Hi, I have recently started building the yellow aircraft A4 skyhawk and was looking for a bit of info, like what retracts to use also I was planning to use a OS91 with ramtec fan for the power unit?

Any help would really be appreicated

I always used springair retracts in mine.

The fan engine combo will work fine.

great and I dont surpose you can guide me on what JR servos's to use for control surfaces?

I flew that exact combo. on Hitec-605's without any problems... This was a while back I would now use at least 645MG's on 6V (133 oz.) or digitals for centering- Just to make myself feel better! I am poor though...

You runnin' the cheater hole? The one I flew had it and it seemed to help over my JHH A-4 Dynamax powered bird- that did NOT have it...

I also like Spring-Air, they are designed to COME DOWN if you have a leak, blow-out or just low pressure! Robarts are fine but remember the air holds them down, not enough air, no locket out LG-

Enjoy!

Like my avatar?

James

Hi ??
I flew a YA A4 on the Australian circuit for some 8 years, it is a great model. Springair retracts are the way to go (never had a problem). use the OS 91 VRDF and Ramtec combo, it suits this model exactly. You do not need the cheater hole in fuse bottom like some A4's, but do cut out the 4" hole where the bifurcated inlets meet, the Ramtec breathes just fine with this.
Do not reef the model of the ground, ease the model off, then push slightly down until it builds up speed, and it will fly great.
i can put you in touch with some jet guys in Qld if you want, they could steer you in the right direction as well.
Dave Burns
IJMC Rep. Australia

there used to be a guy in Oregon I think that made a modified intake for the Yellow A-4, it had a 3rd snout on the bottom.
I fly a friends A-4 that has it installed still and the take off performance is incredible. It will climb straight up accelerating after about a 150ft. roll.

I wish I still contact for that guy. That was back in 96.

That would be great I have also noticed with the OS91 there is a square head and a round head whats the difference?

square head if for pusher fan configurations. round head is for tractor fans

Yellow Aircraft A4 Skyhawk + OS 90 + Ramtec.: I have found installing the JR elevator and rudder servos very difficult due to a combination of my large hands and (we think) a faulty bulkhead moulding reducing the distance available gap. As the model will never be entered for any sort of scale competition, as it has the sport wing, I was considering doing away with the convaluted pushrod snake and mounting the rudder and elevator servos into the fuselage under the tailplane and using external horns and pushrods. My question is whether any one else has done this on this model and if so, what servos did they use and how did they strengthen the fuselage? Also: where is the usual location for the receiver and battery pack? And

yep my rudder servo was way up front in the nose connected by some flex rod through out the aircraft. thought that was stupid so I took the thrust tube out and stuffed a little hs-81mg servo back there.
Works great!!
I figured it would not be a problem with that tiny servo as it's metal geared and has plently of torque plus it's only going to have minimal deflection for short 4 point rolls.

My elevator and throttle servoes are in the tail as well under a pannel on the left side of the virtical fin

Thanks for the info regarding the servo's. So the elevator/throttle servos are mounted in the side of the Vertical stabaliser?

yep

Hi,

A couple of things:

First: If your A-4 kit is the latest one, DO NOT modify the inlet! Advice to that effect applies only to the older design or the JHH one. If you've got the newest kit, with the redesigned inlet, you'll screw it up by cutting a hole in it.

Second: Avoid putting servos in the tail at all cost! If you use the recommended hardware (metal wire pushrods in plastic sheaths), you'll have minimal slop/binding. The servos in the tail 'work' fine, BUT it translates out to a lot of unnecessary lead in the nose. Two ounces in back might mean as much as 6 or 8 up front to counter it. This is a bird that you want to have as light as possible. Trust me on this! If you've got the newest kit, there should be plenty of room for the servos in the forward hatch area. Rig up the linkage first, then place the servo to meet it instead of the other way around. That way, you'll get the proper spacing between the servo and the inner wall of the fuse.

I've built this model six times, four for DF (Dynamax, Ramtec, Viofan) and two for turbine (P-80, JF-50 (for a customer)). I've built the scale and sport wing versions (recommend the sport wing...) stock, and with modifications (split flaps, dive brakes, scale refuel probe) and it's a really super airplane as long as it's not too heavy.

I hope this helps....

Yeah , mine was bought second hand from my buddy Greg Simpsion so I did those mods cause the linkages were old and sloppy, plus I used the smallest but powerfull servoes back there and used standard size servoes up under the canopy area for my nose gear and a micro for the gear. I rigged up springs on the doors so they lean slightly against the struts and close and open very nice!!
my air tank is in the nose in front of the gear door as well as my 7.4 1200mh batt plus I added a 2oz header mounted on top of my 16oz so I hope that helped componsate for the servoes out back but I haven't checked the cg yet but it is only ten pounds wet .

Let me know what you think sean

Mike

I bought my A4 kit secondhand so im not sure how old it is, is there anyway to tell if it is one of the new kits or older?

Once again, thanks for the input. Unfortunately the kit is about 8 years old, I have been building it on and off for about 3 years having bought the kit in a importers (Ripmax) clearance sale here in the UK. The space for the servos is very very tight. The only one that comes close to fitting is the JR specified in the manual. We can only assume that such a powerfull servo is specified is because of the "sticktion" in the push rod linkage. It is due to this difficulty that I keep shelving the build for easier projects, even considering selling it on! With the advent of smaller servos being readily available, I would be prepared to sacrifice weight, balancing the servo position with a slightly bigger NiCad or even a Battery Backer system for greater safety.

Another issue is the availability of a suitable nose leg instead of the Yellow aircraft/Robart unit which is only available by importing it from the US.

Oh why Did I Start This!!!!!!!!!!!!!!!!

Hi,

Of course if you're trying to mitigate the someone-else-already-started build job, then do what you gotta do! As for being able to tell the difference, yes, there is a simple way. Look at your canopy--is it a precut fiberglass frame with a plastic canopy glass interior, or plastic canopy with the canopy frame molded in? Also, look at your inlet--does the rear end of it where it meets the fan look like a fat "C" (due to the center recess for the fan hub) or is it just round, allowing you to see the whole fan face from one side? If it's the former in both cases, it's the newest kit.

BigT,

We can send you that strut via the US Mail. It's really not that bad. If you do end up putting your servos back there, a heavier battery pack isn't gonna do the trick. You'll end up filling the nose cavity with lead. If it has to be done, then it has to be done. The plane will still fly heavy (I had one at 14 lb, 11 oz dry).

Good luck!

Shaun,
From your description this kit must be of the latest design. I assume the "fat c" means that the fan is viewed from the exterior of the fusealage through one of the inlets? If that is the case then I must have got the fan mounting crossmember too far forward as I cannot get a standard Futaba servo + mount in for either the elevator or the rudder without a great deal of pushing and shoving! What I plan to do is put a Hitec mini servo under the rudder and above the tailpipe to work the rudder, dry fit everything else inot the model and see just how far out the c of g is with the elevator servo mounted forward/centre of the fin under the tailplane. If I need a huge lump of lead in the nose then I will think again.

How much is the front strut including postage and is it really necessary if you are flying off a smooth grass strip?

Hi,

That strut is $60, and it's another $10 to post it to the UK.

I wonder if you could shoot some pics and send them to me of your installation. If your fan is in the proper place, I guess I can't understand why there wouldn't be room for the servos. As far as the CG thing is concerned, it's really not going to end up being an experiment. With an O.S. 91 installation and a regular-sized tuned pipe (like the JMP) you will have to add ballast. The only question is how much. With the servos in the tail, it will be a lot. There are advantages to having the servos back there like the smoothness and rigidity of a direct linkage, but you'll pay for it in low-speed performance.

Well I guess this means that mine is one of the newer models, I decided to go with a Rossi90 and ramtec fan, I am just up to installing the elevator and rudder servos now but dont seems to be having all the trouble BigT is having, It's a tight fit but there is enough room to do it.

I wonder if it is the physical size of the servos? I cannot get the reccomended JR4131 as it discontinued, so what would you suggest? the 4131 had a rating of 6.5kg which does seem rather a lot for a DF. I only used Futaba 3001's for the all flying tailplane of my Paul Grey Hawk.
JR servos are physicaly smaller than futaba so which JR would you use now?
Big T

Hi,

On the last DF one I built, I used standard size servos. The elevator was a 4721, and the rudder was only a 531 or something. I wish I could see a pic of your setup...

Hi Shaun
I have e-mailed some pics to the Yellow Aircraft sales e-mail address as well as this one
Big T[img][/img]

Hi,

Well, there's good news and bad news.......


The good news is: I figured out the problem at a glance.
The bad news is: It's a pretty serious problem.

It looks like your fan mounts are way too far forward. This can happen one of several ways. If you use the fan to space things, and the rotor isn't in the fan, you can push the fan housing much further forward than it's supposed to go. That's my guess in this case. Either way, the engine former E-1 should be about 1-1/8" aft of the rear edge of the inlet. It appears, in your pictures, to be about 1/8" on one side; and about 1/4" on the other. The reason it's bad news is that it's no easy task to move those fan mounts.....

Looks like maybe this one is gonna have to have the servos back there somewhere? [&o]

Shaun Hi, OK if I was building another one (which i wont) I would have used my brain a bit more and tacked in the servos BEFORE I fitted E1 & E2. thus ensuring that all was in the correct place. It would hav been really usefull if those dimentions where in the manula on page 15. All it says is Trial fit E1 and trim as necessary. I did use the Fan housing to space things but I did have the fan installed. At this stage of the build you dont have the inlet ducts in place so you have no idea how little room there is. I respectfully suggest that the manula needs to be updated for simpletons like me.
However: the good news is that as I use a space age adhesive to fix all my formers into fiberglass (its a 2 pack adhesive used in the motor trade to fit kevlar and fiberglass body panels onto performance cars), I could have dug out the formers. But I chose a different route. By carefully "notching out" E1 I have managed to fit in the servos in such a way as to allow full movement, easy removal of the servo, clevis and servo arm and maintain the structural integrity and rigidity of the mount. Just to make sure we glassed in a few carbon strands. I will send pics as soon as I am able. Incidently, in case you worry, the brains behind the mod is also a UK CAA inspector for large 20 kg plus models so It should work. By the way having rigged up the pushrods I can see why you need a 6.5kg servo!!! Thanks again for your help. BigT