ground handling loads

[rb1957]

lets have a discussion about ground handling loads. A recent thread "doubler damage tolerance analysis" was concerned with adding a blade antenna to a plane (assume to the fuselage). obviously loads (other than cabin pressure) are a concern, and the poster was looking into aero. loads.

my experience is at unless the blade is very big, or unusual geometry, or the plane very fast, then aero loads aren't significant. so we consider a ground handling load applied to the blade, stricitly anywhere and any direction but nearly always at the tip of the blade and across the blade (nominally blade lift). we use 200 lbs (as an ultimate load).

wotcha think ?

 

[EVdave13]

rb1957, you make a good point. I work at a company just starting to use real blades as opposed to whip and semi-conformal antennae. They have NO criteria set up. I am pushing to formalize some. I have done extensive work on blades in the past, both from the aero side and the structures side.

Normal loading criteria listed in a company design manual is usually something like this:

1. Use aero loads (static) from an aero analysis if you have them (most of the time you will not).
2. If you don't, then use the most critical of the following:
a) 100 lb. (I've also seen 150, and you say you use 200) applied at the tip midchord, normal to the antenna plane. This is a ground "handling" (bumping, falling) load.
b) 100 G load applied at the center of pressure
c) a parabolic distribution varying from 0 at the tip to max at the base, with a total magnitude of 150G (I've also seen triangular distributions used)

You are right in that the handling load is usually the most critical, but there are several conditions on that statement. For small blades on GA aircraft, sure. For large ones (over about a foot tall and a foot in chord), they can be significant, because the area starts to really get up there. Faster aircraft obviously have much higher aero loads, too, as the dynamic pressure depends on V^2.

Another point to remember is that static aero loads can be miniscule in comparison to dynamic aero loads. That is why the last couple of criteria (b and c) have such high load factors.

 

[rb1957]

thx EVdave13,

my experince is small commuter planes. probably our worst experience was placing a reasonable-sized blade antenna behind a largish 3D shape (yes, right in its vortex street) ... it lasted about 3 flights) the problem was the blade stiffness ... but the stiffer blade transferred the aero loads into the fuselage skin.

with commuter (pressurised) planes, our biggest concern tends to be the DT assessment, what with external doublers ... the static strength isn't difficult to meet.

a whine i get somewhat is, why aren't you using AC43-13 ... can we get the FAA to revise that ?

do you have any guide-lines on spacing antennas ?

 

[MikeHalloran]

The word 'blade' brought up this scrap of mental flotsam; cropdusters have a short sturdy cutting blade at the base of the windscreen, for severing the unnoticed telephone wire.

If the aircraft is to survive a _near_- ground encounter like that, you'd want to be sure that the antenna will prevail against a small wire, and shear off without causing major collateral damage when pitted against a larger wire.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[Sparweb]

Rb1957,
I've seen the same thing happen to antennas that got the shakes in icing. One observation that I made from that event was that the cracks followed the contour of the antenna base, completely avoiding the rivet holes and screw holes. Also interesting to observe was the series of stop-drills following the crack along the skin. There were about 10 of them, 1/2" - 1" apart. You'd think they guy would have given up and removed the antenna after the first two or three. I keep the doubler on my desk as a memento (part of my collection of airplane and heli bits broken in many disastrous ways).


Steven Fahey, CET