crasharama

Hi guys,

I am at the stage of balancing my GP Viper, & was wondering if 3 inches is the correct CG as per the manual ?

I have looked back through the old threads but cant seem to see anyone asking this so I'm guessing it's right.....

Tony.

Druce

I put my cg at about 2 1\4 " back. I like it a little nose heavy at first just in case something is not right such as too much elevator etc. After the first flight then you know pretty much what you have to do and make the changes accordingly..

DonStegall

My Feedback: (1)

I consider 3" back on the Viper to be the rear of the range. I have flown them at 3.125", but it is not comfortable unless you are really looking for that feel.

They will fly at 3", but if it is your first Quickie, it may be too much and may not be as stable as you would like.

2.5" to 2.75" would probably feel ok.

IMO, 2 1/4 on a Viper is way too nose heavy, even to start with.

jaka

Hi!
I fly the Viper at the exact Cof G recommended in the manual. Elevator throw is exact as per manual.
Flies very nicely! Very docile! Goes where you point it !

clb

In 2006, I built 7 vipers. All had for the first flight a CG at 75mm as recommended by Great Planes.
75mm is exactly 2.95 inches and all fly great at this CG position. All 7 Vipers need up trim from 10 to 20 steps.
Good flights with your viper.
Claude.

crasharama

Thanks for the help guys, I am going to race this thing, & am looking foward to getting it going

Tony.

DMyer

My Feedback: (2)

I believe they fly best if you adjust the stab incidence to 1 degree negetive - that is the leading edge down a touch... then i will fly with virtually no up trim required.


Dan

crasharama

Hi guys,

I have set mine up 0-0-0, & have done all the standard mods that are floating around on here, plus Trevor Handerson's ply disk around that weak area where the v-tail bolts up.

Thanks for the hepl ,

Tony.

Electriceddie

I think I did the quote thing there right? LOL

Anyway - CLB

I know its been a while since this was posted. Have you ever come across why after building so many vipers that they all need 10 to 20 steps of up elevator? I have 0-0-0 Eng. , Wing, Stab and need to fly with about 1/8th" of Up elevator at full throttle. When I reduce the throttle to 1/3rd or 1/2 stick it climbs quite a bit. So have you ever discovered why the Up elevator comes into play here?

Thanks

Ed

jaka

Hi!
I have put together two Vipers and both of them have required some up elevator trim. On the last one that was powered by a Nelson LS engine the fuselage broke in half just in front or the stab. Luckily for me this was observed before I was to fly the third time that day. I removed the stab and reinforced the fuselage and lowered the leading edge of the stab 3mm. Now I fly this Viper with no up elevator trim.

It seems funny that your Viper climbs when you reduce throttle??? None of my Vipers do that. And I have never experienced that on any of my pylon racers during the 30 years I have been flying pylon.
Are you sure everything is OK with your servos??

djlyon

Boy that sure sounds like down thrust Ed

Electriceddie

Hi Guys,

I know it sounds like I have a degree or two of down Thrust, thats what I was thinking. I have checked and I have 0-0-0 Eng, Wing, Stab incidence. Of course the opposite happens as well if I trim in to fly hands off at 1/3rd or 1/2 throttle thne when I go to full throttle it dives. Unless my meter is flaky or I just cannot get a good gauge set up on the engine it should be right. I wonder how much drag this 1/8th" of up trim is slowing me down as well. All of my years and expertise tell me that the engine has 1or 2 degrees of down thrust. Any body know of any other types or brands of incidence meters on the market other then Robart? I have 2 analog and the lazer one they came out with ( boy what garbage that one was) They have been dropped before on occasion but they test good when checked with a Craftsman digital level and Craftsman lazer level on the workbench. They all come up on zero.

Any other suggestions ?

thanks for the feedback so far.............

Ed

djlyon

Try a different way.

Level your table or surface. Level the assembled plane, without the engine installed, on that surface with the robart on the wing reading 0. Using a true angle from home depot or where ever check the firewall to be at 90 deg from the table. Using a combination square draw a line perpendicular from the trailing edge of the stab to the leading edge. Using a combination square measure up from the table to line on the trailing edge and leading edge. The measurement will be the same if the stab is at 0 deg. I'm not saying this is the best way but is probably very different than how you are doing it and is therefore a good cross check.

Denis

Electriceddie

Hi Guys,

I went back down and started all over again taking readings of the Eng, Wing and Stab and the only thing that I see is that while everything is damn close to zero if not on zero incidence for all thre spots, there is a slight margin for error with the Robart gauges. It is possible there is a margin of about a half of degree for error. If the end result is that the Stab is .5 to .8 degrees negative causing the pitch issue. I am suprise to see that it is not constant thruout the engine range when throttle is applied and reduced. If the plane flies straight and level at 1/3rd to 1/2 throttle and the elevator is nuetral. The when full throttle is applied whay does it need 1/8th" of up elevator. It sounds more like there is too much down thrust, correct? I think I need to take it back out under non race day conditions and do some testing with power ON and Off etc. then maybe I can see if the engine thrust is the issue. But again does anyone concur about the down thrust being too much or maybe I need to add a little up thrust?

Thanks again

HighPlains

My Feedback: (1)

Pitch the Robarts gauge. Not that accurate.

Shim the model up on a flat table, until the wing measures the same to the table from the LE and TE. Then measure the tail, and check the firewall as suggested with a right triangle. I use a steel rule that is marked in 64ths.

daven

My Feedback: (1)

You also could need up elevator trim if your plane is nose heavy. Don't be afraid to try a rearward CG on a quickie, they will turn faster.

You know you've gone to far when you have to push down on the elevator stick to land

freeair

guys, where do you buy the nylon wing bolts ? all i can find are the bolts with the large socket head but i want the counter sunk type like the stock steel ones but nylon. thanks.

Bill Vargas

My Feedback: (5)

Try here,,,

http://www.microfasteners.com/catalo...cts/NYLNBF.cfm


BV

Ed Smith

Wing, Tail, firewall setup, does this help. The work surface is accurately level.

Ed S

Electriceddie

Hi Guys,

I went out to the field again to test fly the Viper. I had previously checked all the control surfaces and everything looked good. I started it up and took it up and flew around about 3-4 laps around the field at full throttle. I neede to add about 2-3 clicks of left aileron and about 3-4 clicks of up elevator, not to bad. I wanted to let me son try his hand at flying the Viper because he wants to start racing as well. I brought it up to approx. 350ft, flying down the center of the field with the wings level ready to hand it off to my son. I reduced the throttle down to 1/2 and as soon as I did the plane started to pull up and into a loop. I said OK...... now this is not good. So I continued to fly and I'm sure my son thought I just wanted more stick time. I had to give it about 12-14 clicks of down trim to get it to fly hands off, straight and level at about 1/2 throttle. Now it flew good, but again as soon as I gave it full throttle it started to dive, so again I had to give about 12-14 clicks of up elevator, which equates to about 1/8th inch of up elevator and very little down on low rates. Anyway, here we go again. I brought the plane home and went over everything the incidence is almost dead on 0-wing, 0-elevator and 0-engine. If any offset on the engine or elevator it is about a 1/4of a degree or less. So little I believe that it equates to about 2-3 clicks of trim at best. I removed the engine and checked the firewall directly and checked the engine and mount as well. They both seem fine. The engine is mounted directly to the motor mount backplate (LCS performance mount) of which I have several. I don't undertand what is going on. I have never encountered any thing like this before. I am thinking about adding about 2 to 21/2 degrees of down thrust to the engine next. I have several other Vipers and not have had any issues like this with any of them.

Please any suggestions!!! HELP, HELP!!!!!

Ed


Thanks

gunfighter

My Feedback: (1)

The symptoms you describe indicate you have down thrust in the engine some how. I certainly would NOT add more down thrust. I would add a shim under the BOTTOM engine mount (to firewall) bolts to add a little UP thrust and fly it again.

freeair

yes this definetly sounds like a down thrust problem, your trimming the model good for full on power but when power is reduced the model becomes out of trim.

djlyon

Is your firewall loose and you haven't noticed it and you are getting down thrust at full throttle

Electriceddie

Hi Guys,

Sorry I meant up thrust, It was late for me last night when I posted. 10:30PM is late when you get up for work at 4:30AM. I think I will try to take some photos and attach them here to show you guys. Its just so strange that since all points are almost dead on Zero degrees that it would need Up thrust. But I guess stranger things have happened.


Ed

DMyer

My Feedback: (2)

This is strickly my opinion, others may say I am nuts, but I and many others have used that particular airfoil on both 424 and 428 racers and these are my observations. You mention you set it up to be 0-0-0 - I will assume you are correct. Unfortunately, that particular airfoil requires either positive incidence on the wing (leading edge higher) or negitive stab incidence(leading edge lower) of about 1 degree. That explains why you needed up trim in your test flight at full power. A quirk of this airfoil is that it will lift like crazy at lower airspeeds and dosn't level out until high speed is acheived which is what you trim for - racing conditions/speeds. Cutting the throttle will cause it to climb. When we used to race 428 racers with that airfoil, on take offs we would use no up elevator and even a bit of down until high speed was acheived and it trimmed out. Since it is a high lift airfoil it works very well in turns and on landing it will glide just about forever! Good airfoil for 424 but it does have it's quirks! Good luck and put the trottle all the way forward for your son - it will fly much better. If he cannot handle the speed,, then he is not quite ready - have him get used to full speed up high under supervision.


Dan