Tail Vibration

Tell all about your Lazair
Post pics --
After a while we wil streamline the forums and see what is most popular.
Want you own forum ? Let us know.

Postby bdiedenhofen » Tue May 22, 2007 9:20 am

Hi,
I've got a question regarding tail vibration. I've got about 100 hours on my series III, and I've got a pod on it. When cruising around at 30 to 35mph, the airplane flies very smooth. But when I get to 40mph, there is a slight buffet in the tail. It's not constant, just a little shake now and then. As I get to 45, the buffet becomes more noticeable. It's not a large shake, and I don't feel anything through the rudder pedals, just through the back of the seat a bit.
It seems to me that due to the way the tail is attached, it is likely that it may shake a bit, so I haven't been overly concerned. Perhaps it has something to do with unclean air behind the pod? I wasn't able to fly that fast without the pod, so can't really compare.
My bracing wires are snug and everything is tight.
For piece of mind, I thought I would see if anyone with more experience might be able to tell me if this is normal or not.
Thanks,
Brian D.
bdiedenhofen
Popular Lazair Member
 
Posts: 65
Joined: Sat Oct 11, 2003 7:55 pm

Postby lazairiii » Tue May 22, 2007 11:31 am

Brian,

It's been present on both my series II back in the 80's and 90's and now on my current plane.

I feel it just slightly on occasion as well, so I'd say go fly and have fun.

George
lazairiii
LazairNUT
 
Posts: 413
Joined: Wed Jan 14, 2004 12:19 pm

Postby JPXman » Tue May 22, 2007 4:45 pm

this is purely a data point for you brian, but with the full EC pod, i flew mine at 70mph in level flight and the shaking just started (full power). is the shaking more prevalent at higher or lower power settings? level or in descent?

Tyler
User avatar
JPXman
LazairNUT
 
Posts: 625
Joined: Sun Nov 02, 2003 10:15 pm

Postby bdiedenhofen » Wed May 23, 2007 8:18 am

Thanks George, thanks Tyler,

I find that the shake becomes noticable at around 45 mph. Power on, I beleive. I can't recall feeling a shake during final with power off, but I can't be certain. I do know that it will shake at any power setting above slow cruise once I get to that airspeed.

Brian
bdiedenhofen
Popular Lazair Member
 
Posts: 65
Joined: Sat Oct 11, 2003 7:55 pm

Postby bdiedenhofen » Wed May 23, 2007 10:19 am

Yeah, I figured a bit of "wobble" might be normal due to the way the tail is mounted to a single long tube. One day, last summer, when I was looking over the Lazair, a fellow took off in a single seat Beaver. I couldn't believe how much his entire tail was wobbling and shaking at the end of his tail boom. I decided right then that my little vibration was no where near as dramatic as his, and I probably didn't have to worry much.

I tensioned the cables as per the manual. They feel even at around 10lbs tension.

Thanks,
Brian
bdiedenhofen
Popular Lazair Member
 
Posts: 65
Joined: Sat Oct 11, 2003 7:55 pm

Postby JPXman » Wed May 23, 2007 1:02 pm

going by memory here, but i think the manual says to tension one to 10 pounds, and then just bolt the other one so its "taught"... as in very little if any tension, but no sags in the wire.... but again, this is memory and its weak sometimes.

**ok i checked the manual and it doesn't mention anywhere that you need even tension, but in section 7 (at least in the 2-seater manual) that its all about making the spreader tube on the tail parallel to the axle, with the "higher" cable attachment with 10 pounds tension..... and the other one with enough tension to make the spreader tube and axle parallel
User avatar
JPXman
LazairNUT
 
Posts: 625
Joined: Sun Nov 02, 2003 10:15 pm

Postby Chappy » Thu May 24, 2007 12:10 am

Brian,

I would suggest upgrading your tail brace cable assemblies by adding turnbuckles if they don't have them. IIRC, I used little AN-140-8S turnbuckles. I found some surplus ones and they were pretty inexpensive. Ultraflight tried to save a little money by skipping them, but it's much easier to tune the tail if you have them.

On my old huge tailed series 1, I ended up fitting a second spreader between the ruddervators (in addition to the turnbuckles in the cables) before I finally banished the tail shake completely. I have redrives and big props that probably disturb the air over the tail more than normal. Even so, it was only intermittent, and at cruise speeds. I never experienced it in high speed dives. Most (but not all) Lazairs I've flow have had some level of intermittent tail shake.

In my experience, good snug tail brace cables also help trim the airframe for a slightly improved glide ratio. Anyone else notice this?

Chappy
Chappy
LazairNUT
 
Posts: 330
Joined: Wed Jan 28, 2004 9:57 pm
Location: Northern Virginia, USA

Postby bdiedenhofen » Fri May 25, 2007 8:11 am

Great advice,
Thanks everyone.
Brian
bdiedenhofen
Popular Lazair Member
 
Posts: 65
Joined: Sat Oct 11, 2003 7:55 pm

Postby lazair3ca » Mon Jun 18, 2007 7:56 am

And I thought it was just my knees knocking because of the speed and the wind rushing through the "cockpit" (no pod). :rolleyes:

Were you using the Halls ASI? I have seen at least 5 mph difference in two different models.

Cheers,
Bill
Bill
C-IHMS
User avatar
lazair3ca
Popular Lazair Member
 
Posts: 60
Joined: Tue Sep 30, 2003 7:48 am
Location: Ottawa, Canada


Return to Pilots Lounge

Who is online

Users browsing this forum: No registered users and 10 guests