alanalex

Can anyone help? how do I plot retract geometry. the leg is sloping 28 deg forward from the datum when down and 11 deg forward when retracted.

how do I plot the angle for the bearers that carried the retract?

Many Thanks

allanflowers

I don't think that is enough info for an answer. A sketch would help too.

Lnewqban

I second Allan, since I cannot understand your description either.

ARUP

Put the retract on a flat surface and block up the rear lugs until extended gear at 28 degrees fwd. Measure the thicknesses of block. Make a wedge from hard balsa using these measurements big enough to insert retract. With gear in closed position rotate for eleven degrees retracted. (The 11 degrees isn't going to affect the 28 degree one much but it will- it'll decrease it a smidge). Mark it for you datum as reference. Now all you have to do is build the retract rail supports from measurements taken off the wedge. You can compensate for dihedral by tilting your reference surface appropriately. I'm sure there is a mthematical formula for all of this but I like empirical methods! This is how I'd do it and I'm sure someone will find fault- after all this is RCU- lol. Regards!

R8893

My Feedback: (20)

Here is the mathematical method taken rom Dave Platt's "Scale in Hand" book. Measure the angle forward from vertical relative to the chord line. I think this is 28 degrees in your example. Measure the angle BACK from the span line for the gear up. This is a NEGATIVE 11 degrees for your case. Add and divide by 2 (28-11)/2=8.5 degrees. Now the mechanism mounts in the wing with 8.5 degrees positive incidence. In the plan view the mechanism mounts rotated 8.5 degrees rotated back from the span line. It is also necessary to mount the leg at an angle to the pivot axis in the mechanism. In this case the leg will be at 70.5 degrees from the axis as opposed to the 90 degrees the mechanism was probably built with. If you PM your email I can draw and scan a sketch. I have used this calculation on a number of models. It works. The hard part is making what I call the "kink block" to rework the angle between the leg and mechanism axis.
Chuck

R8893

My Feedback: (20)

I got curious about whether the method really works exactly. So I drew the retracts per Alan's numbers in CAD. As you can see in the views below there is a subtle skewing of the axle. Reminds me of how a Hurricane looks. Here are the moves and constraints I used. 1) axle is perpendicular to the strut. 2) strut is rotated 19.5 degrees from perpendicular to the pivot shaft in the mechanism. 3) axle rotated 8.5 degrees around strut. 4) mechanism swings through 90 degrees. 5) mechanism rotated up 8.5 degrees from chord line. 6) mechanism rotated back 8.5 degrees from span line. 7) 5 degrees dihedral.

Chad Veich

My Feedback: (60)

Thanks for this post Chuck. I have Platt's "Scale in Hand" book and as far as I can recall the part of your statement I have highlighted in red does NOT appear in the book anywhere. I've pondered Platt's formula many times and could never figure out how it could possibly work. Now I know!

R8893

My Feedback: (20)

Chad, Platt's book is one of the few places where you have to look at the drawing to get all the info. I guess he knows people don't read the instructions I have read the article about a zillion times trying to find where he mentions the angle between the strut and pivot. I guess it really ain't there!
Chuck