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viper - 12/9/2007 8:28:24 PM   
rc.chris


 

Posts: 15
Joined: 11/13/2007
From: brunswick, GA, USA
Status: offline 		I just built a great planes viper with a nelson qm 40. when it gets up to speed it starts to bob up and down, any ideas?
       Post #: 1

RE: viper - 12/9/2007 9:04:05 PM   
Druce



Posts: 79
Joined: 1/6/2002
From: Edmonton, AB, CANADA
Status: offline 		why would you do such a thing?? those planes are not built for those motors or speeds. It's a pretty dangerous thing to do.

(in reply to rc.chris)
       Post #: 2

RE: viper - 12/9/2007 9:07:49 PM   
Super Splatter



Posts: 381
Joined: 3/21/2006
From: , MN, USA
Status: offline 		Balance ? or

On my V'tails I learned to have only 20% down travel as compared to up travel in the elevator. they need very little down when going at that speed

(in reply to rc.chris)
       Post #: 3

RE: viper - 12/9/2007 11:48:19 PM   
Trnnbrn


 

Posts: 4
Joined: 4/9/2005
From: Oklahoma City, OK, USA
Status: offline 		Weve been flying 428 Vipers in District 8 for 2 years now. I can tell you from experience that at these speeds, you dont want the CG any farther back than 2 3/4" . It is really pitch sensitive behind 2 3/4". You also need to shim the trailing edge of tail up about 3/16ths. We also strengthen the fuse in the nose and the tail areas.

Chuck

(in reply to Super Splatter)
       Post #: 4

RE: viper - 12/10/2007 12:32:20 AM   
Jezmo



Posts: 795
Joined: 6/26/2006
From: Spring, TX, USA
Status: offline
quote:

ORIGINAL: Trnnbrn

Weve been flying 428 Vipers in District 8 for 2 years now. I can tell you from experience that at these speeds, you dont want the CG any farther back than 2 3/4" . It is really pitch sensitive behind 2 3/4". You also need to shim the trailing edge of tail up about 3/16ths. We also strengthen the fuse in the nose and the tail areas.

Chuck


Exactly what I had to do. Shimming the trailing edge of the tail up was the last thing I did and it fly's very good now. I have noticed that some folks cut everything outboard of the elevators off of the horizontal. I was told that helps too although from my point of view the only thing it should help with is DRAG. IMHO

_____________________________

Raptor 50 V2 OS 50 Hyper, T-Rex 450XL CDE, Spektrum DX7, SuperTigre OS FOX Saito Enya Jett TT

(in reply to Trnnbrn)
       Post #: 5

RE: viper - 12/25/2007 7:44:21 AM   
ejames7699


 

Posts: 21
Joined: 11/20/2003
From: MURRIETA, CA, USA
Status: offline 		I'm new at speed but I love it and I have the same problem, my viper just dips straight down sometimes. The real problem is I can't shim the tail because its epoxied in, so what or is there something else I can do ?

Need help fast

EJames7699@aol.com

(in reply to Trnnbrn)
       Post #: 6

RE: viper - 12/25/2007 3:19:09 PM   
skull1971



Posts: 745
Joined: 4/24/2006
From: Willis, TX, USA
Status: offline 		The shim works best, SO for your next one...... I've found the nose heavy will help keep it level, just watch the turns (a pylon #1 style turn) it will dive. I started with an ounce and went down from there.

_____________________________

ERIC, Revver Bro. 159, NMPRA 59i, Knucklehead #1
What happen? The ground got in the way of the plane!

(in reply to ejames7699)
       Post #: 7

RE: viper - 12/25/2007 3:52:46 PM   
Electric Delta



Posts: 163
Joined: 1/23/2005
From: Mebane, NC, USA
Status: offline 		It may be worth your while to cut the tail loose, shim and re-epoxy/glass the tail back on. If your stab incidence and elevator trim are "fighting each other", your trim will vary a lot with speed, and you'll have to compensate with the stick. IMO, the Viper tail surface area is on the high side, which can lead to over-stability/over-compensation of the stabilizing moment, which then leads to oscillation in pitch. Trimming the tail surface area down, i.e. cutting off the area outside the elevators may very well reduce the tendency to oscillate and improve overall tracking and handling. As mentioned above, your CG may also need adjusting further forward. There are lots of good tips on the Viper on these forums; just make small adjustments and changes, preferably one at a time, observe what happens, and you'll get it dialed in to your liking.

_____________________________

Auf die Dauer hilft nur Power! Revver Brother #127

(in reply to ejames7699)
       Post #: 8

RE: viper - 12/25/2007 6:35:22 PM   
DonStegall


 

Posts: 615
Joined: 8/8/2005
From: Monroe, NC, USA
Status: offline 		The Viper's tend to have down thrust in the firewall and down incidence in the tail. On the last one I put together, I had to make a firewall shim of 1/8" plywood and bevel it from 1/8" on the bottom to 1/16" on the top. Plus I had to use a 1/16" shim at the back of the tail.

"On your next one" ... This is a key statement. In pylon racing, the planes tend to get used up. It is usually best to stay with one model and figure it out. But while I have stayed with the Vipers, I'm about ready to try a Predator II. I'm getting tired of shimming the firewall and tail on the Viper.

_____________________________

Don Stegall
RCPRO - http://www.rcpro.org - http://www.rcpylon.com

(in reply to Electric Delta)
       Post #: 9

RE: viper - 12/30/2007 5:16:04 AM   
Trnnbrn


 

Posts: 4
Joined: 4/9/2005
From: Oklahoma City, OK, USA
Status: offline 		EJAMES,

Your problem is not the tail. Your plane is too tail heavy at race speed, you cant react quick enough to control it. The plane will fly nice if you rebalance it to 2 3/4. Another tip is to make sure it is trimmed to fly straight at race speed. Reduce your throws. Once you get it flying well, you will notice you are carrying a bunch of up trim in the tail. Dont worry about it, it wont make much diffrerence except for a little drag. It wouldnt hurt anything to cut off the tips of the tails and it may help stability. Viper tails are pretty long as mentioned by Electric Delta. When you build your next one, you can use what you learned on this one.

Chuck

< Message edited by Trnnbrn -- 12/30/2007 5:31:31 AM >

(in reply to ejames7699)
       Post #: 10

RE: viper - 12/30/2007 5:30:52 AM   
garys


 

Posts: 739
Joined: 1/31/2002
From: San Diego, CA, USA
Status: offline 		It's either tail heavy or the incidences of the wing/tail/engine thrustline are fighting each other, or both. Check all the above and correct what is out of whack, don't try to band-aid it by adjusting something else, even if it means cutting the tail loose. They all need to be correct for a good flying airplane. You can probably get away with a little elevator trim, but much more than the thickness of the trailing edge should be corrected.

The tail area is ok, many Q500's have tails as big or bigger. I've never heard of too large of a tail causing pitch issues, but too little area will do it all the time. If bigger stabs cause stability issues, why do they put larger stabs on free flight airplanes?

< Message edited by garys -- 12/30/2007 7:36:46 AM >

_____________________________

GS

(in reply to Trnnbrn)
       Post #: 11

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