HiFly 86" Yak 54 With MT-57

[Mass Balance]

We have received our first example of the new HiFly 86” Yak 54. Our prototype plane was built to use a DA-50 or 3W-50 with an MTW TD75 canister exhaust. In this thread, I plan to show you how we have gone about assembling this beauty and the changes we have made, if any, to enable the use of an MT57 engine and an MT canister exhaust. The MT canisters come in either 60mm or 65mm diameters, and the production Yaks are being produced to fit the 60mm canisters.

The plane shipped in two boxes, one with the flying surfaces and the other with the fuselage and everything else. Out of the box, the plane looks very light, and the framework is highly skeletonized. The covering is Oracover with few bubbles or wrinkles in our example. The fiberglass cowl has a baffled front with a cut-out for engine cooling, so we should need no other baffles, just an exit opening below. The gear is also fiberglass, and the plane is supplied with carbon fiber wing and stabilizer spar tubes. Upon first inspection, the hardware looks good and I plan to use most of it, if not all.

[Mass Balance]

Here are some detail photos of the plane. I began by going over the covering with an iron, then cutting the openings needed in the fuselage covering with a No.11 blade for the wing cutouts, and a soldering iron for the smaller openings. Then I began gluing in the fiberglass control horns for the ailerons, elevators, and rudder. The control horns are the dual-arm design and feel quite stiff and strong. I had to enlarge the slits in the control surfaces by a millimeter on each end, then I roughed up the gluing surface of the horns with coarse sandpaper and glued them in with Gorilla Glue (with which I have had excellent past results). Use care to be sure that both horns in the pair are in alignment and fully seated in the surface. The rudder openings for the two-sided horns needed to be cleaned out with a No.11 and a flat needle file, then they were installed. As the Gorilla Glue cured over the next 30 minutes or so, I made sure to occasionally check it and remove any excess that foamed out.

[Mass Balance]

The canister tunnel in our Yak will accept a 55mm diameter can, but I have the larger, C65LX long canister, which is about 1 liter in volume, and sized for up to an 80cc engine. I want to see how it will work with the MT57. Enlarging the tunnel won't be hard, but the landing gear plate is in the way, and will have to be re-worked.

While thinking about how to accomplish this, I began gluing in the hinges for the rudder, ailerons and elevators. This plane is supplied with flat hinges, and you might think at first that they are pre-glued, but they are not. As recommended by the instruction manual (provided on CD) I used Pacer Hinge Glue. One thing that helped with the long slots for the flat hinges was to run the soldering iron along the slot. This neatly trims the covering away from the slot, leaving a nice uniform edge. The Pacer Hinge Glue is quick to set and the easiest to clean of any I’ve tried. Make sure the hinge slots are not too tight, and use plenty of glue, or else you may find the hinge sticks before being fully seated in the slot.

[littlecrankshaft]

Looking good...subscribing on...can hardly wait to see this bird fly. Are you going to have it ready by Thanksgiving? LOL

[Mass Balance]

If all goes well, she should fly by Thanksgiving. I've got more to post tonight!

[Mass Balance]

Before proceeding with the canister tunnel work, I tackled the engine mounting. I needed to see which type of header would work best with the canister and MT57. The Yak firewall comes marked for the DA-50 or the 3W-50. I had a template that showed both the MT57 and DA50 mounting patterns, so I used it to mark the centerlines of the mounting holes for the 57 in relation to the factory marks for the DA-50. The rudder was not yet installed, so with the fuselage supported vertically on the tailpost, I set the MT57 in place on the firewall. The template was correct and the MT57 standoff centers lined up with my marks on the firewall. While it was in place, I made a few marks to serve as guides for the carburetor cutout.

For a round-cowl plane, the spacing from the front of the cowling to the back of the propeller can be greater than for an Edge or an Extra. The HiFly instructions said to use the 1.25” standoffs if you are installing the DA-50. Looking at the DA-50 side dimensions from their website, the face of the prop hub would be 5.07” from the firewall using those standoffs, and the carburetor would project back through the firewall. Then I compared the MT57 with its stock 2-3/4”” standoffs, which is 7” long overall. If 1” standoffs were used with the 57, the face of the prop hub would be 5-1/4” from the firewall, and the carb mixture screws would still be easily accessible behind the firewall. I decided to use 1” standoffs.

I saw three ways to obtain short standoffs. The MT57 uses four, M5 x 25mm bolts to hold the crankcase together and act as studs for the standoffs, which are threaded internally. If these bolts were long enough, you could use plain oak dowel pieces 1” long for standoffs. You could then simply use locking nuts and washers behind the firewall, as is commonly done with other gas engines. One caution: If these bolts are removed from the crankcase, you have to be careful not to create a leak at the crankcase split, so do one at a time, carefully (or better yet, not at all if your engine is under warranty). I looked for M5 x 60mm long socket head cap screws with fully threaded shanks, but I was unable to find them locally, although they could be special-ordered. The second option would be to make or have made a set of 1” threaded aluminum standoffs. These could be had from a local shop, or perhaps ordered from a supplier such as Forgues Research. These could be made on a lathe and internally threaded with the correct M5-.80 fine thread all the way through.

Deciding to save time and money, I used the third option: cut the stock standoffs to 1” length. It works, and is probably the quickest way of all to do it, but many would wisely not choose this option. Having no lathe, I carefully marked my standoffs at 1” from the big end with a fine point marker. I used the Dremel cutoff wheel to start a fine cut at the mark, then finished the job with a mini-hacksaw (these have thinner blades than regular hacksaws). After cutting all four to approximately the same length, I used a file and a straightedge to even them up. I used my M5 tap to finish out the threads because the MT standoffs are not threaded all the way through like I had assumed. In the end, they look good and work fine. In hindsight, you could cut these to 7/8” long instead of 1” and skip using the M5 tap. The engine to firewall spacing would still work well.

When I re-fit the engine with the short standoffs, I saw that a notch was needed on the bottom edge of the engine box former to clear the cylinder, but this was no problem. Later, after test-fitting the cowl, I saw that the mounting holes I had drilled in the firewall had to be moved 3/8" to the right (facing the firewall when upright) to center the prop hub in the cowl. This was because I didn't account for the right thrust angle built into the engine box before drilling. I plugged the holes with dowel pieces and drilled new ones to the right.

[closetguy]

very good idea on keeping those standoffs short.

[apalsson]

Looking good Mike!!

I have the exact same plane on the way as we speak so I'm very interested in seeing your progress. I had been planning to build ours with the new Australian made YD-A 56cc engine but the MT57 headers will fit that engine nicely.
Then, depending on the outcome of yours, I might stay with the MT57 because they are quite popular here Down-Under

Keep those photos coming!

[marksextra]

Mike, Everything looks very nice so far.

...... is there a background about this plane? How it came to be, who sells/or is gong to sell it, specs, price point, etc.?

[Mass Balance]

Thanks, Mark! I try to make each build better than the last. Southwest RC Engines at www.SWRCEngines.com , is the US HiFly distributor. I've sent you a PM with the commercial info. Here are the basic specs on the HiFly 86" Yak:

Wingspan: 86"
Length: 76"
Wing Area: 1420 sq in
Flying Weight: 15.5 - 17 lb

Tomy Shen with MT Engines has been working with HiFly Models for the past few months to develop several combo packages. I understand that HiFly is a new organization that was able to pick up some of the best skilled workers from a couple of earlier companies. The Yak we have is of excellent build quality, which would seem to support that, and HiFly has been very responsive.

Closetguy, I'm with you on the short standoffs. I like to keep the engine close to the wood structure for the best vibration damping, or use stout wood standoffs if possible.

Apalsson, thanks! I'm sure the YD-A 56 would be a good choice. The plane seems to be adaptable to many 50cc class engines and the round cowl gives you plenty of room. HiFly has video and pics on their site of the Yak with the MT62, and I know I could fit an MT80 in there, although that would be pushing the envelope pretty hard without some serious structural reinforcement of the engine box and firewall. With the MT57, I'm probably going to use the MT Universal Header, which is an adjustable swan-neck design. A rear-exhaust engine should also work well with this long engine box configuration.

[apalsson]

	Quote: Originally Posted by closetguy
	very good idea on keeping those standoffs short.

I agree! I like that aspect of the design. Having the carburettor through the firewall has many benefits and the shorter standoffs are a good idea.

I have just been on the Fedex website and our plane should arrive on Monday ! I can't wait to get my hands on it. You've got to love those tracking websites

I better also declare that I have some commercial interest in this because Model Aviation is the Australian Distributor for HiFly. The relationship is only relatively new but HiFly have been unbelievably responsive and ready to take up development suggestions.

We have for example requested that all kits sent to us come pre-hinged at the factory and the anti-rotation pins installed. They took at onboard straight away.
Another thing we will be requesting is that they include at least a small piece of eack covering colour used.

[apalsson]

Hey Mike,

Have you made any more progress on the YAK?
Keep those photos coming !

[Hose A]

	Quote: Originally Posted by apalsson
	Hey Mike, Have you made any more progress on the YAK? Keep those photos coming !

Mike is working on it, but he has some other "duties" that are taking priority for a couple of days. I'm sure he will be back in the saddle early next week.

[Mass Balance]

Not much time to post tonight but I have some photos of the throttle and choke arms I installed on the MT57. My method involves some risk and may not be for everyone, but here's how I do it.

On this series of Walbro carburetors, the ends of the throttle and choke shafts are peened (or "riveted") after the small arms are put in place at the factory, shown in the first photo. I center-punch and drill into the shafts using a 1/16" drill bit. The shafts are brass and easy to drill, but it's important to start the hole centered, and to drill straight, because the shafts are not very large in diameter. I go slow and use a bit of oil to keep it cool. Drill about 1/4" to 5/16" deep. After that, I follow the pilot hole with a 5/64" bit.

Next I use a 4-40 starter tap chucked in a tap handle to start threading the holes I've drilled in the two shafts. Some light oil helps keep the cuttings coming out of the hole. The tap is very hard and brittle, so be careful to back it out frequently to remove cuttings and to stop when you reach the bottom of the hole. The starter tap is the lower one in the photo. After running the starter tap down both holes, I follow that with the "bottom" tap, so that the threads are fully cut all the way down the hole. This allows a 4-40 bolt to fit properly for the full depth of the hole. Because I had two regular 4-40 starter taps, I cut one off about halfway with a Dremel cutoff wheel and smoothed the cut end slightly to make a bottom tap.

After cleaning the cuttings out of the threaded holes with a pipe cleaner, WD-40, or compressed air, check to see that a couple of 4-40 bolts will thread fully down into the holes. Then you can bolt a couple of the blue Hitec aluminum servo arms (supplied with Hitec high torque servos) onto the throttle and choke shafts. After that, I determine the best way to use a 2-56 bolt and nut to bolt the blue arm to the small Walbro arm, which keeps the blue arm from rotating uselessly. The pics hopefully show this better than I can describe. After all that is done, the user can decide if he wants to use a ball link, clevis, or swivel connector on the ends of the blue arms.

DISCLAIMER
If you don't have experience with this kind of work, seek help from someone who has worked with gas engines for a while. If the holes are not centered and drilled straight in the throttle and choke shafts, you may have to get carburetor replacement parts or a new carburetor, which will not be under warranty. If you break a tap while threading the holes, see the previous sentence. While drilling and threading holes in the throttle and choke shafts, take care not to get cuttings into the carburetor. When assembling the blue arms, use thread locking compound. If you are a machinist, you may be aware of drill sizes more appropriate for 4-40 threads (I just used what I had). If you are Australian, or in another non-US country, you may prefer to use M2.5 (?) bolts and the associated drills and taps in lieu of 4-40

The last pic shows the cutout in the firewall needed to allow full travel of the throttle arm. You can also see the ball link on the choke arm behind the throttle arm, which looks odd because of the angle.

[apalsson]

G'day Mike,

I like your hookup on the choke.
I have for a while used a similar arrangement on the throttle by bolting ALU servo arms as a replacement for the butterfly arm or the short arm that often comes as standard. Because the arms that come with the engines are so short, this allows you much better resolution form your servo.
Here is a photo of my last setup