Is the 12" strip of Fiberglass tape required on leading edge of a Monarch?

From : Don Stackhouse

Antonio Martinez asks:

As I understand it, I am to add 12" wide strip of fiberglass cloth to mid leading edge on my Monarch. It seems a shame to disturb the airfoil. Is there a better way? For example, could I route a slot in the bottom of wing and imbed a CF "spar"?

Antonio, Excellent question!

A carbon spar in that area would actually weaken the wing. The tape in that area is for four reasons, which in approximate order of increasing importance are:

1. It helps absorb local impact loads from hand catches and to spread those loads into the skin aft and to either side of the impact point.
2. It helps reinforce the area where the leading edge dowels are installed.
3. It helps protect the underside of the wing leading edge from chafing by the wing saddle.
4. The highest tensile stress in the wing is on the underside of the leading edge. The peak compressive stress is on the upper surface. There is a high shear stress going around the leading edge trying to connect these two. The tape helps carry the tensile stress, and also deals with the shear stress.

A carbon spar would create a severe stress concentration at its tips , and would not help with the shear stress at all. It would also be of no help with reasons 1-3, in fact it would probably be cut by the dowel hole, resulting in a nasty stress concentration at the dowel location.

In general, you have to be very careful when using materials of wildly different stiffnesses in the same structure. The stiffer materials tend to focus the stresses along themselves, which means you get no benefit from the rest of the material in the structure You also tend to get shear stress concentrations along the edges of the stiff material and tension or compression concentrations at the ends. This is the big reason we use fiberglass for reinforcement in the Monarch wing, rather than carbon.

If you do a neat job of applying the tape to the leading edge the disruption to the airfoil should be minimal. While the airfoils on the Monarch do not generally benefit from turbulation, it couldn't hurt in the area around the disruption from the fuselage. In general, we're talking about a bump of only a few thousandths, well forward on the airfoil in a place where the air is still accellerating. At hand-launch Reynolds numbers the air will probably go laminar again shortly after the bump. You might get a slight tendency for earlier separation at stall, which would actually improve the low speed handling by forcing the stall to occur at the root rather than at one of the tips.

Your attention to aerodynamic detail is commendable; however, in this case the tape on the leading edge is not a problem, and is necessary for structural reasons. Just try to do your very best to apply it smoothly and neatly with minimal thickness, weight and aerodynamic disruption.

Thanks for the question, and good luck with your Monarch!

Don Stackhouse @ DJ Aerotech