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Mar 15, 2017, 10:12 AM
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The big trick with cutting off a nose cone for that whirly thing is to get the down-and-side-thrust right. I"ve done it with a TopModel Gin that had a nice circular nose cone cross-secdtion, but it's a lot easier to figure the thrust angle with a tail-plane out on the end of a tailboom. It's bound to be a bit critical for a plank.

hmm... i need to research "Dodgson couplers"...

ed
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Mar 16, 2017, 08:55 AM
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Carolynne, in going back over this excellent thread, I took another look at your airfoil. While the MH22 has a very good L/D profile (using Profili) it has a fairly substantial negative Cm/4. It's been my experience that using other similar airfoils, I have to add some up elevator to compensate for e negative pitching moment, thus degrading the lift curve. I've never attempted to quantify it, so I only suspect the effect.

I tinker with mixes of airfoils all the time. One that you might try in the future is a 50/50 mix of the PW75 and PW1211. I named it in honor of Peter Wick: the average of the two airfoils' names: the PW631. (Peter, if you read this, I hope you won't be offended at my presumption! I'll rename it if you like.) Since you use Profili, Carolynne, you can check it out. The PW airfoils all have the desirable trait of minimal variation of pitching moment versus angle of attack. The PW631's pitch curve is very smooth, from Re 150,000 to 350,000. Not as "lifty" as the MH22, but might have smoother behavior in the air, especially in turbulence. I'm going to use it on the next iteration of the TigerMoth: I have orders for short kits from my three flying buddies. Now to find the time to cut some foam...

Anxiously awaiting more photos,

Ed
Mar 16, 2017, 06:47 PM
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Hi Ed,

My good friend Tim, was kind enough to assist with the Xfoil / Profili optimisation of the Calliope during the long time in which it was just a glimmer in my eye. During that time, lots iterations were tried and many of the things that you commented on were both noted and examined. One of the things that came up was the calculated polars for the MH22. This did indeed show a Cm that jittered around zero with alpha. This caused me a little concern for about 30 seconds until I remembered a few things:

Firstly, I had spent a lot of time watching Klaus' wing flying very nicely indeed with this airfoil at nearly the same chords and constructions. In fact, the initial drawings for the Calliope used rib profiles taken from the plans for Klaus' wing.

Secondly, I recalled that the the various programmes used to do the analysis are in fact just attempts to simulate the real world. Consequently, while they can be both fun and useful, they should not be taken as gospel. Anecdotally, when Tim and I collaborated on the Anara nuovo canard, Profili et al produced some hilarious predictions that were subsequently shown not to occur in the real world. Similarly, the programme's predictions that the final iteration of the Calliope would have a best L/D of nearly 50:1, I took with a large amount of salt....

Thirdly, experience with the lil Plank, which uses an aerofoil with a positive Cm shows that this can sometimes be a bit of a liability when trying to design a versatile aircraft. So, I was comfortable with the near zero pitching moment of the MH22

Tim also did some analyses for me with varying CG and elevator neutral position and measured resulting stability and performance in terms of span wise lift distribution and best L/D. This showed that with a forward CG, and 3 degrees of up elevator a very stable aircraft would result with a best LD only a couple of points less than the best calculated.
This is the reason for my comment somewhat earlier in this thread that I might experiment with using the ailerons as a trim surface to distribute the trim load, should in this case the computer reflect reality.

I am not very familiar with the PW series of airfoils, but from what I do understand, they are more oriented towards low drag at low alpha and so are not quite what I had in mind for Calliope's 'mission'. Anyhow, at this late stage, the chance of me changing aerofoils is approaching zero asymptotically!

As far as photos are concerned, really only fiddly details have changed since the last photos posted so there is not much new to see. Nevertheless, here are a few photos that I took for Tim last week:

Carolyn
Mar 17, 2017, 07:52 AM
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I didn't mean for you to rebuild the wing, honest I didn't! But I've always wondered how well the MH22 did in "real" life. I've used the M44, MH64 and MH 60, and they did fine, but they were on chevron wings, never planks. So I'm looking forward to the flight tests!

Actually, even if the computer simulations aren't reality, and you can get a 50:1 glide ratio from them, I'd stick with the illusion.

ed
Mar 18, 2017, 06:58 PM
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Thank you Ed,

I was so very worried that I would disappoint you if I didn't remake the wings...

To be very honest, I don't really care about a 50:1 glide ratio or any of the performance related stuff. I just hope that the Calliope is pleasant to fly with not to many 'nasties', and hopefully looks lovely in the air.

Carolyn

Oh by the way, the nose has been cut off, the firewall added and the spinner aligned to the fuselage.
Mar 18, 2017, 10:21 PM
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Ah, Carolynne, I'm so glad to know that 50:1 isn't a valid measure for you! "Pleasant to fly" is the real measure of cost/benefit in this sport soaring game, And yes, "lovely in the air" is on the benefit side of the equation too.

There must be a way to choreograph our flying styles to fit the lift conditions and the airframes. This would of course require a fairly large garage-full of of sailplanes to fit the various flying sites and weather conditions for each one. I must go order more balsa, carbon fiber tow, tubes and rods and some more foam. And find a good videographer...

The latest video of the ResDart soaring the micro-lift in near-zero-lift conditions is a good example. Just lovely!

ed
Mar 27, 2017, 07:34 AM
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Carolynne: I'm about to get serious about a thermal plank, using an electric powered fuselage left from a TopModel 2.3m Gin. The Gin flew beautifully at 14 oz/sq ft. If I increase the wing area of the plank conversion to bring the loading down to 12 oz/sq ft, it should have about the same cruise speed as the original. Bootstrap engineering admittedly.

But this will be the first time I've designed an aircraft that doesn't need inverted flight abilities. You mentioned tweaking the MH 22. Would you care to share the improved version?

Did you preserve the thin trailing edge and reflex curvature of that part of the airfoil? I suspect some performance is lost on many kit planks by using tapered trailing edge stock strips that have flat surfaces.

cheers,
ed
Mar 27, 2017, 06:49 PM
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Firstly, a little bit of progress: a picture of the nose with its elegant bespoke spinner fetchingly displayed.... Fuselage servo arrangements to be made next, and I still haven't settled on a colour scheme.

Ed, as for modifications to the MH22:
I wanted to keep the taper of the main-spar constant across the span, despite the large planform taper break for the extended elevator area. This was because I didn't want to use an enormously deep root airfoil which would otherwise be required. To achieve this, the root chord was initially part of the straight taper. this root airfoil was then stretched aft from the spar position to achieve the extended root chord. Various angles of stretch were modelled and checked ( by Tim { I told you he was good!}) and a version that provided a lovely and unbroken lift distribution across the span chosen.

Regarding your question about the trailing edge: I rather agree that a thick trailing edge, in addition to being aesthetically un-pleasing, is probably also inefficient. The MH22 is substantially flat on the top surface from about the 30% chord point aft to the trailing edge. The reflex on this aerofoil foil is largely seen as a disposition of curvature on the lower surface, with about the last 10% of the chord being increasingly curved to meet a sharp trailing edge. This is being as closely matched as i can manage using a trailing edge construction with a fibreglass composite core. (In case you're interested, this was detailed in Tim's Anara Nuovo thread on RC Groups). this methods allows a tough trailing edge that is literally sharp enough to cut skin! This last reason is why the final shaping of the TE is left until last...

Carolyn
Mar 28, 2017, 08:43 AM
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Thanks Carolynne for the information about the airfoil mods. That was an interesting airfoil design challenge.

I am making up a few TigerMoth kits for friends, and I wanted to keep the construction as simple as possible, but not lose any more efficiency than needed. So the TE is 1/4 x 2 x 34 inch tapered TE stock, with the reflex curvature sanded into the bottom surface, leaving the top surface flat. Just as you describe.

Great minds spin in the same circles...

It just occurred to me that an efficient airfoil TE might not be the most efficient control surface, especially aileron/elevons, where the deflections are often LARGE departures from where the efficiencies are measured...

More spinning circles...
Last edited by EdSoars; Mar 28, 2017 at 08:52 AM.
Mar 29, 2017, 03:33 AM
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Oh Ed! This could open a can of worms!

OK OK, let me get started:

Lets define what we're talking about by 'efficiency'. I like the physics approach which says colloquially, that something is efficient if you get a lot out for minimal input. Aerodynamicists use this idea and refer to L/D ratios as a measure of efficiency as it applies to both aerofoils, as well as whole powered and glider type aircraft. Because I rather like aerodynamicists as a breed, I'm going to go with their definition.

So, moving chunks of trailing edge around is going to change an aerofoil and this may change the efficiency of the aerofoil at that AoA. I am sure that I don't need to recapitulate the old discussion of large surfaces through small angles vs small surfaces through large angles, or even the contribution of control surface aspect ratio and location to control effect. And because I don't need to go into all of that aforementioned stuff, I'm going to throw out another thought:

Here's how I think about a thermal soarer and its use of efficiency; (please note that my thoughts on this subject are heavily influenced by the writings of a chap by the name of D Thornburg, the thoughts of a pilot by the name of G Moffat and discussions with another pilot by the name of D Hobby).

When a thermal soarer is in a thermal, it is far more productive to firstly stay in the thermal and secondly be centred in the area of strongest lift, rather than worrying about flying efficiently and in so doing being booted out of the thermal. Consequently, I feel that it is a reasonable tradeoff to wave control surfaces around as necessary in decisively centring lift before settling down to a less exuberant ride to altitude even though this may invoke a temporary drag increase and hence efficiency loss.

Conversely, in finding lift, I like to have an aircraft that is trimmed for hands-off straight flight, hopefully in an efficient, ( best LD [ and no I am not going to talk about Macready 'speed to fly' stuff here]) ground covering glide, so that any wriggles in its flight path are due only to the action of the surrounding air rather than any random control surface input of mine. In this way I hope that the aeroplane will happily tell me when it is near or in lift.

So, from this huge diatribe, you can see that I don't think that control surface drag is a huge issue in a well trimmed thermal soaring type aeroplane when flown well, and I am not really sure how much or little a sharp or otherwise trailing contributes anyhow. The Calliope will have sharp trailing edges because I think that they will help with the best L/D side of things and because they satisfy my aesthetic tastes for what I would like to hope might be an 'efficient' ( and funky looking ) aeroplane.

Carolyn

( oh by the way, in case you hadn't guessed, I have a background of lecturing to students......
Mar 29, 2017, 05:18 PM
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Ah HAH! So you have a background of hectoring students...

I built several chevron wings using the MH-44 section (the "Talus", else-where and other-time on the Slope forum), with a 50% chord tapered TE stock that let me increase the elevon chord toward the tips. Not wanting to sand any more than necessary, I left the stock with flat surfaces and 1 mm edges. On Profili, the modification actually improved the LD curve: a wider, smoother drag "bucket" than the original with a sharp edge.

Go figure...

And always fly with verrrry small stick movements!

ed
Mar 30, 2017, 08:16 AM
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Carolynne; I looked up those "Dodgson couplers" you mentioned on report #60. I actually remember them from my early thermal RC soaring days in the mid-1970's. They were a mechanical means of mixing two functions before the days of programmable transmitters, used on many of Bob Dodgson's kits, which were the avant garde in those days of early use of ailerons on thermal ships. And they had elegant lines, too. I flew a used Camano for a few years... sigh.

But your application is unclear: Are you using a coupler to link two elevators to one servo, or elevator trim to split rudder?

And could you post a sketch or photo of that split rudder - servo linkage?

ed
Last edited by EdSoars; Mar 30, 2017 at 08:21 AM.
Apr 01, 2017, 10:09 PM
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Hi Ed,

Using the Dodgson coupler allowed me to drive both elevators from a single fuselage mounted servo placed forward in the fuselage. It also allowed for plug in wings as well as accomodating both axial and angular misalignments of the hinge lines. And it can be arranged so that the whole hookup is not visible in the assembled aircraft. Altogether a very neat if slightly fiddly thing.

I haven't finally sorted out the rudder linkage yet, but it will be very simple, with a single servo for each side mounted forward in the fuselage driving the rudder half with a thin piano wire pushrod.

Carolyn
Apr 01, 2017, 11:08 PM
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This build gets more and more interesting....
May 01, 2017, 08:13 PM
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Primer!

Don't you just love the mind numbing joy of sanding and filling?

Seriously though, I think I'm becoming a little obsessive about the Calliope. I have to keep reminding myself that this is an untested prototype of a fairly unusual configuration. Lots of things could go wrong! But, I do love the shape and do want it to look nice, so it may well take awhile for me to launch it when it's finished.....

I wonder how Kent's version is going?

Carolyn


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