Motor Gliders - thoughts?

[nathan_c]

Ok, so cadets is buying a motor glider to complement the other gliders that cadets own, so it got me thinking, what do others think of motor gliders?

 

The glider being bought is a Grob 109, which I have never ever seen in real life, let alone flown in, so it should be interesting what its like. However, I recently flew a Dimona (which wasnt the best flight ever, but im sure you can find my rant thread about it if you are really interested, so Im not going to repeat it), but i wasnt overly impressed by the way it flew. I have been told that it is not the nicest dimona around, but regardless, you had to push the rudder pedals REALLY hard to get any movement at all from them, and once they were pushed in, it felt like they didnt want to center, and you had to push them hard again to center them to fly straight. On top of this, I just didnt like the way it flew either.

 

So lets have a discussion, has anyone flown them? What do you think of the way they fly? Is the way I described that dimona typical of a motor glider, and has any one had any flight time in a grob?

 

 

[Guest TOSGcentral]

Hi,

 

 

Here are some thoughts on motor glider/SLGs – may be a bit blunt but I call the shots the way that I saw them.

 

 

I have flown both the G109A and G109B. I did not think a great deal of them! They were OK but a bit too GAish for my tastes. They have a ‘big plane’ feel to them and a remoteness from the air that is not typical of gliders – you sort of get in them rather than putting them on – which I far prefer.

 

 

They soar OK and glide well enough but I found that the owners got a bit tense when I started doing engine off circuits. The aircraft were basically used as powered aircraft with just a bit of gliding thrown in at vast heights in good thermal conditions.

 

 

I suppose the Oz ATC are taking a leaf from the Pom ATC book – they have used them for years, apparently quite successfully.

 

 

You comments regarding the Dimona I found surprising. I have a fair amount of varied time in these – cross country, training, flight testing etc.

 

 

The Dimona is certainly very much a pilot’s aircraft but I never had any problem with the rudder – which was always very light and positive.

 

 

But the Dimona does have some hidden hassles that keep you sharpened up! There is a very subtle parallax challenge with it that can cross you up on landing.

 

 

Yaw stability on the machine is approaching neutral due to the general pod and boom design layout. The Dimona is the only aircraft that has ever pushed me into rudder overbalance via side gusts. Ok that was in a wave rotor and it was bloody rough air – but! That was a helluva ride and it took me nearly half an hour to get it down safely, and even then I had to land well above the aircraft’s crosswind limits.

 

 

You could yaw overbalance the Dimona just on aileron drag alone – which is why you may have been less than delighted with it. Maybe being a bit coarse on the controls eh?

 

 

The yaw overbalance is also a bit of trap for young players. Outwardly the machine seems normal but is actually sinking like a brick for all its performance! Any training and conversions that I did on it I ensured that I covered rudder overbalance thoroughly (many pilots do not know what this even is let alone been taught it) and thoroughly revised aileron drag and rudder/aileron co-ordination. Then there were no problems.

 

 

You cannot just drive these big SLGs like a Cessna – you really have to fly them to get the best out of them.

 

 

Returning to the G109. As a rather crude comparison I would class the 109 against the Dimona much like the comparison between a Holden Ute and an F1 Ferrari!

 

 

I am stuffed why the ATC mess about with these big glass sleds! If they wanted a practical support training glider then they have to look no further than the Scheibe SF28 Tandem Falke – which is superb both as a trainer and as a soaring glider – and has real ‘feel’ to it.

 

 

Aye

 

 

Tony

 

 

[nathan_c]

Nice Post, I was very interested reading that.

 

Unfortunately I have no idea what yaw overbalance actually is. I have not been taught, or in fact, told anything about this. I do know what Aileron Drag and stick and rudder together is all about though.

 

Mind elaborating on what you mean by rudder overbalance? Im sure I know what that is, just possibly by a different name.

 

It is not the entire AAFC (Australian Air Force Cadets, we are no longer called the ATC), that is buying Grobs, to my knowledge, it is only our section of gliding in the cadets. I actually saw one of the Motor Faulkes on the day that I went up in the dimona, and I was amazed by how slow it takes off compared to the dimonas etc. I also got told that if you got the wings wet, your take off would become a high speed taxi, it just wouldnt lift off.

 

Cheers,

 

Nathan

 

 

[Guest TOSGcentral]

Hi Nathan,

 

 

We will give this topic a go then but I will have to draw some pictures in words – and I may oversimplify a tad for some readers tastes – or even get a bit exotic as I am going to use dinosaurs as examples and also float equipped aircraft.

 

 

Rudder Overbalance or Yaw Overbalance is wrapped very firmly with the principles of Stability – so let us start there.

 

 

The main property of Stability is that it either retains the aircraft in an attitude with respect to the relative airflow, or returns the aircraft to that state if the machine is displaced in space (eg a gust).

 

 

With the exception of stability in roll (which to a great extent is controlled by Lateral Damping), Pitch and yaw stability are usually mainly acting around levers and moment arms. Hence why the tailplane and fin are on the end of a long moment arm that is the rear fuselage. This enables a relatively small surface to exert sufficient force via its mechanical advantage to do the job.

 

 

OK, now for levers to work they have to act around some kind of pivot point or fulcrum. On an aircraft this is typically the Centre of Lift (or the point that the Total Resultant of Lift is said to act through – this is in the centre of the wing span and a little behind the wing leading edge.

 

 

Now I would invite you to visualise a model aircraft (or glider) suspended on a piece of string from the Centre of lift. For simplicity we will assume that the Centre of Gravity is also at this point so there is no tendency for the model to begin tipping from the couple established from any moment arm caused between different C of G and C of L points.

 

 

Stability in Yaw is the easiest to get our minds around as it acts on exactly the same principle as a weather vane. The vane is pivoted at the front with a large vertical surface behind it. As air flows across the vane it will push the vane flat relative to the wind. This is handy on a windmill as a relatively small vane can keep the much larger sails perpendicular to the wind and therefore extracting the maximum energy.

 

 

The expression of ‘weathercocking’ (particularly on taildraggers has it’s origins in this part of mechanics where natural stability aft is acting well aft of the point of main ground contact caused by the main wheels.

 

 

In a cross wind the aircraft will attempt to turn to face directly at the wind. This is a much larger force than most pilots realise. You do realise very promptly the first time you have a float plane on a water surface. Even in only one or two knots of wind (that you would normally disregard on a land plane) the aircraft will weathercock straight into wind and will do so quite rapidly – and you cannot stop it doing so if you have no forward speed! This can be bloody embarrassing if you have something large in front of you blocking your path, so you have to be careful where you park these things!

 

 

I will come back to this but for the moment let us move on a little and talk about dinosaurs.

 

 

Within the various families of these reptiles were flying varieties commonly referred to as pterodactyls . Amongst the largest was the pterodon which was a substantially sized creature that could get up to 15 metres in wing span.

 

 

These things were aerodynamically challenged because they had a large beak like arrangement of considerable area. The creature needed to turn it’s head in flight to spot the next meal.

 

 

So ‘Don’ is out looking for lunch and turns it’s head to the right. The flight airflow then presses against the left side of the beak forcing the head further to the right to a point that ‘Don’ is not strong enough to turn it back again. There was no access to steroids in those days from which neck muscles could be boosted up so instead the basic design grew a big horny vane on the back of the head.

 

 

When Don turns it’s head to the right airflow tries to keep the beak over. But the horny vane is now sticking out to the left and the airflow is hitting that too trying to force the head back in line with the airflow. So the creature had ‘aerodynamically balanced’ itself and could turn its head at will when flying with little effort.

 

 

If the rear vane is not large enough to balance the beak then Don has a problem. It it is too large then there is a different problem as the vane will prevent Don turning it’s head!

 

 

So let us pick up on the Dimona business that Nathan is interested in.

 

 

We are going to start talking about something termed ‘Keel Area’. This is the vertical disposition of area and an important point is where the fulcrum (Centre of Lift) resides amidst that keel area.

 

 

OK we will saw a Dimona vertically down the middle of the fuselage (Don’t do this at home as they are rather expensive aircraft).

 

 

The Dimona is basically a ‘pod and boom’ layout. This means that it has quite a large keel area in the nose compared with the narrow rear fuselage. The Dimona is positively stable in yaw – but only just, it is verging strongly towards neutral stability although there is sufficient aft keel area to re-align it with the airflow in normal flight.

 

 

This can be compared with the Grob G109 which has a relatively small nose and a deep rear fuselage – so is far more yaw stable. Note: You do not actually have to fly anything to find this out – you just have to look at them!

 

 

However if a Dimona is put under a sufficient yaw force then it has the same problem as Don without a big enough rear vane on his head. The big nose swings over, the air has more to push against and that force exceeds the stabilising force of the rear fuselage. So the aircraft locks into the yaw and in the cockpit the rudder pedal flops to the floor and stays there. The machine has now yaw over-balanced or, if you like, the rudder has overbalanced and the airflow is going to keep it where it now is.

 

 

None of this is particularly drastic. The controls work normally and the aircraft continues flying (but may be trying to seriously roll from the further effects of yaw). To resolve the situation simply prod on the other pedal to unlock the over-balance and off you go.

 

 

Significantly however, while the aircraft it in an extreme yaw the drag is huge and it can be sinking very rapidly indeed even while in a normally flat attitude. Bit of a trap for young players there because you may become alarmingly low in circuit without initially realising that you are going there – or why!

 

 

So the Dimona is vulnerable to side gusts that can push it into this condition – although you are unlikely to experience it as they do fly normally and nicely – I love them. But equally the Dimona does have appreciable aileron drag and the adverse yaw can very smoothly pull it into overbalance with no alarm bells going off at all.

 

 

There. That was not difficult at all was it? But to finish off let us briefly return to floats.

 

 

I said that a stationary float plane will rapidly weathercock into wind and you cannot stop it doing so. You can if you have forward speed and you actually use this yaw overbalance to enable you to steer.

 

 

To steer one you need forward speed. You need this to give you elevator power to force the tail down. By doing this you are changing another fulcrum point – the Centre of Bouyancy. This is a point on the floats that the main weight is averaging through. By lowering the tail you move the C of B backwards and so cause more keel area in front of the C of B. The wind now turns you directly downwind.

 

 

Juggle about with the throttle and attitude and you can taxi over the water at any angle to the wind you like. You are actually steering with the throttle and elevator!

 

 

Ho Hum! So what? Just interesting if you are interested in what makes your aircraft tick!

 

 

To conclude – a little about Falkers. What you saw Nathan was probably a side by side seating SF25 or T61 Falke. They are not ‘motor Falkes’. This was the SF25A and is quite different.

 

 

Indeed they are critical in the wet. I once used 6000’ of concrete runway that Comet 4s and B707s used and still could not get airborne in one. The wing section is totally intolerant to water droplets and the low take of speed does not allow them to be blown off in the take-off roll.

 

 

In comparison the SF28 Tandem Falke is quite a different aircraft. Tandem seating, bigger span and faster. One potential hiccup with the ’28 is that it has a cowl flap (or air dam) in the back of the engine bay and if your checks are not up to it then you will cook the motor well and good if you attempt a take off with the thing closed.

 

 

Aye

 

 

Tony

 

 

[nathan_c]

Sorry it has taken so long to reply,

 

Thanks for that detailed explanation, I am sure it took a considerable amount of time to write out. From what you said, that overbalance is most likely what caused the rudder pedal to stay there. It was not a great problem, just more a slight annoyance while in flight :)

 

Interesting about the float planes though, I had no idea they were that touchy while on water. And yes, the Faulke was a side by side seating arrangement, apologies for getting that mixed up, I was just repeating what the name I was told.

 

Cheers,

 

Nathan

 

 

[Guest Flygirl]

gliding freedom

 

I dont know why the older gliders are being left to rot, while the replacment glider is getting out of reach. The old K7 is a great training glider and is very cheap, Some glue problems in the ribs require some work, but much cheaper than a replacment. The old SF gliders are simple to build and maintain, I have just brough a SF27m the first of the self launchers a single seat, and a 34-1 glide. it had been sitting for 8 years, so i did the 40yrly, it may be old but great condition, and very cheap flying $4-5 of fuel and you could stay up all day.I have owned a few old wood gliders, they may not go like a new glass ship, but a practial construction simple construction, any one could work on ,and if you were close to a winch club, a very cost effect way to get airborne.But an RAA motor glider 2 seat at a much more reasonable cost than the inports would have to be my choise, but dreams are free.

 

 

[Guest Maj Millard]

Gee I was going to jump in and tell you about my ride in a Zamango motor glider, but after reading the Late Tonys Hayes's stuff I've realized I'm way out of my league !!..........

 

 

[nomadpete]

Nice to see that somebody has a soft spot for the humble K7. I went solo in one. Had a lot of fun in it out at DDSC. As you said, not fast but fun to fly and they can work weak thermals that the new high speed plastic gliders cannot.

 

PeterT

 

 

[Guest eland2705]

G'day Nathan,

 

You are obviously from 3WG. Do you know if there are any ideas of expanding the idea of a dedicated Gliding Flight to 4WG or are we always going to be outsourced to the Gliding Clubs.

 

Regards

 

 

[nathan_c]

Hey Steph,

 

I am actually from 2WG, not 3WG. Unfortunately I have no idea whether or not there is ideas of a dedicated gliding flight being introduced into 4 WG. I actually rarely fly with the 2 wing dedicated flight anymore, and am about to start very consistently flying with a club after being awarded a Defence Flying Scholarship, so even if there was talk amongst the gliding flight I would not be able to hear it.

 

Sorry I couldnt help more,

 

Nathan

 

 

[Garry Morgan]

Sf 27m

 

After nealy 3 months or crapy weather i got my sf out of the hanngar on saterday and got 1.4 hrs . it has an old 26hp 4 cyclinder Hirth motor. it is slow getting up there but thats all it neets to do. It was to be able to work the lift easier with an off shore wind as we are often chasing the sea brease and broken lift.

 

 

[Guest ozzie]

I liked the 'Silent' that i saw when i was in Italy. it was powered by a Solo engine with a single bladed prop that folded back down into the fuse. there is also an electric version as well. very nice and real glider performance. bit pricey tho.

 

 

[Garry Morgan]

silent

 

In Australian Gliding there is a silent for sale ,no price on it. it has test hrs only for type approval.

 

 

[Garry Morgan]

WE are looking at streaching the Super Daimond mk3 twister fuslarge to a tamden tail wheel, extending the wing to a 4 piece to get into the hangar 17m span, and a 2.2 jab up front . I spoke to GFA and looks OK to build it with them, I can get into more airfields then, and the council here will get off my back operating the sf27m, and i can fly inland more to better lift.

 

 

[kaz3g]

Hi folks

 

It was a long while ago at Gympie but I flew a G109 and really enjoyed it. Yes it is rather GAish as was stated elsewhere but it is also a very stable platform which is why it was used for a long while by the Met Bureau to track weather cells. No doubt that makes it a useful instructing vehicle for those that want to fly with a noise up front/above/behind.

 

It's also nice to read people talking about some of the older sailplanes that are still about, too. I flew and instructed in Blaniks eons ago and even did a Statesman advert in one. Two of us were auto-towed behind the car driven by some young fellow who thought a great deal about himeslf and his ability. That is, until we the pilots got bored after a few shoots and decided to lift just a little bit harder than before.

 

He didn't appreciate his rear wheels suddenly dangling a couple of feet off terra firma at about 45 knots ground speed... it did spin nicely on the gravel :-)

 

I haven't flown any of those later types that people are discussing here but I did survive about 70 hours in a Fournier RF5 without busting it!

 

kaz