TW-742 Sky Hawk Aerobatic Glider

Author: Dmitry Nizhegorodov (dmitrynizh@hotmail.com). My other projects and articles

tw742.jpg

1.   Specs 

Wingspan: 46" 
Weight: 600g
Motor: 380 brushed (some reports mention 480, but it's unlikely: 480 is a *much* bigger motor)
servos: 8g (3 servos total)
ESC: brushed 10A

RTF package includes:
Manual, 4-Channel 27mhz Rx and TX, two 9.6v Rechargeable Batteries (110mAh Ni-Cd 8-packs), 
Battery Charger Adapter, Tools for assembly.

2.   Why I bought it

I chose TW-742 Sky Hawk Aerobatic Glider as my first plane and looking back (I maidened mine in late Jan 2008) I glad I did although this path is not for everyone. New to R/C hobby but with solid model building and sim flying experience, I wanted to start with a complete RTF package - a plane plus radio/electronics - that would not cost me much even if I trash it or lose in the sky. I also wanted my first plane to be a sailplane because there are lots of places to try slope soaring. I also was not interested in 3-channel beginner packages like EasyStar, I wanted ailerons because I've learned to use them on simulators. TW-742 looked right, and soon I snapped two TW-742s on eBay, one for me and another for a friend, for a great price of under $100 each. Soon, even before my first flight, I found that the stock plane has some apparent shortcomings, but this was expected: you can not get fantastic quality and extreme reliability from an RTF package costing under $100. I later found that most TW-742's problems are easy to fix if you have a bit of model building experience and understand electronics, and my TW-742 served me perfectly for many month, survived numerous crashes, high tree landings and resque operations, storm winds, violent thermals, and I still regularly fly it in its heavily modded form I call TW-742 RGY.

3.   Stock TW-742 and its Character

In its stock form, a brand new TW-742 flies surprisingly well for its class and shows good soaring capabilities, thanks to its slim body and folding propeller. By modern parkflyer standards, TW-742 is rather underpowered with its speed-380. Yet it climbs OK, stable and can handle wind, and can slope-glide and soar thermals. Radio gear looks very cheap and is basic, but has a rather good range (over 1 km in-air) and worked *very* reliably for me! The plane is far less durable and crash-resistant as EPP planes, but its fuselage made from some unbreakable plastic polymer is excellent because it warps and even shrinks/folds in crashes but does not break. The nose absorbs crashes well and saves motors, and can be straightened up easily if heated with hot water or even under strong sunlight. Tail "feathers" look flimsy but are in fact reliable. Made from extruded EPS foam, the wing can easily chip or break in a crash, but I quickly learned the art of taping the pieces together, and the plane served me well through my learning curve.

4.   Wing and Tail Improvements

There is long list of improvements possible for TW-742, each fixing one of plane's shortcomings in construction or improving its flying. Some of these mods are easy, some require certain skills. I'll list the most important ones first.

Must-Do: aileron control system improvements. The weakest point of TW-742 is wing and on that, the aileron linkage and then the central trailing area. The whole linkage system in fragile and may break during first not-so-smooth landing. There are two main problems: first, the two torque rods made from bent steel wire end in EPS material of ailerons and the plastic tubing around these designed to protect the foam still ends transfer a lot of stress to ailerons, causing increasing slack/leeway in linkage and breaking the ailerons sooner or later.

The second problem spot is the location of the aileron servo. This servo with plastic arm and two parallel linkage rods are located at the center of the wing, hidden inside of the fuse. When the plane crashes or during a rough landing, the wing held by the rubber bends tends to move around, the sides of the fuse then hit the servo, the arms and the linkage. Soon, the arms or the horns will break or the servo will strip. Next, a related problem is that when the wing moves aside, rubber bends often slip past the protective plastic fold and cut into EPS.

After flying the plane for several months and constantly fixing these problems I ended up with disfunctional ailerons and damaged wing. I decided to completely rebuild the wing. I gave it a second life by installing a separate 5g aileron servo in each wing half. Aileron horns are plastic and the pushrods are z-bent pieces of metal wire.

Optional: trailing edge reinforcement. The trailing edge between the ailerons was reinforced so that the rubber bends would not damage EPS.

Recommended: tail pushrod guides. These prevent tail pushrods from bending. When a pushrod bends, it does not break, but it stops controlling. Add a pair of wire "guides" positioned midway between the fuse pushrod slots and the tail as shown. This prevents the pushrods from potentially very dangerous bending during operation.

The outcome of these improvements was excellent! Once the aileron servo linkage is removed from the middle part of the wing, crashes no longer result in stripped ailerons servo, and you can adjust the bends in such way that the wing is not damaged in crashes. I highly recommend separate aileron servos for any owner of traditional sailplane-style wing.

5.   Brushless Conversion

The next most troublesome spot of stock TW-742 is motor/ESC. After a while, the motor may develop a habit of locking on start - typical for some brushed motors - and that sends way to much current through ESC, burning it. After two motors and two escs, I decided it is time to go brushless. A TP BM2409-18 + 25A ESC combo deal from Hobby City, $18, gave a new dimension to my TW-742. I wanted huge vertical authority and went with BM2409-18 for this reason. However, TowerPro BM2408-21T + 18A ESC combo - a $15 deal! - is even better choice for TW-742, especially if you want to preserve its sailplane character and fly with folded props.

These outrunners cannot fit inside of the fuse and must be mounted in front of TW-742's firewall as shown:

The brushless conversion had transformed my TW-742 into a fun, wind-resistant sports/aerobat with excellent vertical performance, light and floaty.

The best propeller for TW-742 with BM2409-18 is GWS 9x7. You need to use a prop saver for it. I use thin rubber bend allowing the prop to fold when TW-742 belly-lands.

The best folding propellers for TW-742 with BM2408-21T are 7.5x4 or 8x4. Make sure you selected the hub matching your motor's shaft diameter.

6.   Red, Green, Yellow

My RGY mod is all of the above plus some more advanced rework of the wing and the tail. To re-vitalize my beaten up wing core, I stripped it out of everything, removed the factory spar, added my own wooden spars (changed that to much lighter CF tube later), move the EPS part of the wing through a "cloud" of 3M 77 adhesive spray (never spray the 77 on EPS directly) and added it more deliberately onto my EPP leading edge, and covered the core with 2mil colored packing tape. I used 3 colors - Red, Green, Yellow. It is important to leave the taped core under some press overnight. After tapes's glue and 77 cure, the bond is nothing but incredible for such light shell and the wing is very stiff. Some filament tape reinforcement was added, too. So far, the wing hold it shape and flying characteristics very well.

When re-shaping the wing, I reduced the dihedral to barely noticeable (I now wish I left none!) and also shimmed the trailing part of wing mount, substantially reducing the incidence. The stock plane's angle of attack on the wing is too big to my liking.

The tail feather was also reworked, I mainly added some area to control surfaces. The rudder received a counter-balancer. I added new tail servos - HXT 900, 9g servos from HobbyKing,

CG on my RGY TW-742 is substantially aft from the stock location, which together with reduced dihedral and incidence greatly improved performance.

7.   FMS models for the stock TW-742 and for my RGY variant.

TW-742 Sky Hawk 4ch aerobatic glider Download zip archive.

My heavilly modified/improved TW-742 Sky Hawk. Download zip archive.

Author: Dmitry Nizhegorodov (dmitrynizh@hotmail.com). My other projects and articles