Saturday, March 31, 2012

Let's not twist again

A lot of RV airplane projects that aren't nearly as far along as mine have had their first flights, but I'm not ready to fly mine quite yet. I decided some time ago that I would complete the plane as far as possible before its flight test, even if some of the parts I'm working on will be removed for first flight.

Hello, gear leg fairings!

The fairings, which will not be installed for first flight, are usually left to the "I'll get around to it stage" of experimental aircraft projects. They're needed to provide a smoother air flow over the wheels and gear legs and increase speed by 5-10 knots, a helpful thing when you consider the current price of a gallon of 100LL fuel at Fleming Field (KSGS) is currently $5.19.

But the process of fitting them properly is painstakingly slow. By this point in the construction process, however, if you haven't learned patience, you probably shouldn't be building.

My pal, Chris Knauf, was good enough to do some of the construction in the gear leg fairings and he did a great job getting them trimmed and the hinges installed. All that's left to do is fit them properly.

A misfitting fairing can act like a rudder or aileron and change the direction the airplane wants to fly from straight-and-true to didn't-we-just-fly-over-here?

My job this week has been getting those fairings fitted properly. The right-side leg fairing fit pretty well-- a very minor twist that I can take care of easy enough.

Last night, I set the left-side fairing up. The process for this is pretty funky. You mark the centerline of the fuselage, then pick three points on the gear-leg fairing and make an "extended centerline" back to a point on the horizontal stabilizer. I'm using an advanced aviation tool to provide the marking of that point from the tail to the floor -- a broomstick; three actually.

Then I take dental floss (it doesn't sag like string and allows for more precise measurement, plus it makes the hangar smell minty fresh!) and attach it to one of the three points on the front of the fairing and run the string back to the broomstick, so you have two parallel lines of dental floss to the front of the fairing. This, basically, simulates airflow over the fairing.

The distance between the center of the aft end of the fairing and each piece of string should be the same, indicating that the fairing is perfectly centered in the airstream with regard to the centerline of the fuselage.

On this particular fairing, I adjusted the bottom of the fairing to make that distance about 15/32". The mark halfway up the fairing was a little more off, and the distance on one side of the top mark was about 14/32" and on the other side was about 27/32". This, you may have surmised is the dreaded "twisted fairing."

There's no real way to work this out because the fairing is only clamped at the very top. There's nothing -- without an intersection fairing -- to hold the bottom from moving in order to twist the top into position.

The solution? I don't know, but I assume I'm not the first one to experience this problem. I'll begin researching it and eventually solve the problem and include it in an article I'm writing for the RV Builder's Hotline.

I just need a little more patience.

1 comment:

  1. Bob,

    I know you have since flown your plane (congrats by the way) so this has obviously been solved in the meantime. However, other readers may be approaching this task, as I am, and may be interested to solve or avoid a twist.

    I haven't had a twist (yet) so I have no experience to offer, just an opinion.

    It seems to me that the hinge is holding the twist so the only solution is to move one hinge half and reposition it. This means drilling it off but we are dealing with fiberglass so that's no problem, right?

    I think I would immediately fill the old rivet holes with epoxy as the new ones are likely to be overlapping or immediately adjacent. Then I would repeat the vans alignment process with the trailing edge held closed only with spring clamps. Removing the twist will make the two sides of the fairing slide relative to each other. With the fairing aligned in the no-twist position, I would put a tape around the trailing edge in 1-3 positions and then slit each tape down the join line, as per Vans suggestion in the manual. Next, I would take the fairing off and re-drill the holes for the hinge half while ensuring that the two parts of each tape line up perfectly with each other.