Bob,
This is common with a nicely built smooth operating airbrake linkage
that is a little proud of the wing surface. One set on 13669 banged
like hell. I changed out the bearings, sanded the inside of the
airbrake so it would fit into the rebate with no contact, and it worked
OK. Normally I don't deploy the airbrakes above 85 Knots. Pulling it
above that speed is a bit of a workout. The flutter should stop by just
holding the handle. If you are still getting a buzz when holding the
handle, call me, because some thing is loose.
Analysis is:
When the airbrake handle is unlocked and released, the low pressure on
the leading edge airbrake portion helps the airbrake start to deploy
(especially if it is proud). The airbrake leading edge catches the wind
and by the time the flap aft part compensates, it starts to close with a
slap, but wait, it bounces open and catches air again...and repeats.
To compensate for that, the bearings in the linkage are supposed to be
MW4Ts. The T is for tight. When these bearings are installed, the
system is not normally prone to flutter as the friction of these
bearings prevents the ease of motion. The top leading edge of your
airbrake needs a very sharp edge and an aft slope to also break the
pressure buildup, and you probably have a slight contour which
exacerbates the problem. This is a possible manufacturing flaw of the
airbrake core and in the directions in airbrake construction, in my
opinion, as the drawings make you feel that a nice curve is OK. The
floxed corner on the leading edge of the upper surface should be sharp.
After all, the core and photo's have a slight curve depicted and no note
on the floxed corner. Your airbrake most probably sticks up a little
when released on the ground and is forced closed by the linkage. When
the brake is released, the airbrake instantly pops open due to
contacting the inner rebate. This springy action is a problem as it can
cause instant flutter even with the 4T bearings. If the airbrake is too
fat for the rebate and sticks up proud of the surface, this too causes a
low pressure area and a lifting force to the front of the airbrake. It
seems that the rebate is not quite deep enough for a fully filled and
painted surface unless great care is taken in fitting during
construction. Nobody does that, we just build the airbrake and fill
it, expecting it to fit.
Flight Experience:
In flight, at redline and a loose or low friction system, which sticks
up proud of the wing surface will allow the airbrakes to bang quite a
bit when just barely deployed and the handle released without the T
bearings. This is quite startling if you are in a high speed dive and
you bump the handle out of the lock. I have emailed for guidance on
this common problem two years ago. Dave is now looking into the glider
wing aerodynamics. The fix in the past was putting resistance into the
system with the T bearings. I believe we will have to add holes to the
airbrake portion (not unlike a WWII dive bomber airbrake) to relieve the
pressure bubble that forms under the leading edge and the wing close
out. The airbrake core will need to allow for a thinner underside to
the leading edge of the airbrake core to allow a flush, interference
free fit. We also must be very clear on the leading edge shape and do
computer flow analysis and documentation. This is the same airbrake
used on the Slingsby glider, which is supposed to be OK. I do not know
about that personally, as I have not flown one.
Other planes flying that I have tested are: N914PL, which only had a
slight bounce, but no longer has the problem with the 4Ts installed, but
13669 was kind of exciting. Difference is the T bearings and fit, as I
built both control surfaces with the clients, rigged them and test flew
the planes.
I have another motor glider in the shop which will get airbrake
refinements (holes in the forward portion etc.). In the mean time, go
to the hobby shop and tape a piece of 3 foot trailing edge stock (1 inch
by 1/4 inch 90 degree triangle stock) to the bottom of the airbrake
trailing edge. Tape it on with glider wing tape or similar and go fly.
I find the extra up pressure makes deployment tougher, but it holds the
airbrake closed and stops flutter until you can finish testing and get
time to re-contour the airbrake for an interference free fit if
necessary. If your airbrake sticks up on the leading edge, because it
is too fat for the rebate, or springs open due to contact of the
mechanism or rebate fit, I hate to tell you, but, de-skin the inside of
the airbrake as appropriate and sand to re-contour until fit properly.
This is how I fix it for my clients. Once it fits nicely in the slot
and with the T bearings, it seems fine.
Other comments are welcome. As I have only test flown two MGs
extensively.
Bud Yerly
Custom Flight Creations.
shop phone 813 653-4989
----- Original Message -----
From: ROBERT LINDSAY<mailto:rlindsa2@san.rr.com>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Tuesday, December 01, 2009 1:32 PM
Subject: Europa-List: MOTOGLIDER SPEEDBRAKE VIBRATION
During first flight with motor glider wings when speedbrakes were
unlocked, brakes floated partially open. This, of course, is normal.
However, occasionally at speeds btween 60 and 100 knots, the entire
assembly would vibrate or shutter light to moderately as felt on the
handle and visually evident on the speedbrakes themselves. Shutter
frequency was much lower than the buzz of flutter and slight movement of
the handle in either direction would stop the vibration.
Suspect looseness in linikage or brake surface anomaly could be the
cause. Anyone experience this and what was the fix?
Bob Lindsay, test pilot for N911XS, Ramona and Warner Springs,
California
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