So what’s a “spade†the examiner asked ?........:) Now I have heard of balance weights and flutter but have never heard of a spade. This question moved on to a recent crash that apparently broke apart, with the wings being twisted off due to flutter, a Yak of all things. I thought they where built like tanks and that flutter might destroy a control surface, but total wing failure ?
So I went searching for information, in my books and on the web and came up with nothing much, other than it’s a big problem on model aircraft. So any info you guys might have on the subject would be greatly appreciated.
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Hmm... well... yes... I've not heard of that either!
I'll have to ask a few of the other engineers around here and see if it's familiar to them. I know an A&P or two that might also be able to shed some light on it. If I find out a confirmation of that, I'll post it here.
A spade is a device on the control surface that sticks out into the airflow that helps lighten stick forces. You see them on a lot of Pitts on the aelirons - it looks like a little, well, shovel with the blade parallel to the wind. If one comes off, then the aelirons will be assymetrically loaded (i.e. - out of balance).
And for what it's worth, flutter will rip any airplane apart, no matter how tough it is. Don't think flutter like "flutter to the ground" or a "flag fluttering in the breeze." Think violent shaking that, in a wind tunnel, has to be captured in super slow motion to be perceived. Pete Garrison (yes, he of the famous downwind controversy)had a great article about it in Smithsonian Air and Space a couple of years ago. Flutter, by the way, is why aelirons have to be re-balanced after painting, and why I look very carefully to ensure that the trim tab control linkages are properly attached before I fly. And why you should slow the airplane down if you encounter any strange vibrations.
The balance tabs on the elevators of Cessnas do the same thing, although their not really spades. The Frize (someone check my spelling, I don't have my reference books with me) type aelirons on Cessnas can be thought of having spades, as the bottom leading edge of the surface stick into the wind on the up aeliron. This isn't really to lighten stick (OK, yoke) forces, but to help counteract adverse yaw.
Thanks for the information. I will have to take a look at a Pitts, it sounds a little bit like a balance horn, not associated with the balance weights found attached to the leading edge of a Cessna 150 aileron. The one needing airflow to function the other not. I would love to get hold of that article. I have just bought a Jabiru and from what I read, flutter is dependent, amongst other things, on speed, balance and weight of the control surface. The Jab 400 is fast (for its size :) ) and has light control surfaces, I wonder how far the factory took the ASI, and it must be something that the Lanceair folk poked around with. It would be nice to read something that told me not to worry about flutter until 150% VNE or something, it killed the guys in the Yak, both very experienced pilots who would never fly on or near VNE. I would have thought that the first sign of flutter would be a vibration, progressively getting worse, that you would have sufficient time to recognize the onset and slow down.
It’s an interesting subject and I suppose manufactures are not gona tell you what happens after VNE and when…ha-ha. I would however like to know what to look out for and how fast it develops into structural failure…..maybe that’s why they fit a spade to the aircraft, its for digging a hole !!
A few random thoughts on spades, the aerobatic pilot's best friend.
Spades are typically attached near the trailing edge of the ailerons. You may see them on aerobatic aircraft, bush planes and other aircraft types. They help reduce the effort required to contol the airplane in the roll axis, especially at low airspeed or when the wing is loaded at higher than one G. Sometimes they are added as aftermarket modifications to certified airplanes that have heavy roll forces easier to control. They may also be an indication that the airframe designer overestimated the control effectiveness of the ailerons in his/her design.
Do not confuse this with a spar. The spar is the primary structural component the wing is built around, and which attaches to the fuselage. The spar is at right angles to the fuselage and extends from the middle of the airplane, outward to the spar caps, where the wingtips are bolted onto. Fuel tanks, landing gear, control surfaces, all are attached to the spar in one way or another. Sometimes on larger aircraft an additional spar, AKA secondary spar, located aft of the main spar, extends out to a point somewhat outboard of the area where the flaps and ailerons meet. This provides additional stiffness and reduced twisting forces. As the aircraft gets larger and heavier, even a third spar may be needed. When you look out the window of your 747 or A-340 you can see the wingtip rise up just as the airplane rotates for takeoff. That is the spar(s) bending upward as it loads up, to carry the weight of the aircraft.
Douglas M
Surrey, BC Canada
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