New towplane - own design

[pelle_gris]

Hi guys.

I haven´t posted much in this forum previously but I would like to present my new towplane for those that may be interested. It was designed and drawn during a long weekend and the build took a month - from first cut to maiden flight.

It is based on the British SDD Greenley (Large Model Association) meaning that the wing airfoil, tail length and shape of fin was copied from the available plan-set. The construction though was changed completely.
The thought is that this will take all sorts of engines, from 80-200cc, so to avoid problems with the CG, the firewall is located at the wing leading edge and a suitable length engine dome is then installed to keep the CG at the correct location.
The initial 3D model was used for calculating final weight as well as the CG. This was done in Excel and the spreadsheet was updated during the build with actual weights, and when the engine was finally installed the CG was dead on. That is a nice feeling.... :-)

Specs are as follows:

Span: 247 cm (97,2 in)
Length (without engine dome): 170 cm (66,9 in)
Length (with King 140RV): 216 cm (85 in)
Wing area: 1,16 m² (1798 in²)
Take-off weight: 15 kg (33 lbs)
Engine used: King 140RV boxer equipped with KS 90-5 canisters

The fuselage is constructed from 3 mm liteply, with 3 mm aircraft ply doubler in the front fuselage, tail feathers are made from 10 mm balsa sheets and the wing is foam covered with 1,2 mm Abachi. The span was selected so that my existing sheets would cover in one piece.
The wing is full length but Mk.2 will most likely have a two-piece wing instead, as well as larger flaps.

For the maiden flight I used a 3W 32x12x2 prop, but this seemed to produce mostly noise and not much power. Rpm was about 5000 and static pull, measured in high winds from the tow hook attachment, produced 23,6 kg (52 lbs) pull.
Subsequent flights was performed using a Fiala 30x16x2 prop and this seems much better suited to both engine and plane. Rpm´s about 5100-5200 and plenty of speed and pull. I plan on trying a Fiala 30x18x2 later to lower the rpm a bit, as the King engines are timed for low end torque instead of high rpm hp. And the low diameter coupled with the low rpm should keep the noise low. Going flat out either level, or in a slight dive, the prop will still rip quite badly, but most of the noise is produced by the carb intake. This can easily be heard due to the holes in the engine dome.

The maiden flight took place in quite high winds, which is normal for Denmark, and even in such blustering conditions, approach and landing took hardly any control inputs. It flew like there was a gyro on all axis!! I credit this to the slightly high wing loading.

Most 1:3 scale single seat gliders can be towed at quite a steep angle while still keeping the speed up. My own previous experiences using props with low(er) pitch have proven to provide much less power during the tow. In order to keep the speed up the towing angle had to be reduced a lot. With the 16" pitch there is also no chance of the sailplane overtaking the tug and the line going slack.
The largest glider I have yet towed is a 10,6 m (38,8 ft) Nimbus and this was towed to 500 m (1640 ft) in about 55 seconds. This was MUCH easier and faster than the previous tow using a Giant Stinger equipped with a piped G-62.... :-)

I don´t have any good pictures of the assembled model so for now you will have to make due with the 3D model.

[pelle_gris]

I can only attach a few pictures to each post, so there is a little follow-up here.

The airfoil on the SDD Greenley is quite unusual. It has a blunt leading edge which may contribute to its reluctance to stalling. You can fly it with full elevator at idle and it will only decend slowly keeping the wings level.

The tow hook is installed in the wing, at about 70% chord, copied from the SDD Greenley plan, and it seems that even when towing a large glider there is little effect on the trim of the towplane.

The wing and tailplane are set at 0° incidence. The Greenley has the same set-up but also shows 4° down thrust in the engine. I spent some time contemplating this but in the end I decided to also install the engine at 0°. No down or right thrust. And this have shown itself to be perfect, as no corrections seems to be necessary in any attitude.

The Greenley plan also shows quite a lot of dihedral but to keep the build quick and easy I have constructed the wing without. Actually there is a slight anhedral as the table I used to build the wing has a slight twist, but this does not seem to affect the model in the slightest.

Due to the quick build the finish also had to be quick, and it consists of no more than two coats of clear lacquer used for floors. :-) A possible Mk.2 will most likely be glassed and painted.

[pelle_gris]

Apparently the pics didn´t make it.

[lenb]

Hi Tinus

Thanks for sharing that tug design. It's looks good and resembles the might Pegasus.

Looking forward to seeing the actual model pictures, If you are having problems with posting photos, please let me know.

LEN

[pelle_gris]

So, during the last year or so, I have built two towplanes from my own kit as well as supplying two more kits for friends.
v1.0 died in a field late last year as the obechi covering on the wing loosened and was ripped off during a fast flyby. This led to a hard "landing" which destroyed the model completely.
The engine, canisters, most radio gear and the undercarriage survived though, and was re-used on the later v2.0.

v2.0 was upgraded with two-piece plug-in wings, removable tailplane and other small modifications.
This was ready early May of 2016, and was used effectively at a number of events, towing just about everything without much difficulty.

A few months ago though, during an ordinary flight, there was suddenly a weird sound coming form the engine, and then silence as the engine stopped. I did see something leave the model, but had no idea what it was.
So a dead stick landing ensued, and upon landing I saw, that everything ahead of the crankcase had left the engine. Propeller and hub were no longer there!!
The 15 mm crankshaft had broken for some reason, and without prior warning. I must assume, that the crash last year had weakened it, without me being able to detect it.

[pelle_gris]

So there I was, about a month ago, with a well-flying v2.0 but with a broken engine.
The King 140RV used until now is an older version that is no longer produced, so finding a new crankshaft may be a time consuming task.

As luck would have it though, one of the other models built from the kits were up for sale, as the guy who built it, is getting a bit burned out at flying.
Another guy I met during the summer expressed interest in a kit, to be powered by a Moki 250, and used as a training aircraft.

A deal was struck, and we bought the offered model together. I got the engine, and he took the rest of the model, ready to fly only lacking the engine. I also offered to supply him with an engine dome suited for the proposed Moki 250. This will of course be made at the correct length for the weight of the Moki, so it should balance without the need of lead.

The engine that is now installed on my v2.0 is a 3W 210 TS.
Being that the models are built from the same kit, the engine change consisted of removing the 6 M5 bolts that holds the engine dome on my model, installing his dome, complete with engine, throttle servo and ignition, and connecting the servos/ignition wires.
The canisters were different though, being much larger on the 3W, but with a new custom made bracket I could utilize the original mounting holes in the fuselage.

The 3W is quite a bit heavier than the King 140, and even though the engine dome is made for this engine, the CG of my model have moved a little bit ahead due to the fact that my fuselage/wing is a little lighter than that of the donor model.
At the moment the CG is situated at about 26% MAC, where I previously flew the model with the CG at about 29-30%, but this should not present much of a problem I guess. It should only result in a slightly more stable model, which is a good thing for a tug in my opinion.

The fueled weight with the 3W have risen to about 43 lbs compared to about 36-37 lbs previously. But I have seen the donor model fly earlier this summer, and the higher weight did not seem to affect the flying qualities the slightest. On the contrary, seeing that it´s almost always quite windy in Denmark, a high wing loading is a good thing for a tug.

Yesterday I did the first test runs in the drive way, and today I plan on a test flight before leaving for an event in Germany tomorrow.

When I bought the engine I got two propellers with the engine. A SEP 32x14x2 and a SEP 30x12x3. Both in the "Scale" planform, which are quite wide, so the low rpm of around 5.000 was more or less expected, even though I had hoped for a bit more. But with a propeller with slightly less load I guess the rpm´s will come up quite nicely.
But for now I´m sticking with the two props, as the thrust seems promising.

[pelle_gris]

Testflown this afternoon/evening, and everything works perfectly it seems. Plenty of power with the new engine, and a pleasant sound even with the 32x14 two-blade propeller.
It will rip a little going vertical, but not very much. I have not tried going full speed when flying level, as I´m afraid it will be quite loud then. :-)

The extra power is easily felt and is also verified by telemetry data.
The max climb speed has gone from about 17 m/s (3350 ft/min) to about 22 m/s (4330 ft/min) going straight up. Airspeed during the vertical climb have gone from about 50 km/h (31 mph) to 70-80 km/h (43-49 mph).

The landing roll have also been lengthened quite a bit, but I attribute this to the higher weight as well as the more forward CG. The approach to landing on the other hand is now rock steady. Not bad before by any means, but much more stable now. Now it should cope with high wind speeds even better than before.

[pelle_gris]

So, I found that the 3W 210TS and I would newer be a happy couple.
The 3W vibrated quite a bit, and I had to keep checking everything since the vibrations would shake things loose over time. The engine also did not have the nice low idle of the previous King 140RV, and on top of that the noise level was also a bit too high for my taste.

But early in the year I came upon some extra money, and decided to re-engine the towplane yet again, this time opting for a Valach VM-210 fourstroke.

As it turns out the Valach actually weighs a bit less than the 3W, primarily due to only needing one canister instead of the two giant MTW cans needed for the 3W, and I shortened the engine dome a bit to bring the CG back to about 28%.

The recommended prop for the Valach is a Fiala 36x14x2, but due to limited ground clearence, and a desire for more pitch, I opted for a Fiala 34x16x2 instead, and this was used on the test flight.
The rpm´s on the ground are 4.800 after running only 5 liters of fuel through the engine, so I expect this to rise a little bit when the engine is fully run in and adjusted correctly, but at the moment the engine seems to provide more or less the same power as the 3W.
The vibrations are also MUCH less than before, and at all throttle settings below 75% the engine makes almost no noise. Only when going full throttle does the engine start to make some serious noise, and there is also quite a bit of prop rip when flying level.
To combat the prop rip I plan on installing the Menz 32x18x2 I have for my Moki 250, as this should, in theory, spin about the same rpm´s, but with lower tip speed. If this does not help I may opt for a 3- or 4-blade prop down the line.

Due to the crappy winter weather in Denmark I have only done a few test flights until now, and no towing, so I´m looking forward to see how the engine performs with a glider on tow.

[Swiss1]

Nice work Tinus, let us know how it tows!

Jeremy and Ben
SCCAAA TT TN

[Dephil]

Dude, that exhaust is too beautiful for words. Rigid drop headers, flexi SS tube, 2-into-1 canister. So cool.

[pelle_gris]

Indeed it is...:-)
The flex sections makes it easy to install, as there is less chance of putting strain on the headers as is the case with normal rigid headers.
The canister itself is newly developed for the big Valach twins, and according to Toni Clarks website it´s the one producing the most power as well as the deepest exhaust note, the latter being provided by the extraordinarily large exhaust tube diameter of 28 mm. And even then it´s not much louder than the other canisters.

I drilled and installed the Menz 32x18x2 today, and I hope to run the engine tomorrow if the weather cooperates. No flying though as tomorrow also promises to be both wet and windy.

[pelle_gris]

More update for those that are still interested.

The 2017 season was flown with the Valach VM-210 with great success. Perfectly running engine with plenty of power for towing everything.

But this was not to be the case for the 2018 season....
During a tow early may the engine quit at low altitude during a tow, and the resulting heavy landing outside the runway ripped out the gear and hammered the silencer completely flat.
But that was not the worst, as the reason for the flameout was a broken crankshaft. The engine was sent in, and almost all internals was replaced on warranty.
So the trusty old 3W had to be installed again in a hurry to get ready for an event about a week later.

The engine came back some time later, but by then I had lost confidence, and I stuck with the 3W.
It had run perfectly, although with lots of noise and extreme vibrations.
Only other problem with the 3W was the fact that I, on two occasions had experienced complete loss of signal, requiring both TX and RX to be rebooted before connection was re-established. Luckily both happened on the ground with the engine at idle!!!
I suspected the 3W ignitions to be the culprit, and installed two RCexl units instead, with no more radio problems since.

[pelle_gris]

The 3W-210 had taken it´s toll. Everything was loose, hinges broken and servos needed to be replaced often due to the vibrations, so at the end of last year I decided to build a new v2.1 towplane.
The design is almost the same, the differences primarily aimed at making building easier.

I wanted another engine, preferably one that didn´t destroy the structure quite as fast, and if possible quite a bit less noisy.
ZDZ210, DLE222 and other engines were considered, but when I found myself with some extra cash at the end of the year, I decided to install a turboprop. That should get rid of most vibrations, and the noise level would also be much lower.
The price was higher than the other engines I considered, but by selling off the newly renovated Valach and an unfinished Wilga kit, I scraped the cash together, and a Kingtech K-100TP was purchased.

The build of the basic airframe went without a hitch, but when it came to the fuel system and the engine install everything started to go a lot slower.
Normally version two, or maybe three, of a design works as planned, so this was expected.

The first fuel system utilized a normal 1,9L fuel tank in the fuselage, connected to a 4,2L "belly tank" installed under the fuselage where the canisters normally go.
Unfortunately the chosen fuselage tank was way too soft, and turned almost completely round when fueling up, and imploded to a fraction of it´s former size when trying to suck fuel from the belly tank.
So a new stiffer fuselage tank had to be made to fit the already installed mounting system. This was made from a rolled 110mm diameter fiberglass tube fitted with milled endplates. This will hold about 1,1L, which mated to the 4,2L belly tank should give a reasonable run time for the engine.

Also the engine install went through two version before working. The first mounting utilized a normal engine dome mounting for the turbine itself, and tube stringers to hold the gearbox of the engine. But with a very long engine, it was not possible to get this system stiff enough, and the first time I went to almost full throttle the mounting twisted itself to pieces. There is a LOT of torque involved with a 670g Varioprop spinning. :-)

So the next version was a more traditional "bathtub" design with a removable top cowl. Actually it makes the whole model look a lot more elegant, as much as is possible for a flying box, and it was only a little heavier than the failed tube structure.

[pelle_gris]

Due to the large fuel load, the takeoff weight of the TP version is a little higher than the piston powered version, weighing in at 20,7kg (45,5lb) at takeoff, even if the zero-fuel weight is less.
But with the claimed 17hp it´s not really a problem, as the power seems to be plenty. I have only had a few flights dialing everything in, so no towing yet.

Luckily I decided to install Electron electric wheel brakes after the first test runs in the driveway, as it turned out, that even at the 1050 rpm idle the 19" of pitch will drive the model up to 30-40 km/h on a hard surface.

I´m running a 30,5x19x3 Varioprop at the moment, where the pitch can be easily adjusted, so I will experiment with the settings to find the optimal pitch.
The engine is claimed to provide most power at 6.200 rpm, so I may adjust the pitch so this is attained during towing. That way I would need to limit the power during level flight, but that´s what I usually do anyway. At the moment it tops out a 6.000 rpm going level, at an airspeed of about 160 km/h (100 mph), which is more than plenty. The old piston powered version tops out at 100-140 km/h depending on the propeller.

I have added links to two youtube clips showing a full power takeoff and the first landing during the test flight.

https://www.youtube.com/watch?v=D7FpzglTKMs

https://www.youtube.com/watch?v=EBwwqjIPz6U

[BryanB]

Turbine is very cool. Bet the guys in Colorado would love to have you tow even their largest sailplanes with that beast.

[horsefeetky]

can you post some close up pics and info o the jp brakes you used and how you mounted them