- Nov 14, 2009
- Rocky Mountains
My first comment is going to be predictable - Is there history in any of the airplanes that have the slip fit bolts with failures of these joints. If there are a bunch of airplanes flying and there are no problems, I suspect that the practice, as executed and with the FOS used in the successful designs, is pretty darned good. If there are failures, there is some evidence.I see much design with body-bound press fit fasteners or tapered pin joints ....mainly in wing attach fittings and wing carry through fittings or spar caps....What might be the reasons for that considering your statements......So should I not be concerned if I have a slip-fit wing attach bolt connections vs press-fit wing attach bolts?
and weather this should be a separate thread or not....I do not know,...found these posts here so I kept it here.
like it....I imagine the aluminum will turn black....like smoking rivets?Now if you want proof that a new joint is or is not moving, put it together, load it in service type loading, then remove the bolts and look for fretting or any other signs that the joint has shifted
That makes sense to me.Well, if an alternative to welding is sought and the alternative is drilling holes for rivets and bolts, then the issues of drilling holes needs to considered.
So on topic for this thread.
You are correct that prop bolt torque must be checked periodically. (Not necessarily tightened; I've run wood props on Lycs for almost 3 decades, I live in the deep (wet) south, and after the initial retorque on a new prop, I've rarely gotten any nut movement when rechecking torque). But at least on a Lyc, maintaining torrque has nothing to do with maintaining friction on the prop/flange interface. There are six big press-fit lugs in the flange, pressed at least 1/2" into matching holes in the back of the prop. There will be no friction affecting the back of the prop until loose bolts have allowed the prop to wobble on the flange enough to wallow out the lug mounting holes.Prop bolts need to be retorqued periodically for a wood prop, otherwise the prop will move and the friction will char the face and then it gets looser and fails. My flying buddy had the flywheel bolts shear on his Jabiru. While he was installing new flywheel bolts I picked up his wood prop and showed him his charred prop face. He did not know about the need to retorque wood props (50 hrs or something) The loose prop had broke his flywheel bolts, fortunately while taxiing.
The 1/2" lugs are for tight fit in aluminum, I think. I doubt 1/2" of a 4" wood hub is enough. But who knows?There are six big press-fit lugs in the flange, pressed at least 1/2" into matching holes in the back of the prop.
Metal on metal friction is usually well described by a coefficient of friction times the clamping force. This applies until you get to yielding of one or both of the connected members - small faying surfaces (tube on flat qualifies) can reach yield fairly easily, which also results in loss of preload, so they have to be designed carefully. Remember also, that the region in compression around the hole usually extends only one diameter of the fastener beyond the fastener itself. Only in larger surfaces designed as gasketed joints do we spread the loads much further from the fastener itself.Well one issue also to consider....a flat plate on a round tube does not have lots of square inches of bearing for friction.....might want to just consider the shear of the rivet and bearing of the rivet against the sheet or tube. I think in this case the glue might help but given the geometry of the joint (round against flat) and low shear of epoxy .....testing might be appropriate, and it is hard to glue aluminum.