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martynk

Cheshire Cat 100S-E

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Jef Ott

Contradicting my previous post, Martyn's design has still got me well and truly enthused!

Here is how I see your spoiler answer Martyn...

Cheshire Cat with bigger spoilers.jpg

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martynk

That should be achievable. Opening those will be the equivalent of flying into a brick wall (hopefully without the associated damage..)

 

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Whitmore

Hi Martyn,

Just a few thoughts on dihedral:

I'd also recommend Curtis Suter's Sailplane Calc - it includes another check for roll authority 'VvB' which you will find useful for a RE design.

One thing to bear in mind is that when you look at the principles behind Blaine Rawdons B parameter you can see that spiral stability is dependent on CL ie speed. In other words both spiral stability and rudder authority degrade as you slow down And improve as you speed up. You may select a CL value for thermalling but if the plane lands at say CL1.0 you may find it sluggish in roll. It's worth checking VvB for the highest expected CL.

It's also then apparent that flying style plays a part. If you fly fast all the time you wont notice a lower EDA. Airfoil choice also affects things, for instance if the section is capable of high CL, you might want dihedral to be greater 'to get the use out of it' as it were.

Its also good to note that spiral stability and roll control are not the same thing. A model can mildly be spirally unstable yet have adequate coupling for roll control. This point is made by Mark Drela in the article here: http://www.rcgroups.com/forums/showatt.php?attachmentid=3995335&d=1304777459

So it's probable that some of the designs previously mentioned were 'unstable' ie left to their own devices they would eventually spiral in. This isn't a problem if the rate of divergence is moderate.

 

Another, quite important reason for dihedral is how it affects the model's turning ability. In an RE glider we are using yaw to create roll. Excessive yaw hurts the glide ratio/sink rate though so more dihedral equals less yaw in steeper thermal turns.

Roughly speaking more dihedral means the more efficiently it can work small thermals with tighter turns. The lighter the wing loading and longer the tail moment, the more important this is.

I had a quick look at some of the e-soarers on Hyperflight and the aileron models all had from 6 to as much as 8.8 degrees of dihedral. They don't need this for control, it's there for efficiency.

 

The other factor not mentioned yet is inertia. None of the sizing checks figure this in but its worth considering. More inertia means less authority. Drela does detail how to work out radius of gyration but I've never actually done this for a model. It's good to keep it in mind however.

 

Regarding tail moments: The B parameter increases with longer vertical tail moment and dynamic stability is improved with longer horizontal moment arms. I think of the vertical moment arm in relation to the span but the horizontal is related to the mean chord. 3-4 times the chord is the ball park but tails have got longer, perhaps as structures have got lighter and stiffer. This allows longer moments without associated inertia problems.

 

A heads up on measuring tail moment: The true dimension is from the CG to the tailplane aerodynamic centre (quarter mean chord.) Measuring from the wing AC is a very common fudge and will get you close with a normal lay out and tail volumes around 0.3-0.5. As you increase tail size and the CG moves aft of 25% this method will overestimate tail moment arm and tail volume as a result. Its probably not a problem if you stick to gliders - I think Sailplane Calc uses this measure anyway!

 

 

So just a few thoughts, based I ought to say, just on my understanding of the technical principles, rather than experience of 100S! There's not one right figure for these calculations - much of it comes down to personal preference and flying style as well as optimising for whatever aspect matters to you.

 

Jon

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martynk

Thanks for that Jon

I'll have another go with Sailplane Calc and see what happens. The two links are both very useful and I'll extend my spreadsheet with the additional calculations. 

All very useful stuff - much appreciated

My flying style - well TBH, I don't really have one, I am new at this. I do have pretty bad eyesight though (I suffer from double vision following a mountain bike accident about 15 years ago - it only affects my flying - trying to track a visibly small object against a low contrast background) so I need a model that can look after itself while I try and uncross my eyes  :rolleyes:. It has to be safe which is why RE models appeal.

 

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Whitmore

Martyn, I'd have no qualms about going for the full 12 or 12.5 EDA then. I think you'd get solid handling in those sketchy situations and reduced pilot work load all round. Providing the aerodynamics are right in other areas, the drag penalty would be minimal. At least thats what the theory suggests.

 

Jon

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Jef Ott

What is the latest on the Cheshire Cat 100"S, Martyn?

 

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martynk

Hi Jef

Apart from a little more work on the tailplane there has been very little progress :( Sorry about that.

The wing ribs still haven't arrived. :(

I made a jig to help me sand the bevels  for the rudder more accurately. That didn't work either :( I have a plan B though which should work

However, As HAL more or less said " I've still got the greatest enthusiasm and confidence in the mission." :)

I have had a bit of maintenance in the past few days but Bench no.1 is just about clear now as well so hopefully I may get a bit more progress in the next few days

I'll update with some more photos soon.

DSCN2882.JPGDSCN2883.JPG

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martynk

I also have a logo for the wing... :) 

Cheshire_Cat_Logo_v5.jpg

One for a Vinyl cutter to worry about :)

 

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Jef Ott

Nice logo :thumbsup:

If you have an Epson printer, I can recommend the Craftovator waterslide transfer paper for logos etc.

Happy to do it for you if you don't have an Epson printer. Don't know if other printers have waterproof inks.

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martynk

Thanks Jef

 

I do have an Epsom printer but I'll still get it done on vinyl. Tim at ModelMarkings.com is a very good site for this sort of stuff.  He did my Pink Floyd 'Wall' logos for a sport model I built a few years ago. They were great :)

If you don't know "The Wall" you wont understand it..

P7040007.JPG

 

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martynk

A bit of progress

No wing ribs have arrived yet so have decided to continue with the tail assembly

The fin has been built and hinged. I use the Kevlar cloth hinge material as usual

DSCN2898.JPG

And when completed and sanded it weighed this much.

DSCN2899.JPG

Not too bad. (That's grams, not oz)

The tailplane weighs this much

DSCN2900.JPG

A little bit heavier than I would have liked. I wanted to get that down to 12g, but at least it feels quite strong.

Yesterday, I slotted the C/F boom and after sliding the Vladimir C/F tailplane mount on the boom, epoxied the fin into place

This morning the tailplane mount was epoxied. The mount was too loose on the tube so was packed with a strip of 170g G/F cloth and all epoxied into place.

A jig was assembled to make sure the tailplane and fin are and remain at right angles to each other. (Apologies for the slightly blurred photo)

DSCN2901.JPG

and

DSCN2902.JPG

When dry, the next job will be to weigh some components and calculate just how long the nose really needs to be rather than rely on guesswork.

However, it may not happen for a few days, I am working away tomorrow and on Friday I am going away for a short break. Hopefully slope soaring on the cliffs above Barmouth will be part of the break :)

More to come

 

 

 

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Jef Ott
On 01/04/2016 at 09:23, martynk said:

That should be achievable. Opening those will be the equivalent of flying into a brick wall (hopefully without the associated damage..)

 

Letterbox type spoilers are a bit more controllable than the average brick wall, in the same way as you wouldn't go straight to the full extent of the elevator throws, give yourself more time and use the brakes to control the distance to the point that you want to stop at. (The same as you do in your car.)

Give yourself a mental target a metre above the point on the ground that you want to stop at, in case you hit bad air on the way in. If you hit good air on the way in, you will then have sufficient height and speed to go round again.

There are two ways to improve your landings.

1) Practise.
2) Repeat.

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martynk

Cheers Jef. I am away at the moment - camping just outside Barmouth. There is a wonderful SW facing ridge at the moment, however the wind is howling (30-40 kt) in from the North so no practice this week :( Its also blooming cold.. 

 

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martynk

Just a thought, what is a typical glide speed for a 100S sized model? Does 5m/s sound reasonable or perhaps is this a little fast?

 

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Whitmore

Martyn,

I think 5m/s is a bit slow to be honest.

You can work out flying speed using

V = √ (2Wg / ρSCl)

where
V = speed in m/sec
W = Model weight in kg
g = gravity at 9.81
ρ (rho) = air density 1.225 at sea level
S = Wing area in m²

Mark Drela suggests a thermalling CL of 0.7 for larger models and 0.6 for DLGs.

An Elf gliding at CL0.55 would be doing 5m/s (or 11.2mph)

Taking your wing area as 738in² and your lower target weight of 1.25kg gives a speed of 7.75m/s (17.3mph) at CL0.7 and 6.5m/s (14.5mph) at CL1.0. The latter is just a guess at stall CL without looking at airfoils etc. The higher target weight of 1.5kg gives CL0.7 = 8.5m/s (19mph) and CL1.0 = 7.1m/s (15.9mph)

Again, this is just number crunching rather than experience but hopefully useful.

Jon

 

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Jef Ott

Agreed, I would have thought that 12 - 15mph would have been about right, as a minimum for a 100" floater, that's about 6 or 7 m/s.

What is your thinking Martyn?

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martynk

Thanks both

Nothing in particular Jef, just trying to wrap a bit of theory around this model. I have never done a proper design exercise so am planning on working through the examples in Appendix 1 of Simons' Model Aircraft Aerodynamics substituting values for this with the worked example. That was the plan for evenings this past week but I never really got round to it. I am going to have to metricate it though.. Not too much of a problem

You cant take Imperial Slugs seriously

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martynk

Progress! but not much.

The wing ribs that I ordered about a month ago went missing in the post somewhere, so Dylan at LaserCraft kindly cut me a second set and I received them earlier this week.

DSCN2938.JPG

and

DSCN2939.JPG

I could never hand cut wing ribs to that standard.

As a gesture of goodwill, he also kindly sent me a set of 4 servo mounts as well,

DSCN2942.JPG

Thanks Dylan.. :) Very much appreciated

 

While I was waiting for the ribs, I did make some progress with the back end.

The sub fin and been built

DSCN2924.JPG

and finally, I have covered the rear end.

DSCN2940.JPG

and

DSCN2941.JPG

Getting there slowly.

I have also been considering the fuselage construction. My original intention - and still most likely path - would be to make a wooden pod like I did for the Bubble Dancer.

However, as I may end up making two of these (one as a conventional glider) it may be worthwhile carving a plug and making the pod out of glass or carbon. Jury is still out on this though

I can start making some more progress with the wing while I ponder this. The No.1 bench was cleared and vac'd this morning, so the wing should start coming together fairly soon

More to come

Martyn

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martynk

Not moving quite as quickly as usual. Now that its flying season, its also holiday season and I have not only been away for a week but also experiencing some mechanical issues with the motorhome which I needed to fix :( (Like no brakes :frantics:)..

 

Anyway - back to the shed.. :)

Getting the wing panels and bits built in the right order is causing a few head scratches.

Starting with the RH inner panel from the tile printed plan

DSCN2980.JPG

The delimiting ribs for the spoilers are doubled up, the inner rib has the spoiler cut out

DSCN2981.JPG

Main carbon spar caps have been glued up and a taper added to the boundary

DSCN2982.JPG

The first tricky bit is the wing joiner  blocks - cut from Basswood and drilled out carefully making sure that the hole is central and vertical. After the first attempt split the wood, the second attempt has oversized blocks - now need cutting and sanding back to size.

DSCN2983.JPG

Inner and outer end caps are Beech to add a little extra resilience and are about the right size. Outer end caps are only part drilled to retain the carbon rod joiner.

I have never been a great fan of slotted TE so have elected to use gussets at the TE to wing rib boundary

DSCN2984.JPG

More to come

 

 

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