Design Considerations

So, in my last report on Jeticopter progress I was grumbling about not having data about how much a Jeticopter should weigh. Since then I've stumbled upon, online,  a very useful Frank Zaic yearbook from 1950, which as well as giving a wealth of good info of interest to the free flight fraternity, gives me some data on various Jetex powered rotary wing craft of different sizes. 

Now this table gives those weights, rotor diameters and rotor A of A numbers. Now what would be useful to know is how fast the rotor turns under power. Rotor speed would have been hard to gather in 1950.
Here are the Zaic tables and I've added metric parameters to the Imperial units of the original.
In table 1 we see Jeticopter sizes for motors in the range 50 to 300.
In table 1-1 I've shown the nearest equivelances between Tendera motors and Jetex.
In table 1-2 I've shown my current version which is both slightly smaller in rotor size than what Zaic records for a 100 size and heavier by quite a margain. However, mine does have about twice the power! 
But what parameters influence the behaviour of the delta hinge? I've already determined that with a positive rotor aoa effectively locked in that even at this weight the rotor is turning fast enough, even with just one blade, to lift the machine quite nicely . 
But will the rotor spin fast enough that under power centrifugal force it will maintain a flat rotor disc, and thus a positive aoa, during the power phase?
Now the rotor speed is influenced by the power of the motors and the drag of the rotors and hub etc. But, it seems to me, the tendency of the rotor disc to stay flat ought to be influenced by the rotor blade density. The blade and hinge system is somewhat akin to a centrifugal governor and so increasing the weight of the spinning rotors ought to ensure a flatter rotor disc under power for the same rotor speed.
Now, I do intend to measure the rotor speed at full rocket power under tethered conditions - using a cheap Chinese optical rev counter that I found. I would then, on a rig, spin the rotor at the same speed using a variable speed drill. Then I could see what the blade AoA appeared to be.  If it turned out that the rotor disc  was in fact coning up, and thus reducing the blade aoa, when the rotor was turned under power, that would indicate that either more speed is required or heavier blades. But I've not done this yet! Instead I've been concentrating on reducing the AUW and generally improving the design.
Weight reduction
The rotor hub and the parts which I call the cuffs, which hold the blades and are connected to the hub by the hinge pins, were rather larger than they needed to be. Additionally the motor mounts were heavy and the motor boom were heavier than I thought they needed to be. After making changes to all these parts I tried another test flight and discovered that my new lightweight rotor boom, now made of 3mm balsa was not strong enough.  This resulted in the boom arms breaking off as the rotor got up to speed and the still firing motors disappearing into the distance! On this occasion there were no onlookers!!. So I went back to 3mm MDF for the motor boom and was pleased to discover that with all the other changes the AUW was still under 95 gms. Additionally, for the next test I switched to 1 mm thick ply rotor blades figuring that this would help to maintain a flatter rotor disc by reducing the drag and increasing the mass of the blades. 
Hub Design
And I've changed the hub arrangement by copying the  style of my 50 sized Jeticopter that I bought in Finland. 
This style displaces the centre of lift to be further back from the rotor leading edge. I believe that this too is to help diminish rotor disc cone up under power. But I'm not sure. I need to seek out the work of F.G.Boreman who did so much pioneering work with free flight helicopters in the 1950s.
Anyway, this test turned out to be the first real height gain with a freely articulated rotor system. I didn't get as much altitude as on my previous,  locked rotor experiment and it was not clear if the rotor turning on descent was due to the in induced spin momentum from the power phase or actual induced autororation following an intended AoA change during the decent. Had it got higher that would have been clearer. As it turned out the Jeticopter came down quickly onto a hard surface and cracked the rear fuselage. 
So, what next?  I'm trying to add more lightness. And it strikes me that the blades may be bigger than need be. Of course to be a true Jeticopter we need to descend slowly. Reducing the blade area too much may prevent this. I'm also not happy with the hinging system, it's heavy, not as free moving as it might be and takes a long time to produce on the 3D printer. 
So I  now have another variation that I want to try. This is an entire hub cut, using the laser cutter, from 3mm ply. It's possible, by cutting a cunning pattern of slots into the ply to produce a compliant, no hinge pin, hub.  If this works out hub production will be made considerably easier and quicker. And the finished article should be still lighter.
Another idea I want to try is simultaneous electrical ignition. Getting a light up on two motors while holding the model in the launch position is not easy. I wear safety goggles and try and light first fuse at the end and the second fuse about the middle and this is not so easy.
A further test activity, in addition to measuring rotor speed, would be to carry the thing aloft with my drone and drop it. The drone should be able to carry the thing to a reasonable height and it would then be possible to examine the behaviour in the autororation phase and perhaps answer some of the questions regarding necessary rotor size and best AoA without burning through rocket motors.
Another fascinating question is the matter of free flight helicopters stability and how it is achieved. Recently I saw a  video of the excellent Penny rubber powered helicopter in flight. 
This version had two rotors and set at 90 degrees a boom with two small weights. I used to have a slightly different version that had symmetrical paddles instead of weights. Both styles seem to be very stable and can fly well.
Now, as I've seen, the Jeticopter version with locked positive AoA is also very stable, which is to say it maintains a level rotor disc under power. On the descent it was unable, due to the locked in positive aoa, to smoothly enter autororation. Instead, as it descended and the rotor speed reduced it flipped over. Now, with the required negative AoA established (because it had flipped over) it managed a slow and level descent. Perfect, except it remained inverted and the rotor touched down first! But this does mean that, like the Penny helicopter, a free flight helicopter can be made inherently stable even when the CG is above the rotor plane. This indicates that simple pendulum stability is not the driving force here. 
Now, on the Jeticopter, the two motors on their boom might be considered equivalent to the two weights on the Penny helicopter. So perhaps the stabilisation process for the Jeticopter and the Penny helicopter are the same? 
I cannot yet describe this stabilisation process.  Much literature has been written about helicopter stability but I haven't found much on stability without some translation motion. 
Full size helicopter dynamics soon becomes complicated as we move out of the hover and modern helicopters have all kinds of electronic systems to help in the different flight modes so in discussions of stability, pure inherent  stability are lacking. But I'm still looking. 
More next time.

I've now managed several flights with the Tendera powered Jeticopter and although there's still some work to do I am making progress. Additionally, I'm finding out a few things about rotor lifted craft. 

Having decided, after the earlier attempts that the model needed a considerable weight loss program, I created a new fuselage and landing gear. The new fuselage is considerably smaller and lighter than previously. Now, I haven't seen any figures for how much the various designs of Jeticopter/delta hinged rotorcraft are expected to weigh. With kit based designs, the target weight and the precise spec. of all parts is not always stated. One is expected to use the bits provided in the kit and what you get is what you get!

Anyway, with a new fuselage, 3D printed in lightweight PLA - based roughly on the Jeticopter outline and 'lofted' from the fuselage former outlines shown on the plan, I came up with a new all up weight (with used motors installed) of 102gms.

I decided to lock the rotor angle, for experimental purposes, to a positive AoA. From my earlier trials I'd thought it possible that, perhaps either the rotor was not spinning fast enough or the rotor blade mass was insufficient to ensure a positive AoA under power. - The Jeticopter hinged rotor works a little like a centrifugal governor. When spinning fast under power the rotor disc should be flat and the 'delta hinge' induces a positive AoA. At the end of the power run, as the rotor slows down, it should cone up and when it does the delta hinge induces a negative AoA. This to permit a 'autorotation' style - gentle - descent.  

Anyway, with a fixed positive AoA, if the rotor was spinning fast enough and the weight low enough we should at least see an ascent ibut not a autorotation descent. 

I decided to hand launch - to eliminate the possibility of the Jeticopter hanging up on the launch rod. So with two fresh L3s installed, and wearing my safety goggles, I lit the fuses (first one at the end of the fuse, second one half way down) and was pleased to see both motors striking up pretty much simultaneously. 

So, holding the model by the lower part of the fuselage I raised it up and released it. There was pretty much no wind and at first the model, clearly ascending, stayed pretty much directly overhead. This made it hard to judge the height and it was only as the motor run came to an end that I was able to see that the model was maybe 50ft in the air.

Then, as the rotor slowed down the model descended until, with the rotor pretty much stopped, it flipped inverted. Now, with a negative AoA the rotor started to rotate again (!!!) and so the model reached the ground relatively slowly and was undamaged. 

The interesting point here is how stable the spinning rotor is in respect to the horizon. I'd noticed earlier that regardles of launch angles and various ups and downs the rotor plane stayed horizontal. I'd put this down to a kind of pendelum stability but clearly with the inverted autorotation something else is going on. Now there isn't an abundance of technical literature on free flight helicopters (Jetex powered or otherwise) so for me the question of uncontrolled spinning rotor stability remains a mystery! 

And the next test flight was equally interesting. On this one I followed the same hand launch procedure and as I was launching one of the rotor blades came adrift. Perhaps it had clipped the fuselage or my shirt sleeve. I had intended to try and photograph this attempt but at this I didn't even bother to reach for my phone.

Neverthless the model gained height and likely got almost as high as on the previous flight. This time, as the remaining single spinning roto blade slowed, there was no sign of an autorotation (inverted or otherwise) and the model simply plumeted. Much to the amusment of several bystanders „Das ist ein gutes Spielzeug.“
„So soll es nicht funktionieren!“ 

What can we deduce from this? Perhaps that the current rotor size is actually too great? We'd loss more weight and spin faster with smaller blades. 

There's also the matter of the freely moving hinged rotors. Are we achieving a positive AoA under power? I've thought about a way of establishing this without firing off loads of motors but I haven't  had chance to try this yet.   

And there's still more scope in the design for weight reduction.

More on this next time.

 This version weighs in at 92gms - previously 124gms. 





Continuing with the Tendera powered jeticopter version. 

My original idea for the Jeticopter was to have the 'copter ascend from ground level and stabilised by a launch rod sticking up out of a board. The copter has  a hollow rotor mast which threads onto the launch rod.
I tried this a few times but never had a successful lift off. I was wondering if, in fact, the hinged blades were not actually producing enough lift. Centrifugal force plays an important part in the operation and if the blades are not spinning fast enough the blades could simply cone up and then, because of the hinge angle, not have enough of a positive a angle of attack to generate lift.
In order to test this notion I decided to test with a set of blades with restricted cone up.
One of my aims had been to make a rotor head without metallic hinge parts. This arrangement uses the flex of the printed plastic to allow a bend on the required hinge line without the need for a metal hinge pin. I had come up with a design for this and printed it but the articulation wasn't sufficient to give the appropriate negative AoA for a controlled descent.
But, I figured, this would be good enough for the test I wanted to make.
So, I added my new 'stiff hinged' hub and set off to the flying field. 

So, I finally got around to testing the Tendera powered Jeticopter, (or Terricopter as it has now been christened).

After finding the Jetex 50 size Jeticopter in Helsinki it brought it home to me that my ‘copter was considerably oversized for Tendera L2 size, even L2HT. However, I had provisioned for L2 and L3 sized motors so I decided to give it a go on L2 first of all.

It’s been sweltering in Munich these last weeks and I’ve been reluctant to go model flying. I’m without a car at the moment so all models and support equipment have to be hand carried to the flying area. And all the designated flying areas are well out of town. However, I told myself the very first test probably wouldn’t take long!

I’d scouted out a potential launch area within walking distance, an asphalt tennis court. Although it was surrounded by trees I guessed that getting the model stuck in a tree would be the least of my problems.

I set up in the middle of the court and was relieved to find the place was pretty much deserted. With a motor boom sized for L2 in place I threaded the model onto it’s launch rod and checked for free movement of the rotor around the brass tube. 

Our new Jetex Plans section has every plan we have ever published and much more, now in one easy to use location.

Through the years, an amazing collection of plans has built up on Articles were posted with various plans in them as well as pages created to link you to plans on the site based on motor size etc.

As the years went by, Jetex.orgs original site was archived but still available to allow our visitors access to the wealth of Jetex knowledge and plans that were posted. Although there, it made it a little difficult to locate that one particular plan you may have come here to find.

It was clear we needed to put all the plans together in one place, complete with a searchable index and categorised into motor size and model type. But why stop there? We searched the net and found every model we could powered by Jetex and added it to our collection!

To access, and use, this impressive collection, simply click on the Jetex Plans link in the top menu bar. Create a new account in the Jetex Plans Section here ( ) and log in to download any of the plans available. It is separate from the forum and will not work with your Jetex Forum information!

If you have a plan we haven't listed, and would like it added to our collection, simply This email address is being protected from spambots. You need JavaScript enabled to view it. it  to us (as an attachment) with, if you have it, a picture of the model.

As the Jetex store is now closed, we have also decided to make templates of these kits - like the Veron 'Quickys', Keil Kraft 'Shadows' as well as the many original designs like the Draken, Mirage III and Red Arrows Hawk available as downloads so that rocketeers can make their own versions of these unique models.



I  am very pleased that our website has (so far) resisted all attacks and I hope the shop will soon be open for business.  I'm not yet able to give the site the attention it needs - the gallery for example needs greatly expanding  and I've not been as assiduous with the 'blogs' as I should be.  I have also, so far, resisted all requests to go on to Facebook, Twitter, Instagram and the like!

It's not too early to be thinking about models for 2018, and, since we know what sort of motors we've got (powerful L-1's, feisty L-2X's and very nice, if low power (< 10 0mN)) L-2 LT's we can design/ size our models accordingly.  As some of you will know, I have a 'thing' about the idiosyncratic Swedish jets like the Draken,and Viggen and have medelled these more than once.  As to the earlier, rather more conventional  Lansen, read on! Some years ago Howard Metcalfe came across a very colourful ARTF Lansen:

Dr Zigmund, developer and manufacturer of our 'one shot' Rapier motors, delivered a fresh batch of three types of L-2's (L-2 LT, L-2-X and L-2 HP) when he visited Old Warden's model meeting in July.

I have now tested these with the following results