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ATC_RV8OR
09-28-2010, 05:15 PM
anyone out there building or completed a Rocket using -8 tail instead of the -4?

Chino Tom
09-28-2010, 05:51 PM
I'm not building a HRII, but I know of 2 Rockets at Chino with -8 tails.
The -8 tail weighs more (10 -12 lbs?) than the -4. But there are no issues
mounting the -8 tail feathers.

Jeff Brenhaug
09-29-2010, 04:42 AM
Chuck Hanson from Bakersfield has -8 tail on his HR2. If you go here and scroll down the main page you can see it.
http://www.harmonrocket.com/
I've also attached an image I took in Porterville, CA one Saturday morning when Matt Baker flew it over for breakfast.
Hope this helps,
Jeff
http://img269.imageshack.us/img269/9959/p1010003bt.jpg (http://img269.imageshack.us/i/p1010003bt.jpg/)

Kevin Horton
09-29-2010, 06:29 AM
I'm wondering how the various tail options affects the potential for flutter, as the empennage is likely the weak link from a flutter perspective. I expect that the flutter speed for a given empennage is likely not much affected by whether it is bolted on a Rocket or on an RV. For stock tails, Van's recommended VNE for the RV-4 is 210 mph (IIRC) and 230 mph for the RV-8. Both these numbers seem quite low for a Rocket.

On the Harmon Rocket, is the specified empennage a completely stock RV-4 tail, or are there changes that would make it stiffer/stronger? What VNE does John Harmon recommend?

Jeff Brenhaug
09-29-2010, 09:35 AM
Kevin,
Most of your questions can be answered by going to John Harmon's site and sifting through all the information. I'm not sure if he's done any testing or calculations on the -8 tail, though.
Here are a few quotes for the -4 tails from his site,
"Most of the tail surfaces received heavier skins to eliminate any flexing both chordwise and spanwise."
and
"John has flutter tested it in three knot increments all the way up to 320 knots"

There's alot more info out there and hopefully some engineering types can step in and answer some questions for us on this subject.
Jeff

Bill Wightman
09-29-2010, 10:24 AM
One obvious weak link on all the RV horizontal tails are the hinges. They're rod ends, screwed into big nutplates. All of the counterbalance mass is placed outboard on the counterbalance arm. In a situation where flutter dynamic is present, when that outboard mass is resisting motion in the elevator/stabilizer, the first component in the airframe that supports those loads is the outboard elevator hinge. Strengthening this hinge would make the horizontal stab / elevator system stiffer, allowing less flexure at a key point in the load path present in a flutter scenario.

From what I've seen on the rockets, this under designed outboard hinge point has not been improved. Please correct me if this observation is incorrect.

An improvement to the outboard hinge would be to get rid of the rod end and use some kind of solid billet hinge on those outboard locations. Its really not necessary to have adjustability on both hinges (center and outboard). Keeping only the center hinge adjustable still works to align the centers on the elevator hinge axis (edit: although I don't like the rod end hinges at all).

Heavier skins are good, serving to keep a more torsionally stiff elevator and adding resistance to the skin buzzing. (but heavier skins also require more counterbalance mass, adding 2X to the hinge loads I see as problematic in the first place)

RVjim
09-29-2010, 10:32 AM
On my modified HR2, I have a RV8 HS (which uses the RV7 elevators) and a RV4 VS and rudder.

I liked the RV8/7 pre-punched parts for the HS and elevators. I particularly liked the elevator counterweight attachment on the RV7 elevators.
Of course, John Harmon did say that the RV7/8 style counterweights added 2 knots to his HR3, compared to the RV4 counterweight installation.

I had a RV8 VS and rudder ready to install. But I made the mistake of setting my RV4 VS next to it.
The height of the RV8 VS with fiberglass tip and the RV4 VS with my COM antenna fairing installed are the same height.
A brief discussion with John Harmon about the RV4 rudder, and the decision to use the RV4 VS and rudder was made.

Jim Ayers

longranger
09-29-2010, 10:40 AM
On the Harmon Rocket, is the specified empennage a completely stock RV-4 tail, or are there changes that would make it stiffer/stronger? What VNE does John Harmon recommend?

The HRII plans call for adding 0.016 full-span doublers inside the control surface trailing edges just wide enough to catch the aftmost stiffener rivets. I think this is more to alleviate cracking in the bent trailing edge, and less for stiffening to prevent flutter. I don't recall any Rocket that's lost its tail due to flutter, or did I miss something?

Kevin Horton
09-29-2010, 08:31 PM
Here are a few quotes for the -4 tails from his site,
"Most of the tail surfaces received heavier skins to eliminate any flexing both chordwise and spanwise."
and
"John has flutter tested it in three knot increments all the way up to 320 knots"

Thus an HRII with the beefed up RV-4 empennage likely has a higher flutter speed than an HRII with an unmodified RV-8 empennage.

longranger
09-29-2010, 08:47 PM
Originally Posted by Jeff Brenhaug
Here are a few quotes for the -4 tails from his site,
"Most of the tail surfaces received heavier skins to eliminate any flexing both chordwise and spanwise."
and
"John has flutter tested it in three knot increments all the way up to 320 knots"
Thus an HRII with the beefed up RV-4 empennage likely has a higher flutter speed than an HRII with an unmodified RV-8 empennage.

Looking at harmonrocket.com, those quotes apply to the HRIII. Per the plans, an HRII tail uses stock RV-4 skins throughout.

ATC_RV8OR
09-30-2010, 10:11 AM
Thanks for the replies. The reason I'm asking is because I am considering the swap to a Rocket before I press on further with the -8. I'm already working on the -8 tail

Tom Martin
09-30-2010, 11:28 AM
The eight tail does work and fit on the HRII. I have done two of them and have a total of over 400 hours with this set up. I did not go to the published Vne for the HRII and probably maxed out at about 250mph TAS. Both planes were 200 knot aircraft straight and level and so the higher speed would have been recorded in a dive.
The elevators do work but it would be nice to have an extra inch of cord. If the RV7 elevators would fit that might be something to consider. I believe they are a bit larger but I could be wrong. You will not be disappointed with your decision to go to the dark side and build a rocket.

Mike S
09-30-2010, 11:44 AM
Thanks for the replies. The reason I'm asking is because I am considering the swap to a Rocket before I press on further with the -8. I'm already working on the -8 tail

Have you considered a Super 8??? (http://www.mstewart.net/super8/)

ATC_RV8OR
09-30-2010, 10:02 PM
No offense to the Super 8 but it's not what I'm looking for entirely. I realize a Rocket is a Mod to the RV series as is the Super 8. However HRII out numbers Super 8s at least 15 to 1. I find the Rocket to be more tried and true than the Super 8. Also, the Rocket is supported whereas the Super 8 is not. To each their own. Just my opinion.I'm ordering the Rocket plans. Hope to build and see it fly by 2014.