Jabiru Taildragger

[Oscar]

Bruce it was on an old school aircraft that had done heaps of work. The inner bolt of the RH u/c leg broke as I touched down on a bumpy patch of grass. The other two bolts held, but as the leg folded up the right wingtip hit the ground and slid a few metres. The left main and nose wheel described a nice arc to the right and we came to a gently stop at a bit of an angle. No drama.After a long walk to get some tools and and new bolt I put the seat cushion and the wingtip on my head while the bolt was replaced. About ten minutes work. Superficial damage to the wingtip gelcoat only.

Just love fibreglass aeroplanes, and the Jab in particular!

The mains on an LSA55 are JUST behind the c/g. If you have to move one, it requires maybe 10-15 kgs of push on the tailcone just ahead of the fin to raise the nose.

 

In fact, the original 1600-engined LSA55 was even lighter in the nose and the mains where marginal for position. One one occasion, following an extended set of test flights, Rod Stiff offered the test pilot the opportunity to 'take it for a bit of a holiday' - which he gratefully accepted, packed his wife and their toothbrushes and a change of undies and other clothes ( there isn't room for much more aft in an LSA55..) and headed for the Whitsundays. First refueling stop, he filled up (the 55-litre tank, not the later 65-litre), hopped in, reached back to stow the small refueling jerry-can ( steel in those days), and the wee thing sat up and begged, very gently. Moved his arm forward, and it settled back onto the nosewheel.. I suspect Rod slightly changed the main leg profile..

 

(Just as a side-note, and of absolutely NO relevance to this thread: when flying along beside one Whitsunday Island, they saw a Great White cruising along - and were amazed and slightly horrified, to note that not only was it almost as long as the fuselage shadow of the Jab, but its pectoral fin spread was not insignificant compared to the wing shadow!)

 

So, back on topic... it only took a relatively minor change to the main legs moulds to bring the point of tyre static contact forward of the c/g. (remember, that it already is changed forward with the tail on the ground.. motorcyclists who know the rake and trail effect on the front wheel of changing the swing-arm static height will understand).

 

AFAIK, Jabiru have only ever offered factory parts to change to a tail-wheel configuration for the LSA55. I'm not at all sure that they haven't missed out on an opportunity to create with the J170 and a sophisticated application of VGs - as on the Seabird Seeker - quite a competent STOL version of the Jab. as a tail-wheel aircraft. Rod Stiff: if you are listening - you know who to contract to get there..

 

 

[Old Koreelah]

Was this before or after the 500hr life SB?

No idea, Frank. Not my aircraft.

 

 

[Old Koreelah]

... I'm not at all sure that they haven't missed out on an opportunity to create with the J170 and a sophisticated application of VGs - as on the Seabird Seeker - quite a competent STOL version of the Jab. as a tail-wheel aircraft. Rod Stiff: if you are listening - you know who to contract to get there..

At last someone agrees with me. The conversion would not be as simple as many might think. The Jabiru elevator has only a poofteenth of down movement. The gear legs might need beefing up with a cross member under the fuse.

 

 

[facthunter]

My comment about nosewheels is a general one. I know the plane imaged is a tailwheel plane. i'm talking about physics Any time the weight of a plane is carried by something well forward of the Cof G, it's directionally unstable. Land on the nosewheel and find out, if you need convicing. Nev

 

 

[Oscar]

At last someone agrees with me. The conversion would not be as simple as many might think. The Jabiru elevator has only a poofteenth of down movement. The gear legs might need beefing up with a cross member under the fuse.

Well, I was thinking 'quite competent' - not Extreme Smack-down competitor...

 

 

[Bruce Tuncks]

Quite right Nev, and there is also the fact that with the c of g behind the mains, any vertical upload on landing will point the nose to the sky instead of the ground.

 

These 2 effects make a tricycle much easier to land and stay on the runway afterwards.

 

 

[facthunter]

If you hold the nosewheel off the runway, and keep the weight off it, a tricycle is FAR easier to handle . That's why they became so popular until a few mishaps from wheelbarrowing, (touching down at too high a speed by pilots who were getting into the bad habit of approaching too fast) showed nothing is fool proof. Early Jabiru nosewheels were a little weak. I think at one Avalon airshow where a downwind was accepted on final, three damaged their nosewheels.

 

If the surface is rough (rabbit holes etc) a taildragger is far a better proposition.. For flying schools generally tri gear is "normal". The rate of attrition of tailwheel planes hired out would be noticeably higher, unless you insist on selecting your customers carefully. When you insure a T/W, they like you to have quite a few hours on them. (like 100's) in your logbook. Nev

 

 

[Old Koreelah]

Well, I was thinking 'quite competent' - not Extreme Smack-down competitor...

Yes Oscar, that's the sort of performance I'd like. A compromise between extreme smack-down and good cruiser- with the added bonus of an excellent safety cage.

 

 

[Oscar]

Yes Oscar, that's the sort of performance I'd like. A compromise between extreme smack-down and good cruiser- with the added bonus of an excellent safety cage.

The 170, I think, has potential for that role. Larger diameter tyres - but NOT tundra tyres - would absorb quite a bit more touch-down energy by flexing the sidewalls (which also provides a scrubbing, and therefore dampening, action of the U/C from springing back too abruptly).

 

The 170 tail volume and longer tail-boom leverage over the early LSA55's should provide most of the tail-feather power to keep things controllable at a lower stall speed. You might need to play around with vg's on the underside of the horizontal stabiliser, though. I think you would find the advice of the aerodynamasist Rod used to be much against enlarging either the rudder or elevator volumes ( as per the LSA55 in Post #5), as that could introduce flutter unless you very carefully add mass balance ( which isn't used on the rudder anyway, but might be able to be added to the aerodynamic balance tab on the rudder, but NOT without consulting Alan Kerr re the structural considerations! ( I made a small change to the rudder configuration - not enlarging, just to fair it in to the fin better, and the advice I was given, is that you have a margin of about 5% in safety for the c/g of a flight control surface relative to the hinge-line that has been tested to be flutter-free.

 

You'd need to get the wing vg's right. To ensure you have benign stall characteristics at slow speed, the centre-section of the wings has to stall before the ailerons do - and if that happens too soon, then you lose down-wash onto the elevator and that in turn could cause a highly exciting stall... right near the hard brown bit underneath.

 

When I look at the 'extreme STOL' aircraft', I see things that are very, very draggy - so you can fly them down with quite a bit of power-on and still land very short. Even an optimised J170 will be quite a bit more slippery than those... and the old Ke= 1/2MV2 is your enemy...

 

But, as the old saying goes, it's not how big it is but how you use it... I know an old Jab. test pilot ( you know him too), who could get an LSA55 down pretty damn short, using glider tuggie techniques and dumping the flaps at just the right moment. He told me this story..

 

Max Hazelton once visited Nestor Slepcev at Nestor's private home strip. Nestor was apparently somewhat taken aback that Max had arrived in his Auster - not believing that anything other than his (Nestor's) own creation could use that strip - and told Max that he'd not be able to get off again. I believe that Max politely told Nestor that he'd 'give it a try', and left with no more fuss than that with which he'd arrived...

 

 

[Old Koreelah]

Oscar this discussion is one of the great benefits of this forum. I have VGs forward of my elevator and I doubt they do much when I'm near the stall; the air bends around a 25 degree corner and at that position my GoPro footage shows lots of turbulent flow over the elevator.

 

You mention mass balancing isn't used on the rudder.

 

When I extended the trailing edge of my rudder to get more bite at landing speeds I feared this might bring on flutter, so I added a small lead slug to the leading edge. I recall Tony Hayes demonstrating how to induce flutter of the control surfaces of his Thruster; it made quite an impression.

 

 

[giantkingsquid]

A colleague of mine owns this Jabiru at YCCY. I have been up with him and flown it a little, it's a nice handling machine in the air, but after 150 odd hours in it he still doesn't like landing it: tail loop happy apparently.

 

 

 

[Yenn]

I don't understand "VGs forward of the elevator". I thought that VGs were put on the airfoil and usually when used on elevators, they went on the bottom to reduce the risk of stalling the elevator at low speed, Is that what you meant?

 

 

[Old Koreelah]

I don't understand "VGs forward of the elevator". I thought that VGs were put on the airfoil and usually when used on elevators, they went on the bottom to reduce the risk of stalling the elevator at low speed, Is that what you meant?

Yep. I stuck a row of VGs under the tailplane, about 100mm forward of the elevator hingeline.

The idea was to help keep the airlfow attached during the sudden change of direction when the elevator is up.

 

It's hard to say if they helped, because I have no footage of wool tufts before I attached them. I hope they helped, but I can't recall them making any noticeable difference to the stall.