- Oct 18, 2003
- Saline Michigan
First, thank you for your many considered replies. And a BIG thank you all for staying on topic. It usually only takes two or three posts before someone hives off into some obscurely related topic, and the original question/comment gets forgotten entirely.
Now - tdfsks (what do these letters stand for, BTW?) - you certainly got me thinking, and backed up by Billski, I think I finally have the basic principles sorted in my head.
This is the configuration of the HM293:
View attachment 98975
Interestingly, the pivot point is 40mm ahead of the c/4, and the Incidence Control Arm is set back 400mm from the pivot. With this arrangement, although a low CM airfoil is advantageous, the pitching moment of the airfoil doesn't matter nearly as much as the lift of the wing itself. Any lift at all will raise the rear of the wing. This is experienced by the pilot as a pull on the stick, and he/she will have to pull back against it. The so-called "living wing" for which Fleas are famous.
The extreme case (the pivot right on the LE), would see the entire wing pivot upwards, and with the increase in lever arm (i.e. 300mm instead of only 40mm), the pilot would in all likelihood not be able to exert sufficient force to keep the wing from pivoting onto its head, as it were.
If the pivot point is *behind* the CoP, then any lift would swivel the wing downwards.
View attachment 98976
As it is, the Mignet "Formula" (as verified by the plans) seems to place the pivot point slightly *ahead* of the CoP, and to place the Incidence Control point rearward of this creating at least a 10:1 moment arm. This now makes a lot of sense.
My own design also uses a 1200mm chord, but places the Incidence Control point on the trailing edge. This means, to keep to the same "formula" (and hence, "feel"), the pivot point needs to be 10% of the Pivot-Incidence distance ahead of the CoP. Like this:
View attachment 98979
This would require a bracket affixed to the spar extending some 160mm or so forwards, with possibly a number of holes to fine tune the pivot point. I *think* my reasoning is sound. Comments welcome, of course.
You gotta know I am going to try to talk you into another perspective here...
Flying Flea design - With 160 kg (we won't insist on forces being in Newtons right now) supported 0.04 m behind .25c point, the original wing makes 6.4 kg*m of nose down moment. And the mechanism for Incidence control of this foil must counter that moment while resulting in a reasonable force at the stick and within a reasonable stick travel for the entire operating range of that foil.
Now we go to your design - Same chord of 1.2 m, and same lift of 160kg but the distance to pivot from .25c is now going to be 100 mm. The moment to be balanced by stick force will now be 16 kg*m, 2.5 times as much moment as was carried previously. I contend that now your control system must control a device that has the same amount of rotational travel but operates at 2.5 times as much torque. This absolutely means that either your stick forces at the same travel have to be 2.5 times as high or if you play with the gear ratio to get the same stick forces, you will need 2.5 times as much travel. There are an infinite variety of design options between the two mentioned, plus more outside that range.
The location of where the connector attaches to the foil is immaterial to the force and travel choices being presented. You can change ratios anywhere along the system to get a good trade, but if you increase the arm that lift uses to create a moment, the moment must go up. I strongly suggest that if you place your pivot point 100mm forward, you will have heavy controls or inadequate travel or both. If you are really married to placing the pushrod at the trailing edge, set the control forces and stick travel you want and then work the mechanical advantages to have that match the forces and travel of the foil.