Product Review- Thunder Tiger eHawk 1500
Posted on April 17, 2009 by Bannerman
Thunder Tiger has been producing ARF’s for many years. In the late ’80’s, I learned to fly with a .40 size nitro high-wing trainer by Thunder Tiger. More recently, Thunder Tiger(TT) has entered the powered glider market. The eHawk 1500 is a higher performance evolution from the smaller eHawk 1400. TT states that this model is intended for intermediate to advanced pilots. With an RG-15 airfoil, lightened, built-up wing, low wing loading and brushless power system, my expectation is for a plane with “warm-liner” type performance. I expect to have brisk clime-out, good thermaling with the ability to move fast and efficiently between thermals and fine aerobatics.
After opening the box, I found that the components were thoughtfully packed. Hardware was grouped logically and sealed separately so checking the parts count against the manual was a snap. I verified that all parts shown were on hand. One thing I really like about this ARF is that TT includes a brushless motor and folding prop with spinner. I have to assume the manufacturer has found the ideal motor/prop size combo for this glider, which takes, what sometimes feels like, the guess work or trial and error out of choosing the right set-up.
Required components are 3 servos, receiver, speed control and battery. I chose Hitec HS-55’s, Castle Creations’ Thunderbird 18 esc, FMA’s M5 receiver, and 3 different batteries to try. The 3 batteries that I planned to test were 3 cell lithium polymers with capacities of 1000, 1350, and 2200 mAh.
Before any gluing or screwing happens, I read through the instructions to check out the components for any issues that might complicate construction. The hardware looks to be good quality. The construction of the airframe is well done with clean, mostly tight and smooth joints. The notches at the rear of some of the ribs are slightly too long and don’t quite meet up with the sheeting, but this should not affect airflow or strength with this type of construction. The wings look strong with attention paid to keeping the weight low with nice lightening holes in the bottom the the D-box sheeting, trailing edge and ailerons. The wings have no warps and only need some minor tightening of the covering which I will do with a trim iron. Both ailerons are warped, but that will be resolved by taping one end of the aileron to the wing twisting the other end in the right direction while going over the surface with the iron. Most ARF’s need to have their covering tweaked a little out of the box. The V-tail is sheet balsa with the control surfaces pre hinged with covering; a very nice touch. The fuselage is a pod and boom design. The pod is strong a lightweight gel-coated fiberglass with a flawless finish. The boom is an exceptionally strong spiral-wound, tapered carbon fiber tube.
Assembly could not have been much simpler. The wing assembles without glue and ailerons use CA hinges. Wing bolt holes need to be drilled in the fuselage. I took my time to carefully align the wing before marking for the holes by temporarily sliding the boom onto the fuse pod and measuring the wing tip to boom tip distances. The HS-55 servos fit perfectly in the servo tray, but I needed to shim the tray to fit snugly in the fuselage. Thunder Tiger has a convenient design for the tail assembly that requires no glue. The balsa v-tail feathers are sandwiched between plastic v-shaped pieces that slide and screw onto the boom. The firewall was already epoxied in place with what looked to be appropriate thrust angles. I used blue thread-locker on the two motor mounting screws, which the manual didn’t mention, but I know from experience that even electric powered aircraft can shake screws loose.
Bench testing the power system revealed 12 amps and 134 watts at full throttle. Unexpectedly, the box listed the weight with the recommended 1100 mAh battery at 600 grams(1.3 pounds), but mine only weighed 479 grams with the 1000 battery. Power to weight ratio looks very good at 127 watts per pound with the 1000 mAh pack. With the heaviest of the 3 batteries and a flying weight of 558 grams or 19.7 ounces, the ratio is 110 watts per pound. Wing loading ranged between 7.14 and 8.28 ounces per square foot with the different battery packs.
I maidened this glider on a overcast day with moderate winds. Wow; the eHawk climbed vertically, which was a nice surprise, because some ARF planes with included motors are less than impressive. After leveling out and motor shut-down I only needed slight elevator and aileron trim. There was something wrong; the plane seemed to be sinking way too quickly and I as I flew it back closer to me I realized the problem. The folding prop kept spinning, although I had the throttle off. After landing I found the default setting on the speed control was break off. Folding propellers need to have the motor brake activated to actually fold in flight and if it doesn’t fold and keeps spinning, the drag can be considerable. The second flight with brake set to “on” made a huge difference with the glide. I had a small issue with the Thunderbird esc not providing strong enough brake force. For example, I had to bring the plane to a near stall to assist the brake to stop the spinning propeller. I would recommend a speed control with an adjustable strength brake, such as the Castle Creations’ Phoenix 25 esc. Stall testing showed no tendency to spin, even with full up elevator. I found the plane to have trouble penetrating the wind with the 3 cell 1000 mAh Li-poly. A heavier 1350 mAh pack helped penetration and glide speed, but the glider still seemed to need more weight. I settled on a 2200 mAh 3 cell which really improved the glide. Climb-out speed was a bit lower with the heaviest pack, but the glider flew best with it.
Another day with some sun let me more fully explore the eHawk’s capabilities. I spotted some buzzards working a thermal and joined them at a lesser altitude. The thermal was light, but I was easily able to find it and stay in because the wings react quickly to air movement. Finding thermals with a tipping wing was essential on a light day as this one.
After gaining some altitude I wanted to play. Mild aerobatics are easy with the high end of the recommended control throws. I wanted to do some power-on aerobatics and really crank it, which almost lead to disaster. I tried to level out the plane and it didn’t feel or look right. I flew close into myself to get a look. The v-tail had rotated some 45 degrees. With gentle controls, I landed her. It turned out that I had not tightened the set screw that holds the tail to the boom enough. During the build, I hesitated to over-tighten the screw as not to crack the carbon tail boom. I tightened it and continued to fly and have a lot of fun with this glider. Later, I drilled a hole through the tail mount and boom for a small self-tapping screw to keep the tail from rotating again.
This glider seems to fit more into the thermal, rather than the hot-liner class. Straight down terminal velocity dives yielded some flutter. I found a bit of play in the aileron torque rod where it exits the wing, which is the likely source of flutter. More flying showed the airframe not to be quite as slippery as it might appear. In power-off dives from a good altitude the plane did not accelerate as quickly or retain as much energy after leveling off and climbing out as I had hoped. Despite this, for its class, the eHawk performs quite well.
Overall, the eHawk was an enjoyable experience. Good quality components, an easy build, attractive lines, response to thermals, no bad habits and mild aerobatics are highlights of this model. A person who skillfully flies one or two beginner planes, glider or otherwise, would feel fairly comfortable with the eHawk. Someone looking to zoom will find that it tame because it won’t cut through the air like a heavier, molded sailplane. I will take the eHawk to the field with me whenever I want to have a relaxing flight that I can also wake up with some aerobatics.
Average street price is $120. Other colors availabe include, purple, yellow, and red.