Graupner Sukhoij 26M
A review by Trevor Hewson first published in 1995/96 (in two parts) in 'Sloping Off', the newsletter of the Christchurch and District Model Flying Club
This model was built and reviewed by Dave Chinery in the January 1995 edition of Silent Flight. If you do decide to build one for yourself, I would recommend you read Dave's review - I can lend it to you. It certainly helped me avoid one or two constructional pitfalls - like the servo plate that definitely cannot be manoeuvred into position at the specified point in the building sequence! If, on the other hand, you have no intention to build the Sukhoij, then constructional details are not of too much interest, so I will confine myself to those points which could conceivably influence this key decision in your life.
This is a Graupner kit, and Graupner have a quality reputation. By and large this kit lives up to this reputation but, by Graupner standards there were a few niggles. The wood was of variable quality, particularly the 1.5mm sheeting. This comes in a variety of different widths so there was little scope for matching the grades of left and right hand parts. One wing rib (I'll tell you which one for a small fee!) was incorrectly profiled. There was a shortage of 8mm square strip wood and although for once the wing tip blocks were big enough, the tailplane tip blocks were not! Somewhat surprisingly there are also a few errors of part labelling on the plan and in the instructions.
Don't get me wrong, this is a good quality kit. The things that matter are right. The parts fit, the ply parts are precisely cut and fretted out to save weight, the plastic cowl, dural undercarriage and combined cockpit/rear turtle deck moulding are all excellent, and, at under £100, you can't quarrel with the value for money.
How's it made?
All flying surfaces are built up in a fairly conventional manner. The fuz starts life as a fairly odd looking square sectioned box of interlocking liteply parts. A bit of a three dimensional jigsaw puzzle, but it does make it fairly easy to get straight (or should that be 'easy to get fairly straight'?) without the aid of a jig. The fuselage sides are added next and pulled in at the rear - then it gets interesting!
Curved formers are glued to the top, bottom and sides, providing a cylindrical sectioned front end which is then sheeted over. This is the sort of operation I approach with trepidation, but it was in fact remarkably trouble free. Indeed, the circular front section blends into the slab sided rear with the minimum of filling required - just as well since I was planning to cover it with white film, which hides nothing! The wing saddles and fairing to continue the circular shape under the wing required a lot of trim and try to get them just right, but the end result is worth it.
Juggling everything to fit at the rear is made a little more tricky because of that moulded plastic turtle decking which fits directly to fuselage, tailplane and fin and is somewhat unmodifiable in shape - so everything else has to be made to fit it - and each other - and be vertical or horizontal as well. A real test of patience (and the art of compromise!)
For a film man like me, the turtle deck is again a problem. After much umming and ahhing (that should fix the spell-checker), I took my courage in one hand and a hottish iron in the other, along with some profilm, and set about finding the melting point of the plastic! Only when I turned up the heat in an attempt to shift the last stubborn wrinkle did the turtle decking begin to distort, so the iron was turned off and the wrinkle stayed.
Then there's that plastic cowl. Armed with my new-found confidence from using acrylic paints on the cowls of the Junkers 52, I tackled this job with a little more enthusiasm than usually accompanies such adventures. I should have known better - see here for the gory details. Actually, the cowl came out quite well in the end, but the garage will never be quite the same again!
The rest of the model was covered in white profilm and decorated with a rather gaudy 'JAC' sticker sheet that I had been wondering what to do with for over three years. (This is why this model is filed under Sport Electric, rather than Electric Scale). There were stickers provided with the kit for a scale colour scheme, but they were not to my taste.
The cockpit is very prominent on this aircraft and so I took a bit more trouble than usual in making an instrument panel. The model is flown once again by a pilot painted by David (since the other two have done such a good job of flying the Junkers), but this one seems to have a rather negative outlook on life whenever the elevator is moved (something to do with the 5mm dowel stuck up his neck, I think!)
What makes it tick?
I don't use reversing switches - my pre-flight discipline isn't up to it. So, studying how the servos were to be fitted in the wings, I carefully worked out that I needed to use JR or Hitec servos rather than Futaba in order to get the ailerons to work the right way round. Hitec HS80 servos were therefore duly purchased, installed, connected up, tested - and went the wrong way!
What to do? Answer: crash an Aerojet - hey presto, two Futaba 133 mini servos, ailerons for the use of. I would like to say that nothing was wasted, because the HS80s were used for rudder and elevator. Indeed they were, the only trouble was I had already bought four of them!
What makes it snarl?
Now we are really in uncharted territory - at least for me. Dave Chinery's Sukhoij performed well on 14 cells and a home grown 'Team Gear' unit consisting of two 600 size motors driving a common shaft through a 4:1 gear drive. Dave has put the Team Gear into production, so it seemed that I could avoid some uncertainty (and some of the cost of the alternative high tech, rare earth magnet motors) by following his lead. Simple though this decision sounds, it took me several weeks to reach that conclusion, but in the end the Team Gear 40 was duly ordered. (see Log Book for update)
After a similar amount of hesitation I also ordered the AI Robotics FX35D speed controller. This is a very flexible device and so far has performed faultlessly. If you choose to venture out of the tried and tested 7 cell electric territory, this controller is well worth considering - providing you are prepared to spend the time reading the 20 page book that comes with it and don't mind doing a bit of soldering to attach plugs and leads of your choice.
The next problem was switches. The speed controller has a tiny three position slide switch and it is recommended that a master switch be fitted in the lead from the flight pack. The nature of the fuselage construction makes fitting switches in the fuselage side more than a little tricky and the idea of an external switch capable of carrying 20amps plus was also pretty unattractive (literally!). Equally there is no point installing a safety switch inside - the only access to the innards is by removal of the wing, and that means turning the model upside down on a stand (ah yes, that was another problem!).
This conundrum of how to make the switch inconspicuous yet easily accessible was eventually solved by fitting the switch inside the motor compartment, operated by sliding the plastic air intake on the underside of the model. The mini slide switch was also mounted on a flexible mount in the motor compartment, poking through a hole in the cowl. By pressing it in with a screwdriver, the cowl can be removed without disturbing the switch. Phew!
The result of shoving all this electric gubbins (and the receiver battery) up front is that the space for the flight battery is uncluttered and enormous. two 7-cell packs drop in leaving room for three more, so a large foam offcut was fashioned to wedge them firmly in place.
Will she, won't she?
Right, it's paranoia time now! At a mere 57" span and an all up weight of 3kg (alright, which units do you want?) this is a heavy model. By some sleight of calculator, Graupner manage to claim, at this weight, a wing loading of 55g/sq dm (about 18oz/sq ft), but I make it over 25oz/sq ft. As far as I can tell, the Graupner wing area calculation seems to be based on root chord, ignoring the 50% wing taper completely - and then they add the tailplane area in for good measure. The question is, whose calculation will the model believe?
I decided to check the model against the guidelines published by Paul Rossiter in August's edition of Electric Flight International. This is suitably reassuring about there being enough thrust - I was amazed to measure a full 4lb - and also suggested that the wing loading was not out of the way - for a fast flying fighter type aircraft ... hmm.
The one worry raised by Paul's rules was propeller pitch. The calculated stall speed of the model was a little over 18mph - should make for interesting landings - and he suggests a theoretical 'propellor speed' of 3 or 4 times this. My 14" x 8" prop was turning at just 6000rpm giving a propellor speed of 48mph. Not quite enough. However, since I had over 1lb of static thrust in hand, I decided not to change the set up unless it really did run out of breath at speed - I was more concerned at this point about a secure take off and climb out!
No, that's not a typing error! I booked the day for the maiden flight well in advance and, for once the weather obliged with a light North Easterly wind.
I arrived at the field with Jim, Peter and Dave all ready and waiting. The preliminaries didn't take long (teaching the Chairman how to work the video camera was the longest part!) so, after a quick range check and cg check, away we went.
Acceleration across the grass was brisk and the Su26 was soon at what I judged to be flying speed - but it wasn't flying. A touch of up and it lifted off and immediately tip stalled to the right. The subsequent porpoising climbing left turn showed that the model was in need of a little up trim and also extremely elevator sensitive (I had rated the ailerons but not the elevators). Having completed the first circuit and reached a safe height, I throttled back slightly to see if life would be a little less exciting. Seeing the model begin to lose height and speed, I eased the throttle forward again and nothing happened, so the second circuit had to become an emergency landing!
After an untidy descent I got the model pointing into wind on finals and, apart from the extreme twitchyness, all seemed well. However, the moment I touched the elevator to begin to flare out, the flight ended as it had started - with a tip stall to the right, and that was that. Mercifully, the only damage was a detached undercarriage, cracked motor bearers and a broken propellor - how the wheels managed not to come up through the wings I will never know.
A post mortem showed a dead cell in one battery pack. The motor mount was rebuilt with a little added weight, the u/c re-attached with steel screws in place of the original nylon ones and the elevator throw drastically reduced. Then I went to Lanzarote for a week to recover.
Same field, a fortnight later. Say what you like about this aeroplane, it really knows how to order the weather - another sunny Saturday morning with a light easterly.
Jim and Dave were again present for moral support and Dave kindly tested the air with his Lightfighter. Having pronounced it silky smooth (I think that's how he pronounced it), there was nothing for it but to get on with it.
Once again the Sukhoij, untroubled by the longish grass, scooted away, straight as an arrow. Once again though it seemed more interested in the land speed record than in flying. So once again the flight started with the control input that caused all the trouble last time - up elevator!
Now we had daylight under the aeroplane, but no climb and not a lot of field left, so a touch more up was called for, and aileron as well (gulp). The resulting low level, high speed, huge wide circuit was frightening to watch (at least for me) but rock steady. Now back into wind and enough height to dial in the required up trim and ease the throttle back a bit.
At last I began to feel in control. After a few more circuits at a less frantic airspeed, the handling could almost be described as docile - a real transformation! Resisting (with ease) the temptation to try anything in the least bit rash, I decided that it was time to land. Still suspicious that the tip stall might be waiting to bite, the landing circuit was high, fast and well within the confines of the field. The resulting approach was therefore fast, flat and very long. Determined not to overdo the flare out, the first touchdown resulted in a long hop, but perfectly controllable, the model coming to rest almost at the far end of the park. At least I had the satisfaction of a long, relaxing taxi back - the ground handling is impeccable.
There is clearly a long way to go before I can report fully on the flight characteristics of this model, but I must say I am now eagerly looking forward to exploring its capability. I'll keep you posted.
In the meantime, if you do build a Sukhoij, do watch out for the cg and the elevator travel - else make sure there is a cardiac unit close at hand!
Part 2 - the Sukhoij Sequel
Last time in the story of the Sukhoij...
"...the flight ended as it had started - with a tip stall... mercifully the only damage was a detached undercarriage, cracked motor bearers and a broken propellor".
...elevator throw drastically reduced, added noseweight, steel u/c bolts. Fast, frantic, but stable.
Same field again, a little breezy this time. Dave again kindly agreed to check out the air but encountered some glitching on the climb out circuit. "Better get it down quickly" I suggested. "Well, it's not going to come to any harm up there is it?" was the typical response. For a while it seemed he was right, then "It's funny," he said "I often get glitching here and it always seems to occur ... over ... there. Oh S***!!"
The Lightfighter spiralled in, controls apparently locked hard over. When the bits had been retrieved, all worked well and there seemed little doubt that external interference was the cause. Does anyone have a map of the telecom microwave links in the area?
But back to the Sukhoij - what was I to do? I was in a pretty paranoid state before witnessing all this and my first reaction was not to fly. However, I knew that this would result in my being even more nervous next time, and I have never had any radio problems here so, after a very careful range check, the Sukhoij was lined up for take off.
Flight no.3 started as per no.2. Smooth take off, climb out very fast and shallow for fear of pulling too much up, but the first circuit completed with no problems. Then, at the downwind end of the second circuit, the nose began to drop and the speed built up even more. A touch of up - nothing. A bit more up - still nothing, still diving, still accelerating, ground rapidly approaching. Yet more up (all I had, in fact) - and the Sukhoij levelled out at eye level heading straight towards me. I swear I heard it say "Ha ha, had you going there!".
I tried to ignore the knocking noise from my knees and think. Was it interference? Was it turbulence? Did I stall it? Or, with the elevator rated, was there now really not enough movement? Whatever it was, I was sure I didn't like it and the time had come to follow my own advice and get the model safely down as soon as possible. So down we came for a carbon copy of landing no.2 - too high, too fast, a long hop and then the real landing a long way down the field. Once again the taxi back was the most enjoyable part of the experience.
So ended outing no.3. Three flights in the log book, total air time now about one minute!
Perfect conditions. No frights this time. Once I got up to a safe height I throttled back - and the climb rate increased! I still do not know why this occurs because there is no downthrust built in. I can only assume it is something to do with prop-induced turbulence affecting the efficiency of the wing. My Taylorcraft does the same thing. Anyway, I stayed airborne long enough to begin to get the feel for the model, but landed in good time - not ready for a dead stick landing yet!
Boxing day. Bright, sunny, perishing cold and the ground frozen hard. Worse still, it was clear that, shortly before freezing, the ground had been churned up somewhat by a football training session.
Fortunately, from my viewpoint (directly behind the model) I could not see the vibration suffered by the undercarriage during the takeoff run, but I could hear it! Once the model lifted off and the rattling stopped all went well. However my enjoyment of the flight was spoilt by the prospect of trying to get down safely onto that rock hard moonscape. To my great relief the landing was the best yet and no damage was sustained.
Flights 6 & 7
Yes, two on one day! By now I had gained enough confidence to unrate both the ailerons and elevators and remove half of the noseweight that I had added after that ill fated first outing.
After a couple of circuits to check that the handling was still okay, I plucked up courage to try the first roll. Brilliant!. Good roll rate, no elevator correction required. A couple more rolls were flown during the flight just to prove it wasn't a fluke (and because some present hadn't been paying attention). Unfortunately the landing was the worst yet. The approach was too short and, to quote Paul, "looked like a fly by". However, thanks to the steel u/c bolts, the model survived with no more than a couple of muddy wing tips. Thank goodness the ground had thawed out!
The second flight of the day was to have contained the first loop, but by now the mist had closed in and so we were restricted to more rolls and, thankfully, a much better landing.
That was new year's eve. Then came 1996...snow, rain, wind, snow, wind, rain etc.. Despite keeping this article open until late February, I am unable to report on any further exploits and the publication deadline has beaten me. Still, there's nothing wrong with a little bit of suspense, is there?!
(Five years of suspense is long enough - check the log book for an update - TJH April 2001)