UIUC Low-Speed Airfoil Tests

Wind Tunnel Model Construction Notes - Supplement for Flapped Models (Version 5f)

Materials Supplied

These instructions
Drawings of the flap endplates (A), flap mounting brackets (C), wood insert drawing (D), additional detail and alternative flap bracket (E) and two pages of photographs.
(2) CNC machined flap endplates (right and left sides) donated by John Spitz and Ernest Trent, Jr. of Design & Manufacturing, Ltd.
(2) 2-56 flat head machine screws with nuts
(4) 8-32 flat head machine screws with wood inserts (nuts)
1/16 inch music wire and (1) 3/32 inch O.D. brass tubing (K & S brand)
Brass sheet stock for flap brackets

Important Note

The flap endplates will be reused on other models, so do not permanently glue them to the model.

Construction

The Basic Model

Build the model "whole", that is without cutting out the flap. Remove approximately 0.090 inch from the sides of the model where the endplate will be positioned so that in the final assembly the endplate will be flush with the end of the model. Use the endplates as a guide to determining how much material to remove. As an alternative the model could be built shorter, and then material could added so that the endplates are flush with the model. Flap Endplate Installation
A key step is the alignment of the flap endplates on the model. On the flap endplates, small alignment holes are used to indicate the hinge point location (S) and another is for the trailing edge (R).
Temporarily position the endplates on the model such that hole R is at the trailing edge and hole S is at the hinge point location. (It is important that hole S is accurately located where the hinge line will eventually be located.) Select two of the 8-32 holes for mounting the endplate to the model. Note: The 8-32 hole close to hole S will interfere with the hinge.
Notch the sides of the model where the wood inserts (nuts) will be located. Secure the 8-32 screws to the endplates using the wood inserts on the other side as a nut.
Position the endplate on the model side with the wood inserts still attached. Once satisfied with the alignment, secure the endplate to the model with tape or some other means. Carefully secure the wood inserts to the model by building up a bed of epoxy or alternatively mount the wood inserts into a small block of wood first and then gluing the block of wood to the model once the endplate is aligned. Important: Before installing the wood inserts (1) ensure that the 8-32 screws are tight and (2) check to make sure that the endplates are perpendicular to the model. If the screws are not tight, then once the endplates are removed and reinstalled, the position may not be the same. If the endplates are not perpendicular, then the flap will not travel the full range.
Continue building up epoxy to fill in the notches. Mask off the endplates so as not to damage the endplate since they will be reused.
If something goes wrong, start again with any of the other holes if the initial ones cannot be reused due to having the inserts in the model. The wood inserts can be obtained at any hardware store like True Value or Ace.
Repeat the process for the other side of the model. Flap Bracket Installation
Make two small 90 degree angle brackets (drawing C) out of brass or some other metal material. This bracket is used for securing the flap and also for setting the flap angle via an indexing pin. The brackets should be fixed to the bottom of the model as shown in the drawings.
Cut and bend the bracket to fit.
Mount the bracket to the flap preferably with screws for easy removal.
Drill the hole in the bracket for the 2-56 screw. Use the slot to mark the hole location, but do not use the slot as a drill guide so as not to damage the reusable endplates. When locating the 2-56 hole, note that the flap must be deflected down to 40 deg.
Spot glue the 2-56 nut to the bracket with epoxy. Once satisfied with the position, build up a fillet around the nut to secure it. Alternatively, the nut could be inset into a thin piece of plywood and this could be glued to the bracket. It is important that the nut be secure. If 2-56 blind nuts can be found, they would work better since they could be secured to plywood and then that assembly could be glued to the bracket. Note: The 2-56 screws are long and will stick out the side of the bracket. In the wind tunnel we have short ones, so the 2-56 screws that were provided do not need to be shortened.
Use a pin through hole F (0 deg setting) to score a mark on the bracket.
Remove the bracket and drill an oversize hole through the score mark. The hole should be slightly larger than the 3/32 inch O.D. brass tubing (supplied). Do not use the any of the alignment holes as drill guides.
Reinstall the bracket. Push the 1/16 inch music wire (supplied) through the endplate and though the oversized hole on the bracket. Round the tip of the 1/16 wire first to avoid ... (you guessed it) damage to the endplate. From the other side, slip the brass tubing over the 1/16 inch music wire and through the oversized hole in the bracket so that the tubing is flush with the endplate. Any gap between the brass tubing and the endplate will result in play in the flap setting. The shortest possible piece of brass tubing is best; a length of 3/32 inch is reasonable. Carefully spot glue (with epoxy) the brass tubing to the bracket. Do not use CA glue since it could seize the entire arrangement and damage the reusable endplate! Remove the bracket and build up a glue fillet around the brass tubing. This a another key step since the pin/tubing will be use for setting the 0 deg flap deflection, which in turn sets all other angles of attack. Note: During the wind tunnel tests, the alignment pin will be removed from the model once the the 2-56 screw is secured.
Streamline the flap bracket / nut assembly to minimize the flow disturbance.
Repeat the process for the other side of the model. Note: If there is time and interest, the 2-56 screw could be located so as to screw directly into the flap (drawing C).

Alternate Flap Bracket

Important: This method should only be attempted by experienced builders who are sure that the endplates will automatically be located in the same place every time they are removed and re-assembled. See drawing E for an example of an endplate recess that will ensure the plate can only be mounted in one position.

Bend a piece of music wire (included) as shown in the sketches on drawing E.
Cut a slot for the music wire and fit it so that the flap is at exactly zero degrees deflection when the music wire is located in hole F on the endplate (drawing A).
Fill the slot and area around the music wire with epoxy resin or similar.
When all is set, make sure that when the endplate is removed and replaced again, the plate goes into exactly the same position with the flap returning to exactly the same angle. This is extremely important since we will remove the endplates every time we adjust the flap angle. If there is any doubt, consider using the bracket described in section (3) instead.

Flap Installation

Cutout the 21% chord flap, which is 2.52 inch for a 12 inch chord model. Use a standard V-type cutout as shown in drawing C. The V-cutout should only be large enough to allow for a -15伆 reflexed flap deflection. All models will use the V-type cutout to be consistent; we will use a mylar gap seal on the model during the tests.

Inspection

Check to make sure that all flap positions can be set with the alignment pin and secured with the 2-56 screw.

Final Comment

In the wind tunnel, we will tape over all holes in order to maintain good flow integrity. For this purpose, we have a special tape that is easily removable, so you need not do this step.

Non-Standard Flap Chord Lengths

For flap chords greater or less than 21%, the endplate will still work. The only change is that the aft alignment hole R, will not line up with the trailing edge. It is necessary, of course, that the hinge line remain at point S. If you contemplate using a non-standard flap chord, please call us first.

COMMENTS?

If you have any questions, please contact Prof Michael Selig.

If you have any suggestions for improvements, please send them to us. If you would like to provide us with a sheet of notes: "lessons learned" we will include them with these instructions. On your notes, please list your name and phone number in case a builder would like to ask you questions.

Shipping of Models

See notes here: modelNotes.html

Example of standard flap bracket

Detail of one flap endplate installation.

We prefer this (below): Example of alternate music wire flap bracket - see text before attempting this method. (Model by Jerry Robertson). Note recessed plate.

===>MORE pictures of a flap endplate install.
Flap retrofit by Jerry Robertson
Model by Yvan Tinel
(2.4 MB)

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