yes. Me too. That's the method I've used on all my ailerons and flaps but with film rather than tape. The ailerons on this model (photo at top of thread) are top hinged like that too.
The reason I started this thread is because I don't need the flaps to go up (so top-hinging is not needed) and I want them to come down about 80 degrees (which top hinging wouldn't allow). That's why I'm looking at bottom hinging for the first time.

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19 hours ago, Simon Chaddock said:

David

All my flaps are on power planes where I have used bottom tape hinges in the style shown in eflightray's diagram.

The biggest properly shrouded aileron I have is on my Ballerina. It is 240 mm long.

AileronC.JPG.80da7bcbf556d388057004fc9880c74c.JPG 

The wing section is quite thick which makes the aileron pretty rigid so it gets away with being pivoted only at each end.  The offset axis is thus no problem although once it is all glued in it is 'permanent'.

It does gives a very smooth joint and it is absolutely free moving with a narrow 1 mm gap over its full range of travel. I would say the trouble and effort to set it up is not really worth any minute performance gain unless you ae simply trying to prove a point which in my case I was. It is all done in Depron foam! ? 

A full size Cessna 172 flap manages with just 2 hinge brackets.

Cessna172Flap.thumb.jpg.6fb21c23b89adcbffef83dfcb7be0653.jpg

But of course it is big enough to allow access panels so you can get at the pivot points buried within the flap.

By comparison a bottom 'tape' hinge for a flap is so simple to do.

I hope this helps. 

 

Thanks Simon,
With my long thin solid balsa flaps I agree "trouble and effort to set it up is not really worth any minute performance gain".


All posts considered, I think I'm going to go for a dead simple bottom hinge (as per the sketch at the top of the thread) and use a cover tape like this https://www.sussex-model-centre.co.uk/ripmax-gap-tape-20mm-5m (on the top) cut down so that it only just covers the <3mm gap that will open up at 10 degrees down-flap.

If the tape is a bit 'soft/floppy' then cutting down the gap-cover to only a few mm should reduce the chance of it being sucked up into the reduced pressure over-wing air flow. 

I might just round-off the top edge of the flap very slightly so that if I go from full-down to full-up there's no danger of the top of the flap catching on the rear edge of the tape.

 

Thanks everyone for the advise. Much appreciated.

David

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David

The flap catching a cover strip is always a danger.

If the cover is reasonably stiff longer is actually better than shorter as provided the strip is beyond the edge of the flap as it comes up it cannot jamb.

 

 

TopShieldFlap.jpg.c73a4b337b9a6ed25b3cc0ef0f63542d.jpg

At big flap angles the effect of the open edge of the cover strip will be insignificant.

Look forward to seeing the result.

No cover strip on this but it does generate a lot of lift.

https://www.youtube.com/watch?v=5drTnj4nnqY

 

 

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53 minutes ago, Simon Chaddock said:

David

The flap catching a cover strip is always a danger.

If the cover is reasonably stiff longer is actually better than shorter as provided the strip is beyond the edge of the flap as it comes up it cannot jamb.

 

 

TopShieldFlap.jpg.c73a4b337b9a6ed25b3cc0ef0f63542d.jpg

At big flap angles the effect of the open edge of the cover strip will be insignificant.

Look forward to seeing the result.

No cover strip on this but it does generate a lot of lift.

https://www.youtube.com/watch?v=5drTnj4nnqY

 

 

Great advice Simon. Point taken. Thanks.

I noticed that Hyperflight also do one. https://www.hyperflight.co.uk/products.asp?code=F-RMXGT1&name=gap-seal-tape-20mm Might be better than Ripmax.
I'm a bit incapacitated at the moment but when I've finished the flaps I'll post a pic.

Have a great year.

David

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