Viper 500 - As Experienced by Don S.
by Don Stegall
Click on the photos for full size images.
9/8/2003
I got my Viper 500 on Thursday afternoon. I assembled it on Friday and flew it on Saturday.
It was very easy to assemble. Because I wanted to use a bubbleless tank on the CG, I had to rearrange the radio components some.
Since the nose feels plenty strong even without the cover plate, I am temporarily using screws to hold it in place.
The wing tips are Metallic Purple, and Neon Orange.
Here's a view from the bottom.
Since I plan to carry this one around and demonstrate 424 or Q-500 Novice (aka Slow Quickie  ), I made sure I can move the 650
mAh NiMH battery almost anywhere in the fuselage.
This is the rear compartment.
The V-tail servos are Hitec 85MG, and the throttle servo between them is a Hitec 81.
I cut out the middle webbing in the servo tray so that the 650 mAh NiMH battery could nestle down and not interfere with the pushrods.
Since I wasn't concerned about getting the n'th degree of speed, I simply mounted the switch to the side.
The receiver is a 1.1 oz Futaba R148DP in a JTEC foam rubber cover. I may have to remove the cover because the white spots you can see in the blue tape are where the cut-off aileron torque rods are rubbing just a little.
Space is very tight in the area under the aileron servo, push rods, and control horns. In fact, I had a piece of 1/4" foam under the JETT CG tank, but I had to remove it. I used the basswood servo rails provided (5/16" square I think), but I should have used 1/8" plywood instead. If I had used the plywood I probably could have left the foam rubber in place.
Here is the engine area view.
The sides are thin enough in the area where the doublers are cut out that I was able to use grommets on the tank lines.
I left the fuel line long because I plan to have a variety of engines on the plane.
The engine has a JETT BSE exhaust mounted Remove Needle Valve Assembly. The BSE version works well with small to medium sport engines, as well as the 428 engines.
The black stuff is Velcro so I can move the battery pack up front if needed for Slow Quickie with a OS .25 FX engine.
Even though the Viper 500 feels much stronger and better built than the Predator 500, it does have some issues.
* The most serious issue is the firewall alignment. It seems to have significant downthrust, at least on the one I received.
* The fit for bubbleless tanks looks like a concern, to me anyway. I don't have a Tettra to try, but the only JETT tank that will work in my opinion is the CG tank. There are two reasons. The aileron servo creates a tight fit at the rear of the tank. Because of the way the front wing hold down plate is installed, there is not much room at the former. You would basically have to grind out the middle of the hold down plate to get a Slim in far enough. I put a 1/4" foam pad around the neck of the CG tank and placed it against the former.
* Radio gear placement will require some thinking and trying different configurations unless you use the included tank.
* Weight is an issue. To get the extra strength, the Viper comes out of the box a little heavier than a Predator. Even with very light radio gear, mine came out effectively at 3 lbs 13 ozs. Maybe an additional ounce, two max, could be shaved on the radio gear. My receiver weighs 1.1 oz and the ultralight receivers are around 0.5 ounce.
* The landing gear is very short. And with the wheels installed, they don't actually meet the 7" width requirement. I flew off of closely cut grass, and the prop did some extra mowing as I took off. Unless you are taking off from a hard surface, a new taller landing gear is in order. The landing gear bay chord is 1.125", but you should easily be able to get the LCS LG 125 CF gear
(http://www.lcsperformance.com) to fit. This will also give you an extra inch or so of height needed for grass fields.
The plane flies and tracks well. And with the same engine, I believe it will be faster than a Predator.
After I get the firewall issue worked out, I will put a 428 engine on it and report further.
9/9/2003
My first flight on my Viper required 17 clicks of up trim on my Futaba 8UAS.
After I got home, I checked the incidence, and the firewall had almost 2 degrees of downthrust.
After requesting info in the forums on how to shim the backplate, and getting great feedback, I made a shim out of 3/32" aircraft plywood. The shim tapered from 3/32" at the bottom to 1/16" at the top.
When I re-measured the downthrust, it showed 1 degree.
I test flew today. Since I had taken out a significant amount of downthrust, I took all of the trim out. On low rates, I couldn't get off the ground. So I added 8 clicks of up trim. I was then able to get off the ground, but still needed more up trim. I wound up at 12 clicks of trim.
I should have trusted myself when I measured the adjusted downthrust at 1 degree. On landing, I didn't think to trim at idle or on approach. I had to hold in down until flare time. Not being used to flaring from holding down, I ballooned and landed in weeds beside the runway. I thought the airplane was ok, except for a tiny crack at the back of the servo compartment cover. However, on taking the plane apart, and wiping it down, I found out that the stab area had suffered some damage. This is a little distressing because it was not a hit that should have caused damage.
I talked to the designer of the plane yesterday and today. The design spec was for everything to be 0-0-0. However, some of the production planes were pulled and they measure 0.5 degree of downthrust. This is supposed to be fixed in the next run of planes.
Check the incidence of your engine/firewall. A Quickie that is not straight is hard to fly and land, especially for us lesser experienced pilots.
9/10/2003
Here is a photo of the damage to my Viper 500 tail. The glue came loose around the plywood former. From the crack in the side, and from what I can see through the crack, there is no doubler on the fuselage side in the stab area. And it only looks like thin CA was used on the plywood former.
Remember that Predator owners were having problems with their tails breaking because of not removing the covering from the front and rear of the stab filler block and gluing it in properly?
I think that the tail is going to have to be glued on for the tail area to stand up to much use.
I'm even considering pulling the covering off of the fuselage and stab block, and putting 2 oz cloth on the fuselage and painting it.
9/14/2003
I used some .007" carbon fiber laminate on both sides of the tail to increase the strength of the tail in the very weak area at the front of the stab.
I used thick CA to put it on, and the CF got pretty warm all over, so I know I got a good bond, except at the very tip of one corner. I hit the corner with some thin. Then I recovered the area with MonoKote.
Yea, it would look better on the inside, but then the bolt on tail structure would be a problem. Since we're still trying to get the incidences right on the plane, it's best to leave the tail as a bolt on for the time being.
The CF greatly increased the strength in the area, and didn't change the CG hardly at all.
9/16/2003
I went back to my 2 degree firewall shim (had tried 3 degrees) and shimmed the rear of the stab about .070" and got every thing at 0-0-0.
Now it flies great. Grooves well and turns well.
Since all of the testing had been done at the club closest to me, it was time to try it out on the PylonWorld field. I took off and flew 5-7 laps, then I handed over to Mike Hill (1st Place in both NATS 2002 Intermediate Pattern and NATS 2003 Advanced Pattern) for his first laps on a 3 pole course.
My field is tight with trees at the end, so you have to do a circling approach. Mike went long but kept the plane nice and level during touchdown ... until we saw something black come off. I immediately knew that it was a wheel and figured the axle just came loose.
My field gets rougher the further you get from the more groomed area. The black plastic part of the wheel came right off of the bushing that is in it. The bushing is the same O.D. as the retainer. There is no groove or anything significant to keep the plastic wheel on the bushing. I think these wheels will be problematic with age. Especially for those on grass fields.
Here are some photos of the wheel that failed on my Viper this afternoon.
The first photo is the complete wheel with the bushing.
The second photo is what the departed plastic part left behind on the landing gear. The bushing from the wheel is still on the axle.
The third photo is from the inside/backside of the wheel. The bushing is between the wheel and the axle.
The fourth photo is from the outside of the wheel.
 
 
The complete wheel in the first photo was reassembled by just pressing the bushing back into the wheel.
I was very surprised by the construction of the wheel. I had guessed that the wheel was molded around a bushing with a groove or some other kind of surface treatment to keep the plastic on.
I checked the intact wheel and the bushing in it is moveable with just finger pressure.
So if you're going to use the stock wheels, you better make sure your wheels never encounter a side load.
9/17/2003
Here is a NOT TO SCALE drawing of the wheels.
The wheel on the left is the classic Glenn Lee wheel that is molded from urethane.
The wheel in the middle is a simplified diagram of the Viper wheel without the streamlining recessed area on the backside. You can see that the bushing is just a press fit in the hole in the wheel.
The wheel on the right shows the recessed area along with the bushing.
The outside of the bushing doesn't have anything other than the press fit to hold it in place. On top of that, the Viper wheels are much harder than the common wheels that we have been using.
Ed Smith: Is it too simple a solution to hot stuff the bushing back into the wheel, or use a screw and a large washer to hold the wheel on?
9/18/2003
Ed,
Thin CA (ok, Hot Stuff ) got right in there ... I could see it wicking in. Did the other side (flange side) without getting any on the bearing contact surface. Ahhh ... the nice aroma of CA kicking off.
I tightened the wheel down to some 1/8" thick aluminum and did my best to get the wheel off, but it stayed on.
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