Dave McD's
Predator
Kit:
Tufflight Models
Aileron throw: +/- 3/4" hi
rate (needs more), so low rates are never used.
Elevator throw: +/- 7/32" low rate
for combat, +/- 3/4" hi rate for thrills.
Expo: -40% on both high and low
settings.
CG: 8 5/8" from elevon
hingeline (5/8" behind recommended CG)
Downthrust: 2 degrees.
Deviations from plans: Removable
hatches over receiver and battery.
Added internal antenna tube. Servos screwed to plywood rails instead of
wedged into place. Coroplast fin was covered with Ultracote. Changed
balance point to 8 5/8" from hingeline to allow nearly hands off
inverted flight. Added 2 degrees of downthrust to engine to eliminate
pitch changes when throttling up or down.
Yes, I've got it balanced 5/8" further back than the recommended balance point of 9 1/4" forward of the elevon hingeline. In fact, I started with the CG at exactly 9" forward of the hingeline, but kept moving the CG back until the Predator began to require very little forward stick pressure to maintain level inverted flight. Even at 8 5/8", it's still requires just a little forward stick pressure to maintain level inverted flight, but not nearly as much as before. So I'm going to stop there at 8 5/8". With the more rearward CG my Predator now loops tighter than before in both upright and inverted flight, and it rolls much more axial. With the 3/4" elevator throws, the extra travel lets the Predator enter into a deep stall and results in some cool looking aerobatic maneuvers. For instance, half power and full up elevator will cause the Predator to nearly stop all forward motion and begin to rapidly flip end over end with the wings staying level. If you do this and add left aileron at the right time, the Predator tilts it's wings to a vertical position and begins falling in a knife edge position while quickly rotating nose over tail. It looks cool!!!
Although I have my high rate elevator throws set to +/- 3/4", I have my low rate elevator throw adjusted to +/- 7/32" for combat. The 7/32" throw was found by inflight trial and error adjustments of the low dual rate setting on my 8UAF transmitter until the Predator began to stall at full elevator stick travel. The low dual rate value was then programmed to be 1% less than the travel it took to stall the Predator at full up elevator stick travel. That's why I use the low elevator rate setting for combat, so the Predator won't stall out during aggressive maneuvering. But the aileron rate stays on high at all times, whether I'm flying combat or just goofing around with my Predator.
The need for 2 degrees of downthrust was determined by flying the
Predator straight and level at just above stall speed with the engine idling. Then
full power was applied while returning the elevator stick to neutral at the exact same
time. Without the downthrust, the Predator would begin to climb steeper and steeper,
which required down elevator stick pressure to control the climb until it reached normal
cruising speed. With the 2 degrees of downthrust, applying full power and returning
the stick to neutral simply made the Predator accelerate straight ahead. No elevator
adjustments were needed.
I believe the reason for the need for the downthrust has to do with two factors. The
first is that the engine thrustline is actually located slightly below the centerline of
the wing. This creates a slight nose up force when power is applied, which causes a
climb. The second reason is because of all of the drag producing stuff located on
the top side of the wing and directly in the path of the propeller slipstream. This
drag force increases as the propeller spins faster and faster, which again causes a nose
up pitching force as the engine power is increased. The downthrust counteracts these
upward pitching forces from the engine
and the drag and cancels them out......resulting in the Predator simply accelerating or
decelerating straight ahead without any change in elevator trim settings.
I'll be the first to admit that not everyone bothers to take the time to trim out their combat airplanes for aerobatics. And I'll also be the first to admit that I tend to go to extremes on trimming out all of my planes to fly their best.....even the ones I combat with. I don't expect Tufflight Models to change their 9 1/4" recommended CG back to 8 5/8" just because I said I liked my Predator balanced that way. On the other hand if you build a Predator, you might want to experiment with a more rearward CG and 2 degrees of downthrust to see if you think a Predator flys better that way. I do. But I also realize that it's my opinion versus your opinion, and there is no definitive answer.......sort of like the 2000 presidential election. ;-)
So do I like the Predator? You bet I do! With the OS 40 FP, the Predator is a fairly fast, agile, rugged, and competitive RC combat machine. With the more rearward CG, it loops and turns tighter than before, and rolls cleaner. With 3/4" control throws, the Predator is capable of all the normal aerobatics, plus spins, flat spins, end-over-end flips (waterfalls?), and some other cool looking aerobatic maneuvers.
This Predator has already proven itself in combat by taking a first place finish in the Outlaw class at the 2001 Misery In Missouri RC Combat Meet. It has also proven to be an extremely durable airplane by surviving several mid air collisions that would have destroyed most other combat planes, including my own DaWing 40 design. Once when an elevon servo failed inflight, the out-of-control Predator crashed into a field. The crash broke the motor mount assembly loose.....just as designed......with no other damage! I swapped out the bad elevon servo at the field and resumed flying the Predator about 15 minutes later. If I ever manage to destroy this Predator, (which is looking more and more unlikely), you can bet that I'm going to build another one!
NEWSFLASH!!! My Predator has acheived fame by getting it's photo published in the Combat Ribbons column of the January 2001 issue of RC Report, page 129. The Predator (and me) are also pictured in the Radio Control Combat column of the September 2001 issue of Model Aviation, page 135. The MA photo was taken at the 2001 Misery In Missouri combat meet immediately after a 4 cut round of combat, with the streamers still stuck to the Predator after it landed. Thank you Pete Rundel of RC Report, and Greg Rose of Model Aviation!!!
Comments?
E-Mail me at:
dmcdnld@yhti.net