Skyfox AD

[James]

She still looks sharp and flies well, I'm the third owner, the chap I bought it from did his training in her. Do you have any photos of her from around that time? What was the rated MTOW when you flew her? the reason I ask is when I bought her it stated 520kg in the flight manual and placard but since the audit by CASA at RA-Aus I,m now flying with MTOW of 450kg as per type Cert. virtually impossible to fly 2 up. Any info you have would be great.

 

Cheers James

 

 

[facthunter]

Austers carried a belly tank underneath. You could probably do similar for special purposes. Nev

 

 

[Dafydd Llewellyn]

I'd heard you mention that before, apart from the problem in the first instance the airframe held together till they got it on the ground. I find it interesting that the Kitfox makes the claim of never having an inflight break up, and is considered a very strong design, why has the skyfox had this issue, being overstressed is one thing,,,,maybe the reputation has grown out of proportion to the problem, I know after 8-90 hours in one I can't see why they've got the reputation they have, I love mine and reckon it's no harder to land than a Eurofox, and a third of the price ,Matty

I suggest you look at the CASA website, www.casa.gov.au under "airworthiness directives" - "under 5700". You'll find the CASA AD under "wing carry through" I think. It shows very clearly why the fuselage cross-member cracks; the design of the wing fold hinge at the base of the lift strut applies a substantial bending load to the fuselage tube; it's a piece of execrable detail design. That AD was to perform an inspection for cracking; it came out in 1996. The reinforcing strut, which helps react the bending load from the lift strut, was my design, and was later made mandatory by RAA. It was a result of the Bribie Island incident, and the aircraft stayed together solely because the bottom longeron on that side had previously been replaced with a much heavier-wall piece of tube in the course of repair following a landing accident. I don't have an electronic copy of the drawing or the Engineering Order still on file; I may be able to turn up a paper copy, but it would help greatly if you could look in the log book of a Skyfox (any model) that has the tube fitted, and tell me the Engineering Order number.

 

 

[Blueadventures]

I suggest you look at the CASA website, www.casa.gov.au under "airworthiness directives" - "under 5700". You'll find the CASA AD under "wing carry through" I think. It shows very clearly why the fuselage cross-member cracks; the design of the wing fold hinge at the base of the lift strut applies a substantial bending load to the fuselage tube; it's a piece of execrable detail design. That AD was to perform an inspection for cracking; it came out in 1996. The reinforcing strut, which helps react the bending load from the lift strut, was my design, and was later made mandatory by RAA. It was a result of the Bribie Island incident, and the aircraft stayed together solely because the bottom longeron on that side had previously been replaced with a much heavier-wall piece of tube in the course of repair following a landing accident. I don't have an electronic copy of the drawing or the Engineering Order still on file; I may be able to turn up a paper copy, but it would help greatly if you could look in the log book of a Skyfox (any model) that has the tube fitted, and tell me the Engineering Order number.

Hi Daffydd

I've emailed you some info and the Engineering Order number details.

 

Cheers

 

Mike

 

 

[Dafydd Llewellyn]

Hi DaffyddI've emailed you some info and the Engineering Order number details.

 

Cheers

 

Mike

Yes, got it, thanks. I've just emailed back to you, the Engineering Order covering the manufacture procedure for the back-up tube. It's no longer of any commercial interest to either myself or B & R aviation, who were the manufacturer at the time. Whoever needs to use it will presumably need to get it approved for his aircraft serial number by a current CASR 21.M authority holder - but I suggest you check with RAA TM on that point.

 

 

[metalman]

Mine, along with most of them have been done, I have a B&R aviation tube in mine, although I've heard that if it's not done its hard( read near impossible) to get one made nor B&R are no longer. It's interesting that the aircraft is a Kitfox copy and yet the Kitfox doesn't seem to have any issues ,

 

 

[Dafydd Llewellyn]

Mine, along with most of them have been done, I have a B&R aviation tube in mine, although I've heard that if it's not done its hard( read near impossible) to get one made nor B&R are no longer. It's interesting that the aircraft is a Kitfox copy and yet the Kitfox doesn't seem to have any issues ,

The CA 21 Skyfox started out as a licence-built Kitfox 3. However, it had a number of really nasty problems (e.g. aileron flutter) that had to be overcome before it achieved certification - in all, 35 major engineering changes. That information was fed back to Denny - part of the licence agreement - and was incorporated to some degree in the Kitfox IV and later; so it would be more correct to say that the Kitfox - after the Kitfox 3 - is a copy of the Skyfox - except that the Kitfox never had to address fatigue issues (since it's not certificated - at least as far as I am aware); and it never adopted the segmented aileron that was necessary to prevent the aileron circuit from locking-up at limit load. The later Skyfoxes (CA 25 & later) were not tied to Kitfox, but developed independently.

The failure at Bribie Island was aggravated by internal corrosion; however it showed up a design weakness. Denny may have used a heavier tube in that area in his later models - I don't know. However, anybody who flies either a Kitfox or a Skyfox that has the original high-camber wing, should be aware that the high camber causes the load on the wing to shift aft as the flight speed increases; at around 70 knots, the whole load is carried by the rear spar; and above that, the load on the front spar is actually downwards, and that download adds to the burden of the rear spar. So it's definitely NOT an aeroplane that one should try to get speed out of. Great for floating around; just don't fly it fast.

 

 

[metalman]

Interesting, mine isn't a Skyfox, it's a VH exp but I've used some components from a CA22a so I'm very interested in any info on them, I'm getting a cruise of 80kias with a 912uls which puts me into the area your mentioning, the slightly higher cruise speed wasn't my aim but rather a much better climb and shorter takeoff .

 

The type has gone through an interesting evolution , I've got a few hours in a eurofox and always wondered about the way the wings bend from the strut attach outwards, quite a bit, till I had a look at a wing with the covers off, absolutely no vertical rib in the spar ,and very lightly built , the fox spar on the other hand doesn't have anywhere the same movement .

 

I have another fuselage that I'm keen to build another from, wider and longer in the cockpit, a bit more rudder area, and a different airfoil, Dafydd, I'm keen to hear what changes you would've done to the Skyfox to make it better, frankly I love the type and reckon with the time to develop further it could've been a great plane.

 

 

[Dafydd Llewellyn]

Interesting, mine isn't a Skyfox, it's a VH exp but I've used some components from a CA22a so I'm very interested in any info on them, I'm getting a cruise of 80kias with a 912uls which puts me into the area your mentioning, the slightly higher cruise speed wasn't my aim but rather a much better climb and shorter takeoff .The type has gone through an interesting evolution , I've got a few hours in a eurofox and always wondered about the way the wings bend from the strut attach outwards, quite a bit, till I had a look at a wing with the covers off, absolutely no vertical rib in the spar ,and very lightly built , the fox spar on the other hand doesn't have anywhere the same movement .

I have another fuselage that I'm keen to build another from, wider and longer in the cockpit, a bit more rudder area, and a different airfoil, Dafydd, I'm keen to hear what changes you would've done to the Skyfox to make it better, frankly I love the type and reckon with the time to develop further it could've been a great plane.

Blimey, where to start?

The original Skyfox certification was an exercise in shoehorning an aircraft that was really marginal in a number of areas, into the "box" defined by CAO 101.55 and BCAR S (preliminary issue). The manufacturer had largely spent his investment money and needed certification with the least possible modifications. So the end result was necessarily a compromise.

 

The main things that did not work properly and had to be changed were, from my recollection as the test pilot involved, firstly the aileron flutter - that was caused by the way the wing ribs were fretted out with rectangular lightening holes, which just about destroyed their beam strength. The fix was to glue 1 mm ply webs into the ribs that carry the aileron hangers - but it might not have been necessary to do that, had the lightening holes in the ribs been triangular, so the rib had the form of a Warren truss (no, not the political one). The ply webs work, but they prevent proper internal ventilation of the wing, and cause water traps in the trailing edge - which probably had a major part in the fatal Darwin crash (I was an expert witness on that case). The second issue was that the lift-strut tubes had to be connected to the wings further out than on the KF3, to control the upward bending of the rear spar due to the load transfer problem They got longer and a lot larger in cross-section. The original round cowl of the KF3 had enormous drag; the eventual cowl shape of the CA 21 cowl made it possible to meet the climb gradient requirement (just). The original idea of the Junkers-style ailerons was that they could also be used as flaps - that proved to be a complete failure. The "flap" lever on the KF3 became the trim lever on the 'Fox.

 

There are so many things wrong with the original wing design that it's really not worth fixing; it needs to be scrapped and a decent wing designed for it.

 

Bob and I looked at the possibility of coming up with an entirely new wing for it, using a better airfoil (less extreme camber) and conventional ailerons, and a proper spar and D-box, still fabric-covered - and with better fuel tankage. The torsional stiffness of a proper D-box would allow the use of a single lift strut. We called it the "Springbok" However it needed at least 20 owners to get together and form a group to order sufficient wings to make it worth-while for a manufacturer to tool up for them, and that's an impossibility with Skyfox owners, so that idea went nowhere. The original ailerons are excessively heavy; using conventional ones would save enough weight to get some more fuel in there. I suppose you could do this if you are building a -19 machine and you don't mind building proper wings for it. If you want to pursue that track, talk to Bob.

 

Otherwise, you're stuck with the wings. You can make the wings less worse by using an airfoil with less camber; I think we were looking at 4415, but I don't know whether that will work with the existing spars - and by re-designing the ribs to have a proper form of truss design. If you stay with the external-airfoil ailerons, at least use the developed version as fitted to the Gazelle, with the mods that were designed to correct the poor manufacture of the drive, and the problem of water absorption by the foam filling. And FFS use a better form of aileron hanger. Take particular care with wing drainage & ventilation.

 

The CA 21 had too small a vertical tail, and the thrust line was too low, so it went extremely nose-high in a power-on stall. Both those things were fixed in the CA 22 & 25; I'd leave well alone there.

 

The other major improvement would be sealing around the doors so that you do not get jets of air squirting out into the main airflow; they act as invisible dive-brakes.

 

 

[Dafydd Llewellyn]

Matty, I suspect the reason many of these problems did not show up in the original Kitfox was that it had an enormous airspeed system error; the first thing one does in flight testing is to calibrate the airspeed system; and that immediately showed that the KF3 was really flying at about 70 KCAS when its ASI was showing about 83 KIAS - and the error got worse at higher speed. However, certification requires that one tests the aircraft to a flight envelope whose Vd is not less than 1.4 time Va - and Va is not less than twice the zero-flap stall speed - in knots CAS. This meant it had to be taken to 112 kts REAL airspeed, and I doubt most KF3s ever got there; they'd be showing about 130 kts IAS if they did; and with the original lift-strut attachment position on the wing, the wings would have screwed themselves off.

 

This ASI system error was mainly caused by using "cabin static" - and it showed that there was a lot of suction around the cabin door, which was reducing the pressure in the cabin (and thus causing the air leaks I mentioned). So the Skyfox got a proper static pressure system, which was very accurate. I strongly recommend that you calibrate your airspeed system ASAP; see CASA AC 21.40

 

 

[djpacro]

Matty, ...... I strongly recommend that you calibrate your airspeed system ASAP; see CASA AC 21.40

Surprised if it got a CofA without an airspeed system cal but seems to be the state of knowledge these days. Give me a shout if you'd like some help with it.

[djpacro]

I should've added: unless he was convinced the airspeed system was identical to an approved type and you could copy and paste the PECs to your AFM.

 

 

[Dafydd Llewellyn]

I should've added: unless he was convinced the airspeed system was identical to an approved type and you could copy and paste the PECs to your AFM.

Yes. But I don't think there's any requirement to do this, for a VH experimental aircraft. However, given the critically small safe flight envelope of the Skyfox, knowing that the ASI system is telling the truth is a life & death matter. I never cease to be amazed that people make such heavy weather of ASI system calibration; a trailing-cone static and a pivoting-vane pitot are things anybody can make for not much over $10. Sure, you need two ASI instruments whose errors are known, but that's no big deal for an experimental aircraft, you don't need a certificate from an approved instrument shop; a simple manometer check of the instruments will suffice. You can do the flight recording with a kneepad & pencil.

I use an instrument box that straps on the spare seat, and records the instruments by a video camera. With that setup, the ASI calibration is all over in about 5 minutes of flying - it hardly needs more than an extended circuit. It takes a bit longer, and is not quite so precise, by pad & pencil, but still quite OK for the purpose.

 

 

[metalman]

Thanks DP, the pitot/static system is all Skyfox ,and was checked as per inst 8 before reg, I don't doubt it's pretty accurate,,,,the plane feels slow as :)

 

I've wanted to look at a better wing with a friese aileron and convention type flap,,,,I'll PM you dafydd, a few of the things you mentioned I've done as I built the plane, the aileron hangers are much better ,it has plenty of drainage, and is always hangared , but I would like to do something better with the wings ,I built my ailerons as per the gazelle ones and so far all good, but I am very careful with turbulence ,always slow and steady in the bumps ,

 

Matty

 

 

[Air rafters]

Matty

 

I'm currently rebuilding my wings for my Gazelle/kitfox/raven/avid. They will be modified for flaps and ailerons as per the highlander. Possibly slats as we'll. Lots of work yet to do such as flap actuation but will most likely keep with the highlander design. My aim is STOL performance so the wing will be rebuilt with that in mind.

 

I have a private strip in Dereel, Vic and if your ever over this way drop in. The invite is open to all as we'll, the strip is 650 Mtrs grass north south just give me a call first so I have the kettle hot. ( or beer cold for those not piloting).

 

Ph 0417455244, h 03 53461248

 

Cheers

 

Mick

 

 

[metalman]

MattyI'm currently rebuilding my wings for my Gazelle/kitfox/raven/avid. They will be modified for flaps and ailerons as per the highlander. Possibly slats as we'll. Lots of work yet to do such as flap actuation but will most likely keep with the highlander design. My aim is STOL performance so the wing will be rebuilt with that in mind.

I have a private strip in Dereel, Vic and if your ever over this way drop in. The invite is open to all as we'll, the strip is 650 Mtrs grass north south just give me a call first so I have the kettle hot. ( or beer cold for those not piloting).

 

Ph 0417455244, h 03 53461248

 

Cheers

 

Mick

Cool, always looking for places and people to visit, I'll put it on my to do list, and keen to see your wing design ,

Matty

 

 

[metalman]

 

Would this be your place???

 

 

[Neil_S]

[ATTACH=full]27765[/ATTACH]Would this be your place???

Hi Matty,

 

Speaking on Mick's behalf - yes it is! I have flown in a number of times. Let me know when you plan to go there and I will try to get there too for a coffee/chat.

 

Best to choose a day with nil wind or northerlies as there are tall trees at the northern end of the strip which can make landing southwards a bit tricky.

 

Cheers,

 

Neil

 

 

[metalman]

Sounds like a plan, will have to tee it up for after Easter some time ,:-)

 

 

[aircrafters]

Matty, Neils right about the trees at the north end and just to help out for the flare the strip drops away to the south. Not such an issue if you have flaps and a constant speed prop but bit of a challenge in a floater.

 

Mick

 

 

[Bob Llewellyn]

So I am bit confused now, is this a problem with the normal Gazelle's?David

The fuselage carrythrough between the strut pickups is beefed up in all Gazzles; most, if not all, 'Foxes came out of the factory with the lighter tube, which should be reinforced.

The Blackwater accident investigation concluded that the aeroplane had flown threw horizontal "rotor" off the mountains, and the combination of reversing acceleration and extreme pitching moment of the airfoil caused the front lift strut to first buckle, and subsequently fail in tension. BCAR-S (CAP-482) does not specify a Vb; the gust factor is assumed to be catered for by the 4G limit manouvring factor, but add the highly cambered airfoil and the margin of safety can disappear. The Gazzle has essentially the same airfoil, and very similar structure in the wing, so I'd be taking it easy in turbulence.