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Outdoor Free Flight Forum => Peanut Scale => Topic started by: Prosper on October 11, 2014, 12:19:05 PM

Title: Miles M.35 Libellula
Post by: Prosper on October 11, 2014, 12:19:05 PM
I've been cut down to size where flying space is concerned, and with just a small patch to play in for possibly a long time, it's Peanut time again. I printed out the drawings of the M.35 Libellula a couple of years back but never made use of them. It's an arbitrary scale - I guess I wasn't concerned with scale size at that time. The span is 186mm or 11 1/4" (which puts it somewhere near 1/21 scale). It's at an advanced stage as you can see but there's still plenty to do.

Yesterday afternoon I made the prop (5"), and kept telling myself 'left hand, left hand, anticlockwise'. . .That way I got the blades shaped correctly. . .then after that triumphant achievement I went straight back to autopilot and fixed them to the hub in the tractor sense, not the pusher sense, so the hub is wider at the back than the front! Clever, huh? Not that the hub taper matters mechanically, but it looks pretty daft. The prop on its own weighs 0.9g which is good - I deliberately went for lightness because the way it's mounted should mean it's reasonably safe from damage.

The weight of the model so far is nothing to dance around about, but not tragic either. Being a tandem-winger the overall wing area is quite large. The construction is as per previous models I've made. Although I've skimped a fair bit for lightness, in some areas I think it's still overbuilt. One thing I recall about models this size is that they seem to stay in one piece pretty well. [Uuhm, should I have said that out loud?]

The one and only Miles M.35 Libellula was knocked together in six weeks from conception, in 1941. You can see that the rudders are M.14a Magister units, and I'm pretty sure that's also true of the main landing gear units and the wing centre-section. Apparently they did no wind tunnel testing and didn't know where to put the CG - so that makes two of us. This ugly duckling led to the beautiful M.39b Libellula of 1943.


Title: Re: Miles M.35 Libellula
Post by: TimWescott on October 11, 2014, 12:40:03 PM
A quickie thumbnail calculation tells me that the neutral point is around the leading edge of the rear wing, or a little bit forward.  So I'm guessing that you want a balance point about midpoint between the LE of the rear wing and the TE of the front.

But I think that what you REALLY want to do is to make a glider out of meat-tray foam or depron that has the same planform as the model, and experiment.  Then once you know what glides well, you can finish the model to match.

Title: Re: Miles M.35 Libellula
Post by: billdennis747 on October 11, 2014, 01:05:03 PM
Doug McHard's diesel version had the cg about 1/2" in front of the rear wing

Title: Re: Miles M.35 Libellula
Post by: billdennis747 on October 11, 2014, 01:13:18 PM
..the rubber and diesel designs are here


Title: Re: Miles M.35 Libellula
Post by: Prosper on October 11, 2014, 02:27:38 PM
Thanks for the advice, Tim, Bill. I saw the 1960 plan on the interweb thingy when I did some research, Bill, but didn't study it because it has flat plate wings which will behave very differently to the 18% thick Miles jobs I've copied. For the same reason I wouldn't make a 2-D glider, but thanks for the idea Tim which is certainly useful in some instances. Also, I think this model should be so light and relatively non-fragile that with some long grass it should look after itself. I was hoping the CG would be just slightly in front of the rear wing L.E. because that means definitely no noseweight, but if it's about equidistant between wings as Tim suggests I may need to add weight, depending on the rubber motor I choose. One reason I went for this design was a certain fatigue with scale tails that are too small and workable CGs that require tons of noseweight :). The tail of this model is a bit heavy because I didn't have a thrust button and matching piano wire to suit a Peanut; the installed kit is suitable for a much bigger model. . . never mind: with a name like Miles it should fly a long way. . .


Title: Re: Miles M.35 Libellula
Post by: billdennis747 on October 11, 2014, 03:28:55 PM
I was hoping the CG would be just slightly in front of the rear wing L.E. because that means definitely no noseweight,

Well, as I said, on Doug's model - which flew - that's where it is

Title: Re: Miles M.35 Libellula
Post by: TimWescott on October 11, 2014, 04:23:24 PM
Hey Stephan:  I'm pretty sure that for the purposes of determining a good starting CG point, flat-plate wings will give you essentially the same answer as thick ones.  It's a matter of where the aerodynamic center is with respect to the CG, and the aerodynamic center doesn't change with wing thickness (the required trim angle, however, probably does).

None of which matters at all, because if there's a successful free-flight model out there that balances a bit ahead of the leading edge of the rear wing, then you should probably just start there.  An RC model may want the CG farther ahead, and a CL model even farther, but that doesn't matter to you.

Title: Re: Miles M.35 Libellula
Post by: DHnut on October 11, 2014, 05:15:48 PM
            I knew Don Brown the author of the Miles book and on one visit there was a discussion of the M35. He was not positive about its flying characteristics. Don thought the M39 was a much better solution citing the forward wing position was in the right place.  They built a model that was towed and did not behave properly and the first take off was an accident when the power was reduced but allowed the CG to be sorted out, this was in Don's book. I also talked to Doug McHard at an RAFMAA championship about his model, as I was building one at the time, and he was not enthusiastic about its behaviour but it did fly but lacked consistancy. I have also seen one at a Peterbrough meeting and it was flying well. The build of the Aeromodeller plan was a nice exercise and the model which was destroyed in the move to New Zealand was never trimmed so I cannot comment further. I will watch this one with interest.

Title: Re: Miles M.35 Libellula
Post by: Prosper on October 12, 2014, 07:22:52 AM
Thanks for the very interesting post Ricky. As a kid I pored over that Miles book, borrowed many a time from the library. Wish I'd bought more of those Putnam books when I could afford 'em - they seem to fetch a very high price 2nd hand nowadays, too much for my budget. I've got the civil Miles stuff in the Putnam 'British Civil Aircraft' volumes, but nothing on the military.

I've read the story of the accidental liftoff on the web  - I suppose the particular thrustline of the engine was rotating the nose into the ground. Perhaps I'll be witnessing the same effect soon! I agree that the M.39b seems a much better worked out proposition - that's what happens when you build an aeroplane in six weeks from bits found in the back of the hangar! Another reason I wanted to try an M.35 model was that it would lead naturally to the M.39 which I thought would be a good choice for a first twin - however, although I knew the M.39 was small, it was only recently that I found out that it was actually super-small. A 1/24 scale model would have just a 17" span, and the wings are 20% thickness-to-chord at the root, and they're very high aspect ratio to boot :(. Maybe I'll try a larger-scale effort one day. It's certainly a lovely aeroplane (except perhaps for the angular auxilliary fin they seem to have tacked on as an afterthought. . .).

Bill, I hope that CG works for me too, and I understand what you're saying Tim, but as I see it, it's not just the thickness of the wings but how they behave as pitch changes. Flat-plate wings are more predictable in this regard, I suspect. I would think that as pitch changes the (scale thickness) front wing with its high aspect ratio might behave quite differently to the rear wing  with its broader chord and endplates. Then there's inertia to consider. . .anyway it's not too far from being ready for test chucks, so let's see . . .


Title: Re: Miles M.35 Libellula
Post by: Prosper on October 12, 2014, 11:52:01 AM
When I started mucking around with aliphatic-coated balsa I was aware of the potential for scribing panel lines, but it took me a long time to try it because I was skeered it would weaken the material too much. This doesn't seem to be the case. I also realised that embossing or scribing the sheet from the balsa side (the inside) was a possible technique, but a quick experiment showed that even using heat, there was a problem - it looked good just after doing the work, but after a day or so the marks would have almost disappeared! I suppose that's down to the rubbery quality of aliphatic glue plus the ability of balsa cells to recover their shape somewhat after being crushed.

I was disappointed by this outcome because I'd already a couple of projects in mind that might have exploited the technique. Anyway I thought I'd try it again on the Libellula. The rudders are Miles Magister units, fabric-covered, so I spent a few hours trying different approaches to imitate the fabric covering by embossing.

I had four rudder faces to play with and tried various tools (bits of piano wire; jewellers' screwdrivers etc) to try to achieve both the rib effect and the rib tape effect, and then the stitching effect. I used a candle flame to heat the tools as required. In the end it seemed that if the rib lines were orientated with the balsa grain then heat was unnecessary. Although I was daunted by the thought of impressing all those stitch marks, in fact this is a quick job. I judged the spacing by eye - no measuring - and just went dit-dit-dit along the line. The 'ribs' need to be strengthened with CA on the balsa side; this is not so much because the scoring and embossing is crushing the wood and weakening it (although this is the case) but because without some CA to lock the detailing into place, it will disappear over a period of time as mentioned above.

Pic 3 shows today's final effort (the outside stbd face), prior to a last trim and cleanup. In general I'm very pleased, though more practice is needed. There are several - perhaps many - styles of taping and stitching. This imitates the style where the 'stitches' are in fact knots, but looking at pic 1 you might just about see, on the lowest 'rib', where I tried to imitate stitches running across the rib.

Each fin/rudder assembly weighs 0.45g.


Title: Re: Miles M.35 Libellula
Post by: Prosper on October 16, 2014, 09:53:51 AM
Okay, I take it back about this being an ugly duckling, Miles old boy. It does look that way when you  see only the side view or the plan view, but in the flesh (at least without the gawky undercarriage, as seen in these pics), it looks very appealing, to my eye.

This is as far as I've got, and I may get no further. As seen it weighs 16.2g, nearly 3 of which is rubber, and 1.5 is noseweight. That gives a wing loading of 0.07g/cm2 or 0.46g/in2. Paint and landing gear plus sundry details will add to this. At this early stage of testing, the unfortunately large amount of lead in the nose seems necessary - it puts the balance point about midway between the T.E. of the front wing and the L.E. of the mainwing centre section.

The reason I may get no further is that it seems to display a profound death-wish when launched. Admittedly this is early days and I'm out of my depth with the trimming process on this one, and it was breezy. . .but it doesn't look too good. It's not pitch so much, but yaw. It has on one solitary occasion done a nice flat-turn into wind, but otherwise if it yaws it crashes, in the minimum time possible. When looking at the design initially I thought . . .oooh goody, it's got dihedral and sweepback. . ! However, it looks to me as if the effect of the two together is so powerful that there's no way the model can save itself from rolling violently and diving in. Alternatively, or additionally, it could be something to do with the thrust behind the CG? Although the thrustline is fixed relative to the CG, I wonder if inertia could cause an off-centre component of thrust at the start of a yaw/bank. . ? Someone please let me know if that's ignorant rubbish.

I made the front wing such that it should release in a crash, since it's one of the most vulnerable bits. It does release after a fashion, but can cause 'collateral damage' in the process, so the model's already picking up scars.

I hope I'm wrong, and have just got the basic trim out of whack in some way - I'll learn more when the weather allows. I've been fiddling with ways to make acceptably light-but-scale-looking landing gear - the photo shows a maingear leg from bamboo wrapped with tape and silver mylar, and a fork made from al. tube - weighs 0.15g. The oleo links will probably be plastic card - haven't worked out how to fix them yet.


Title: Re: Miles M.35 Libellula
Post by: Pit on October 16, 2014, 11:19:39 AM
Squeak in a bit (maybe, a lot) more dihedral in the canard/foreplane.  Make SURE that lateral balance is equal and incidence on both sides of both wings is exactly the same at each station along span.  Check that the fins/rudders are perfectly aligned.  Thrust line - shaft end where it exits the prop should point UP a bit (go easy here, the thrust line will have to be worked out gradually).

How much decalage do you have (about 4-5° might be necessary with 90% on the foreplane), but at the small scale you're at, the airfoil used on the wings will play a very big role (flat bottom - Clark Y'ish for both) - I doubt very much that scale airfoils will work in sub-peanut size.  Post dead-on side, front and rear view photos and one dead-on plan view.

This model is quite similar to the "Shark" canard pusher I've been working on and off for the past couple of years.  Initially had similar "rolling off" issues that were solved with added span in the canard along with a flat bottomed airfoil (original canard was semi-symmetrical).  The "Shark" is 18" span with raised wingtips (dihedral) but very close coupled.  

Title: Re: Miles M.35 Libellula
Post by: Hepcat on October 16, 2014, 12:53:41 PM
I can quite believe what you say about the lack of yaw stability and subsequent roll.  We are not used to looking at Canards with their rearward CGs and consequently short fin moment arm and are always surprised at how much fin area is needed.  This is worsened on the Miles with its bulky fuselage giving a lot of side area at the front.  Further than this however I think that the fuselage is causing much more trouble if a yaw is initiated.  Imagine that the aeroplane yaws with its nose to starboard and consider what the airflow is doing over the starboard wing.  I think behind that big, flat sided, fuselage it will be a turbulent mess with loss of lift on the starboard wing.  Conversely the port wing may actually gain some airspeed from the adjacent fuselage.  In addition to this of course, with the acutely swept back main wing, a yaw to starboard will promote a roll to starboard because of the increased efficiency of the advanced (port) wing.

Removing the dihedral from the foreplane may help a little but it is so small already I don't think it would be worthwhile.  As it is a scale model there isn't really much that can be done.  I suppose, in the interests of science, you could replace the rudders with two enormous ones to see if a side area balance looked possible.  Perhaps some form of pendulum control is needed. ;) :) :)

Title: Re: Miles M.35 Libellula
Post by: Prosper on October 16, 2014, 02:17:51 PM
Hi Pete, good to hear from you. Thanks for the analysis. I think increasing the incidence of the front wing could be a good bet - it's hard to gauge the true incidences of the wings because it's such a curvy aerofoil section. I should've paid more attention to the 'line of mean camber' or whatever they call it! The lateral balance is spot on, but thanks for raising the point, I'd rather taken it for granted and only tested it on your prompting. I agree about thick wings on small models but that's the dictates of scale. Miles aircraft (excepting the Gilette Falcon and the M.52 I guess!) were largely characterised by their absurdly thick wing sections. The matter is worse on the Libellula rear wing where there are large gaps between wing and control surfaces. BTW I think the model is quite 'true' in terms of surface alignment (as my models go :o ) - but will try to get some telephoto shots tomorrow when (if) there's more light.

Hullo John, those are very valid points. I had worried about the forward keel area but then forgot to worry about it, because I couldn't estimate the leverage of the endplate fins (doesn't the fact that they're a long way from the CG, stuck on the end of a wing, give them more leverage than an equal fin area mounted on the centreline of the fuselage at an equal distance aft of the CG in side elevation?). Also I hoped that the pusher prop would add to yaw stability. However, the potential disruption of flow in the fuselage 'shadow' hadn't set alarms ringing - probly should have.  Anyway, a further forward CG might ameliorate both the yaw and fwd keel area issues, and matched with the correspondingly greater incidence on the fwd wing might improve pitch stability too. . .

Lor, pendulums on a tiny model that's already rather too heavy - no thanks ;D


Title: Re: Miles M.35 Libellula
Post by: Prosper on October 17, 2014, 10:47:43 AM
Lorks, it seems to be showing some sulky inclination to fly :o (in a straight line only). I increased the front wing incidence (thanks Pete) and added a bit more noseweight. If it ever looked likely to need more rubber then that would do for noseweight as 2/3rds of the motor is ahead of the CG. I also added a bucket of left-thrust and fiddled with the rudders a bit.

There's a significant but daft problem with the rudders. As far as I can make out, each rudder travelled outwards only - the way the fins are mounted to the wing would have precluded inward rudder travel unless the wingtip itself was cut away over a considerable part of its chord. Not impossible but unlikely I'd have thought. Now, my rudders have bag-tie wire hinges. As bag-tie wire afficianados will probably concur, to get this stuff to bend to the right point you have to over-bend it, and it springs back a bit, you hope to the exact point you'd estimated. Well you can't over-bend these hinges - there's a wingtip in the way :(. So once the rudders are bent outward it's impossible to get them back aligned with the fin. I had to demount one and reglue it in the neutral position.

More when the weather allows.


Title: Re: Miles M.35 Libellula
Post by: Prosper on November 07, 2014, 09:16:31 AM
Update: the weather has been combining wind and/or rain in imaginative ways for a long time now, but I did manage to fly the model in breezy-but-dry conditions a few weeks ago.

The good news is that the model finally showed some promise, by demonstrating some basic essential skills. It managed a nearly-full orbit several times, and in doing so showed its ability to negotiate the downwind turn. Also it showed positive recoveries from some 'unusual attitudes'. The best flight time was 8-10 secs (counted "manually" if you see what I mean) but we're still on low turns, far fewer than the motor can store. I found that resorting to a large sidethrust angle (pictured) improved yaw/roll stability greatly. I can't add any more without the prop striking the T.E. It still lost control in a right-hand yaw/roll sometimes (quite a gusty little breeze) but was much better mannered most trips.

The bad news is that the 'detachable' fwd wing was cumulatively damaging the fuselage as it detached in crashes. I decided to glue it in place, whereupon it just started damaging itself in crashes, ho-hum. With the weather set wet'n windy for the foreseeable, I'm putting the model aside until I think of a solution to the fwd wing weakness.


Title: Re: Miles M.35 Libellula
Post by: FreeFlightModeller on November 07, 2014, 10:50:31 AM
Hope that you don't mind me posting this .... I took this shot on my second visit to Old Warden in 2003 for the Scale Day.

This was one of the entries in the McHard memorial event .... wish I had bought one of the McHard models that was on sale that day.

Title: Re: Miles M.35 Libellula
Post by: Pete Fardell on November 07, 2014, 11:47:28 AM
That's a very evocative picture, Russ. I rather like the way the windsock's in shot.
(And yet another aero-modelling legend I wish I'd seen in action.)

Title: Re: Miles M.35 Libellula
Post by: billdennis747 on November 07, 2014, 12:11:44 PM
Looks like Steve Bettney

Title: Re: Miles M.35 Libellula
Post by: Prosper on November 07, 2014, 12:33:40 PM
Hope that you don't mind me posting this ....

Of course not Russ. Like the man said, a very evocative shot. I think from what I've seen of my M.35, that it will fly if set up right, and it appears from what's been said in earlier posts that others have made it fly. I could well believe Ricky DHnut's report that it "lacks consistency" though! I see that the model you shot has increased fwd wing dihedral - don't know what bearing that might have on things.


Title: Re: Miles M.35 Libellula
Post by: FreeFlightModeller on November 07, 2014, 12:42:48 PM
It's a while since that photo now Stephen .... but I do remember that there was a 'struggle' with the model!. As I remember, this and other launches ended with a descent into the ground. Could have been lacking power, but as I say, a bit too long ago to give the full story.

Thanks for the possible identification Bill ... I have wondered in the past. Am I right to think that he is deliberately dressed in a 'McHardesque' way?

Title: Re: Miles M.35 Libellula
Post by: Pete Fardell on November 07, 2014, 02:27:11 PM
I just assumed it was McHard himself, and that Bill was referring to the other fellow. Shows what I know.

Title: Re: Miles M.35 Libellula
Post by: Prosper on December 29, 2015, 11:26:29 AM
Update: I got this down for repair. It needed a new front wing, and if ever I caught sight of it on the shelf I winced because I'd made the left-hand prop with the hub
back-to-front, i.e. wider at the back than the front, and this looked really stooopid. The model was very tail-heavy so first I removed all the prop's tackle which was
unnecessarily heavy for a tiny model like this, and replaced it with light-duty stuff, including the most basic bent-wire-in-ramp style freewheel. Bearing in mind my
stupidity in having made left-handed prop blades only to cheerfully fit them into a right-hand hub, I was very watchful. So you can imagine how furious I was when I
realised that on filing a ramp into the brass prop bushing, I'd filed it into the front not the back (gnashing of teeth). I now had a left-hand tractor prop, not a
pusher. . . I squared-off the ramp and double-triple checking that I was about to attack the correct end of the bushing, I filed a ramp into the correct end
of the bushing. Only trouble was, I filed a right-hand ramp not a left. There was steam coming out of my ears at this point and I was on the verge of placing the Libellula model tenderly and carefully on the floor AND JUMPING UP AND DOWN ON IT. . .

Luckily, there was just enough brass bushing left to get it right. This is only because the prop is at the back and completely protected from stress - it would be too weak
for a nose-mounted prop. After all this nonsense I had saved 0.55g from the tail, which I guess is a significant amount for an 11" span model.

I made the new front wing with a much better 'knock-off' than the original. There's a wire clip at the centreline and a strip of rubber fixed across the fuselage is hooked over this clip.

I first tried a very aft CG, with which the model weighed 14.2g. The model didn't do anything that looked much like flying (but did stupendous vertical torque-rolls and slow
barrel-rolls very well :) ). In the end I've got the CG well forward. This is a shame in that it needs a gram of lead in the nose, but the model flies! Today I got a proper flight (in very breezy and blustery air). About 15 sec from about 1/3 of the available turns, at which point the model hit a tree and dismantled itself as the photo shows.

Apart from today's crash which will leave scars, the model has taken a bashing during trimming attempts and is very lightly constructed, so it's looking rather beaten-up. I
think I'll repair it and see if there's any consistency possible with it, and then if it does fly predictably I might make a new one at some point. It's a very easy and quick build.


Title: Re: Miles M.35 Libellula
Post by: Prosper on March 10, 2016, 12:45:09 PM
Well, I finally gave up on this one. I tried hard but the broad facts are that:-

1)it needs large 'decalage' and a very forward CG to be pitch stable. In this condition the huge drag of the 20% thick main wing and 18% thick forward wing make it descend (very stably, mind you :) ) even under full power. To contrast, the same power with an aft CG will have it launch like a rocket ship. The forward CG implies noseweight - lots of it - and the whole initial premise was that here was a layout that wouldn't need noseweight!

2)there is little roll/yaw stability. Trimming is complicated by the fact that the hz control surfaces are both elevators and ailerons. The model will fly straight or turn either way; it seems impossible to persuade it to turn one way and one way only. Also, it will roll out of control as often as not. It has shown itself able to make repeated tight circles, on the one flight it's made in dead calm air, but 'calm air only' is not a useful restriction for an outdoor model.

Another thing I found was that although the model could (usually) withstand full-power dives nose-first into the ground, it would often hit on a wingtip. I was quite proud of the fin/rudder assemblies because I think they were my first attempt at all-sheet jobs with rib tape and stitching embossed into them. They were scale in appearance, light, and I would call them strong - but stuck out on the wingtips of a model that crashed viciously and often, they got mashed.

I think the layout might work with thin wings, but a - perhaps the - major characteristic of Miles a/c was their exorbitantly thick wings.

Title: Re: Miles M.35 Libellula
Post by: Art356A on March 10, 2016, 01:12:44 PM
My M39b always flew better than expected, which is not to say that it flew really well. I think it has more potential than I was ever able to get out of it. 

The 39b also grew a big fixed wire braced vertical fin, evidently as an afterthought, and they must not have gotten that far into the development of the 35 to arrive there. The 39b flew for several years and was only broken up after the war.


Title: Re: Miles M.35 Libellula
Post by: Prosper on March 10, 2016, 02:30:58 PM
Interesting, Art.  I wanted to build a 39b but that's at the bottom of the list now. Was (is) yours well behaved in yaw/roll?