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A4 Skyhawk build

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Old 09-11-2011, 04:14 AM
  #1
Alex48
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Default A4 Skyhawk build

A project for myself which makes a change I had wanted something bigger but this came up and it was too tempting to refuse.

This kit is the prototype pre production model that Skymaster and a number of other builders have started to put together. Loads needs to be done/re done but I'm impressed with the detail and think it should make a great addition for next years fleet. Progress will be slow as I have some more important projects to do but it should plod along nicely.


Length: 85.5" (97" with refueling probe) (2225mm). Wingspan: 61' (1550mm).

Power: 25~32Lb (12~15kg) thrust

dry weight: 14.5-16.5kg (32-38Lbs)

The model weighs 20lbs add 4lbs of electrics 3.5lbs of turbine and ancillaries and 7lbs of fuel we're talking 34.5lbs wet. I'm hoping a Merlin 140 will work as this gives around 33lbs of thrust here in the UK which is more than its rated for but I'm finding Gasper at Jets Munt is conservative in his specs which makes a change for a manufacturer. If it needs more of a push then a Jetcat 160 will be used but this is over a pound heavier so my hope is that keeping it light will work better :xx

I spent an evening cleaning the model with meths and degreaser as it was covered in cack and red over-spray from somewhere. I like to do this as it does show up the areas that need attention and in this case there are five or six small crush marks and hanger rash that needs sorting before anything goes in it.
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Old 09-11-2011, 04:16 AM
  #2
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Default RE: A4 Skyhawk build

The gear is worth talking about. As usual the quality is as good as it gets with the higher end stuff that Skymaster make. The mains are twist and turn with lovely wheels and brakes. The nose gear is interesting in several ways... The steering is accomplished using hydraulics and is completely proportional and the nose oleo can be lengthened and shortened via the transmitter to change the AoA of the aircraft on the ground which is similar to what the full size did. The nose oleo automatically shortens when retracted by releasing the pressure in the reservoir, when it retracts the second slender cylinder pulls the oleo in. It will be hooked up to a multifuction valve for retraction with a separate single electronic valve for the oleo adjustment.
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Old 09-11-2011, 04:19 AM
  #3
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Default RE: A4 Skyhawk build

The wings need work unfortunately. The gear doors have been installed but clash with each other when opening. The forward main gear door needs to be hinged about 10mm further outboard using robart style hinges and two small slots to get the distance which is what they did on the production airframes. The gear doors have no internal structure so are very flimsy, I'll either need new doors or I'll build something up myself. There is also a small hatch that's been cut just outboard of the forward gear door probably for an air cylinder but I can put one forward of the wheel for the gear door actuation. Luckily the hatch has been cut with care so its just a matter sticking it back on.

Anyway I'm looking forward to getting this one ready especially if it flies as nicely as Ali promises they do
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Old 09-11-2011, 05:52 AM
  #4
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Default RE: A4 Skyhawk build

I'm glad you got yours! I can now follow along when you pass me! A couple of notes.....
1. The flap servos in mine went in a ply mount pre installed. I used jr8411s. You will have to sand, esp the glue on the wingskin to get them in. Also I put a small servo horn facing straight down so the linkage doesn't bind with the scale "pod" cover.
2. The Stab also has a pre installed ply plate and I installed the servos "face down" (towards the stab tip), you will need small servos Jr3421 is what I used.

The airelons are pretty straight foward. I'm sure you'll pass me soon enough I build slow! I am looking to possibly change the rudder set up. Let me know what you decide.

Peter
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Old 09-11-2011, 07:15 AM
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Default RE: A4 Skyhawk build

Hi Peter

This pre production model doesn't have any servo mounts in for the flaps, ailerons and stabs. I'm able to fit a standard size servo in the stabs either a JR8411 or I might go all Savox for this model and use their 12kg servo on the elevators and the fast 10kg on the ailerons as I want good response for the gyro. I'm with you on the rudder... this requires a bit of work as it damages the top of the fin as it moves so I'll re profile the leading edge of the rudder towards the top. The servo arrangement is my least favorite method for rudders and if I was doing it initially I would have done it the conventional way and cut a hatch on the side of the rudder. I wont need a gyro on the rudder with this model so resolution wont matter as much. I'll just see how I get on... Be good to see some pictures of your build if you have time.

The servo will be the other way round once installed.
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Old 09-11-2011, 07:20 AM
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Default RE: A4 Skyhawk build

The fin and stabilizers have now been cut back with 600 flatting pads and imperfections filled and leveled. The slots for the servos horns had already been cut along with the horns on the surface being installed, they just needed neatening up. I'm keen for the fin and stabs to be removable mainly to help me with winter storage plus it's useful for transport if not necessary. To do this I've cut a few holes under the stabs that will enable be to get a ball driver at the bolts.

The fin post and stab attachments. The stabs required some fettling to get a really nice flush fit to the fuz. I'll also have to make a small former to place in the fuz towards the front of the stabs to stop any compression thats evident when you move the stabs by hand.
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Old 09-11-2011, 08:26 AM
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Default RE: A4 Skyhawk build

I will try and post some more pictures. I had measured for my servo horn location three times! But had to make the slot way larger than I had wanted (read search and destroy mission to find servo horn). I still have some final shaping to do on it. Interesting on the control horn for you stab, I only recieved the large horn as in the flaps, not the two on you set up.
I have not cut for the horn yet.....
I did not get any of the linkage shown on the rudder install and there is a slot on the bottom of my rudder?
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Old 09-11-2011, 09:18 AM
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Default RE: A4 Skyhawk build

Looks Good Alex............. Are you permitted to build one for yourself

Dennis
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Old 09-11-2011, 09:53 AM
  #9
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Default RE: A4 Skyhawk build

Thanks Dennis, I think so.... I haven't heard of any complaints yet, as long as the customer models I have get finished before this I should survive

I'll take some photos of the rudder set up for you Peter. Bear in mind my model had a few previous owners so things like the horns and servo linkages were probably done by them and not Skymaster. Other than those few things its basically untouched. I like to use double horns on the surface for these medium sized models with a decent clevis on the servo side. It distributes the load better than a single horn. (assuming your using a ball link on the other end)
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Old 09-11-2011, 05:48 PM
  #10
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Default RE: A4 Skyhawk build

How did they do the proportional hydraulic steering? I'd love to know how they did that so i could use it on the spoilers of my F-14. Would be allot easier than trying to gang 4 surfaces to 1 or 2 servo's.
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Old 09-12-2011, 02:39 AM
  #11
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Default RE: A4 Skyhawk build

Having now done the rudder it actually works rather well which has taken me by surprise as the other models I've built using this method have been less than satisfactory. The only slop in the rudder is that of the gear train on the 8411. Still need to see what its holding power is like but feels promising.

The steering uses two cylinders on either end of the system the servo moves one and the other moves proportionally to it, have a look at post two and you'll see what I mean.

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Old 09-12-2011, 02:51 AM
  #12
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Default RE: A4 Skyhawk build

The stabs had seen better days, part of the structure looked like it had been yanked out to make room for a servo and the slots that had been cut are in slightly different positions. Its important the linkages are exactly the same length and the servos are in the same position on either stab. I cleaned up the inside then made two 1/4inch ply mounts which I epoxied in. I'll just fill part of the longer slot with balsa and seal with filler to make good.

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Old 09-12-2011, 02:59 AM
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Default RE: A4 Skyhawk build

Alex can you let me know the breakdown on that rudder set up? Dimensions of servo horn, and length of bolt?
I still haven't decided on the rudder set up yet....
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Old 09-12-2011, 03:48 AM
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Default RE: A4 Skyhawk build


Hi Peter, no problem... I'll have a look.

I like the cockpit to be easily removable without any screws or fixings other than an engineered fit. The benefit being I'm able to quickly take the cockpit out at the field to get to valves and switches etc... which will be necessary with this model as access space is very limited. I spent some time accurately trimming the tub to shape. I've also cut loads of material off the inside frame work on the fuz to make it look a little more believable. The cockpit now slides in and is held in place by a cross brace and two ply tabs at the back, once the canopy is on its held in place.
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Old 09-12-2011, 04:48 AM
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Default RE: A4 Skyhawk build

looking good alex. Any idea how they got all the air out of that steering system?
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Old 09-12-2011, 06:07 PM
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Default RE: A4 Skyhawk build

I'm waiting to decide on what elevator control horns to use and how to mount them, so after reading Alex's posts tonight I started messing around with the cockpit and one thing lead to another.......
monkey see, monkey do. I used some bvm canopy hooks for the front and some ply wood triangles for the back. Thanks Alex!
I still have to twist my cockpit to get it in. What width did you cut the fuse frame (rail) down to?

Peter
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Old 09-12-2011, 10:47 PM
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Default RE: A4 Skyhawk build

Do you guys think a smoke tank can be added? I'm thinking about getting this jet.
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Old 09-12-2011, 10:55 PM
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Default RE: A4 Skyhawk build

Hi Peter, at its widest point where the angle changes its 115mm wide, I do have to twist the cockpit initially to get it in.

You should be able to fit a smoke tank.
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Old 09-12-2011, 11:14 PM
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Default RE: A4 Skyhawk build

After a rather frustrating evening of trying to install the stab servos into an F5 I decided to do a little of the A4 at the end. By anyone's standards the fit of the two different refueling probes is poor and also the intake ducting is badly fitted with loads of glue and a large canyon running around the seem line. The advantage with gelcoat version is I can fix all of this before paint
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Old 09-12-2011, 11:55 PM
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Default RE: A4 Skyhawk build

Quote:
looking good alex. Any idea how they got all the air out of that steering system?
They didn't but its good enough for its application. I'm trying to think of a way to bleed it myself but still working on it.
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Old 09-13-2011, 12:38 AM
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Default RE: A4 Skyhawk build

I cant believe that you are making me want this plane all over again already.... Hang on! Seeing as you have yet to pay for it....
Fancy a swap for a lovely F-5?? Pro built and painted and all that jazz
Regards Al
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Old 09-13-2011, 02:53 AM
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Default RE: A4 Skyhawk build

Quote:
ORIGINAL: Alex48

Quote:
looking good alex. Any idea how they got all the air out of that steering system?
They didn't but its good enough for its application. I'm trying to think of a way to bleed it myself but still working on it.
It will be a little messy; but without a reservoir and bleeder, I imagine the best way to bleed the system would be by disconnecting the line from the closed end of one the of cylinders and put both the line and the end of the cylinder into a cup/bottle of fluid and "exercise" the cylinder to bleed the system. Once the air is purged out, you'll have to connect the line back to cylinder while it is submerged in the fluid to ensure no air is introduced back into the system by the line or cylinder draining. Of course you would have to remove whichever cylinder you choose to use from where it is mounted (strut or servo mount) to make it easier, but it should be simple enough. It probably wouldn't be a bad idea to secure all the lines on the fittings as well when done.

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Old 09-13-2011, 03:02 AM
  #23
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Default RE: A4 Skyhawk build

Alex it wont make you feel better, but those following along considering an A4, the prepainted one has no intake or probe issues.
So far everything fits very well including the canopy and hatch.
Thanks for the measurements
Peter
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Old 09-13-2011, 07:03 AM
  #24
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Default RE: A4 Skyhawk build

Quote:
I cant believe that you are making me want this plane all over again already.... Hang on! Seeing as you have yet to pay for it....
Fancy a swap for a lovely F-5?? Pro built and painted and all that jazz
Regards Al
lol, I wouldn't want to deprive your shop ceiling from a nice new ornament and besides after JP this A4 will be distant memory. The funny thing is, your the 2nd person today wanting to buy this off me once complete......anyway I wouldn't worry I'm even installing JR servos for you as I did promise first refusal to yourself enjoy JP.

Quote:
It will be a little messy; but without a reservoir and bleeder, I imagine the best way to bleed the system would be by disconnecting the line from the closed end of one the of cylinders and put both the line and the end of the cylinder into a cup/bottle of fluid and "exercise" the cylinder to bleed the system. Once the air is purged out, you'll have to connect the line back to cylinder while it is submerged in the fluid to ensure no air is introduced back into the system by the line or cylinder draining. Of course you would have to remove whichever cylinder you choose to use from where it is mounted (strut or servo mount) to make it easier, but it should be simple enough. It probably wouldn't be a bad idea to secure all the lines on the fittings as well when done.
Good idea, I'll give that a go...thanks

Quote:
Alex it wont make you feel better, but those following along considering an A4, the prepainted one has no intake or probe issues.
So far everything fits very well including the canopy and hatch.
Thanks for the measurements
Peter
No worries...

Glad your pleased with the production version Peter, you hear so many horror stories on these forums its nice to see someone happy with something I should probably say that the kit I have won't be very representative of the production models as its the pre production version. The last full pro kit I did from Skymaster was an A10 and this was good so I'm sure the A4 is no exception.
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Old 09-13-2011, 12:46 PM
  #25
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Default RE: A4 Skyhawk build

Peter, the rudder servo horn from the centre of the screw to the centre of the bolt is 10mm and the bolt length is 20mm, I'll not take credit for doing it as one of the owners before did this rudder set up and very well I might add! It must have been someone different who had a go at the stabs. The only problem is that under load the rudder and servo are rock solid but it moves the back quarter of the fin in the other direction. I'll have to beef up the support and make a ply former to tie the servo mount into the aft fin spar, if that makes sense? I'll take some pictures when I do it. This is how I've done the elevators if it helps..

Cheers, Alex

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