Mailing List lml@lancaironline.net Message #67492
From: Gary Casey <casey.gary@yahoo.com>
Sender: <marv@lancaironline.net>
Subject: Yaw
Date: Mon, 21 Oct 2013 07:36:45 -0400
To: <lml@lancaironline.net>
The idea is to put a drop(s) of oil, presumably ugly black oi that is highly visiblel, on the spot you want to evaluate.  Then go fly for enough time for the air flow to "streak" the oil in the direction of airflow.  It doesn't always work very well.  Gravity will affect the trajectory of the streak, and in the back of the plane the boundary layer is sufficiently thick that little streaking may occur - at least in a reasonably short time.  And then, of course, the time spent in the desired flight regime (say, cruise) must be enough longer than the other conditions (taxi, climb, etc) so that the streaking is accurate.  The method doesn't cost anything, so it's usually worth a try.  And then different kinds of oil will produce different results - I haven't experimented with this.

And I agree that a single dorsal fin would be better than the twin ventral fins.  Ventral fins are used when there is limited vertical clearance, like on a jet when a tail "rub" is possible.  They are also used when increased pitch stability, especially in high AOA condtions, is desired.  Neither is a factor in our planes.  I looked hard at building a dorsal fin, but finally gave up and just bought the ventral fin from Custer.  The thing that tipped me over the edge was the difficulty of providing a tail tie-down.  In retrospect I should have stuck with it.
Gary

How do you execute the "oil drops" method?

FWIW I ended up with a single strake on my IVP .
(What I really wished for was a longer rudder but I didn't have the =
engineering/flight test budget for that....)

On Oct 18, 2013, at 7:56 AM, Gary Casey wrote:

I certainly concur with the comments below.  I installed them on my ES =
after doing an A/B test.  They increased the "feet-off-the-rudder-pedal" =
stability by about 30%.  I measured a drop in cruise speed of about 2 =
knots.  Painful, but I'm still glad I did it.  There seemed to be no =
difference in low speed pitch trim.  And I agree that the alignment of =
the strakes with the local airflow could be better.  If I were doing it =
again I would carefully evaluate the local airflow, probably with oil =
drops, and take a wedge-shaped slice out between the strakes, most =
likely toeing them out by maybe 5 degrees.  That said, I have evaluated =
the airflow with oil (eventually the engine blowby oil will reach the =
strakes) and I don't see any real problem.  Maybe I'm imagining a =
problem when there isn't one.  I was told by someone (professional test =
pilot) who has flown all types of Lancairs that on a IVPT they are =
"absolutely required," on a IV they are "essential" and on an ES they =
are "nice."  FWIW
Gary Casey
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