Chicago Chopper

Introduction

I always wanted to build a helicopter. When I was about 8 years I designed a helicopter powered by human force (nobody took me seriously) and during summer vacation I sneaked into my granddad's workshop and used his precisely cut, sanded and polished pine wood for the construction of my helicopter. I was discovered at the time when the rotor was almost finished (surprisingly, everybody looked pretty serious then).

Talk about the consequences. I was cured from the helicopter design for more than 20 years. The idea of building a helicopter came back in the form of necessity just before the MWRC99 contest. Our team (T#087/CHEDAR-1) was looking for a competitive helicopter model because all the helicopters that we own and fly are more what you'd consider 'a backup' then 'competitive'. The result of four weeks of research and construction struggle finally resulted in the Chicago Chopper design (ChiChop for short) which is presented below.

The Game of the Name

Why Chicago Chopper? First of all, it seems to me that all Cyclons, Whirls, Spins, Rot-A-Somethings are already taken. Chicago Chopper looks different enough.

It's a quite obvious why chopper, but what has the Chicago to do with that? To tell the truth, I do not know. I just like that name (sometimes I wonder how my brain works).

Model Description

DRAWING: BOOST AND RECOVERY

Construction

Level of Difficulty

Pictures that May Help

Tools and Materials

Blades

Drawing: BLADE DIMENSIONS

Cut the blades and sand the airfoil. The high point of airfoil is 1/4" from the leading edge of a blade. I agree that sanding airfoils is a big pain, but there is no way around. However, you can find the procedure, that I use, somewhat helpful:

1. Draw the line at the high point of airfoil. Be careful not to push too hard on a pencil.
2. Using a masking tape, mask the leading part of the blade. This will protect that part from an accidental damage and also shows you where the airfoil high point is (the pencil line gets sanded off quite fast)
3. Use a sand block to sand the trailing part of the airfoil. I use block which is bigger than sanded blade, to cover the whole blade at once. During sanding check often that the bevel is even along the whole blade.
4. When you are satisfied with trailing part, remove masking tape from leading part, mask the whole trailing part and sand the leading part.
5. Finally sand the airfoil to final shape.
6. Repeat for all blades.

Once done with the airfoils, check that all of them are same (again, you want three identical blades). Now, you have a good opportunity to put a new blade on your X-Acto knife. You are going to cut the blades in the half and you want a really clean cut. Measure 1/2" from the leading edge of each blade and draw the line (do not push on the pencil). I cut from the upper side (the blade is laying on flat side) to avoid wiggling. Use a ruler to guide the knife.

IMPORTANT: Immediately after cutting the blade, mark both halves with the same letter or number, so they do not get mixed up with others halfblades.

Now take a black pastel pencil or a marker and color the bottom surface of each blade (except the area where you wrote identifying letter or number). Black blade is much easier to spot against the sky than the 'natural balsa color'. This will help you when you'll chase your rocket drifting down the wind.

Halfblades will be held together by the 'flexhinge' which can be either Crystal Scotch tape or a mylar tape. Flexhinge runs along the whole blade (see drawing: BLADE CONSTRUCTION) except the are where hinge will be glued (see drawing: BLADE CONSTRUCTION again).

DRAWING: BLADE CONSTRUCTION

Now you need to install a rubber bands, which will unfold the blade and keep it in a unfolded state. From a rubber band, cut nine pieces 3 mm wide and 6/8" long (3 pieces per blade). If the rubber band you use is a little wider or little more narrow, do not worry about it. But try to have all nine pieces the same (again, you are trying to create three totally identical blades). Use a CA glue and glue rubber band pieces to each blades as shown on the picture. When the blade is unfolded, there is almost no tension in the rubber hinges. Make sure that you do not stretch the rubber bands when gluing them to the blades. Also be very careful with CA glue and do not glue the halfblades together (that could easily turn into sanding yet another airfoil for a replacement blade).

Refer to the drawing: BLADE CONSTRUCTION (or photo) again and cut off the clearance for hinge. If you use 1/2" hinges, you need to cut about 5/8" from the trailing half of every blade (cut on the side that will be closer to the hub).

DRAWING: ANGLE OF ATTACK

Now you need to do add the angle of attack. Blades must be mounted under certain small angle if the rotor should rotate well and create the lift. Get a scrap of of 1/8" balsa (you'll need a strip about 2"x 1/2") and sand it to triangular profile exactly as indicated on  drawing: ANGLE OF ATTACK (check the dark pink triangle named beveled hinge mount there). It is easier to send the profile to the whole strip and cut the pieces from it (this will ensure that all bevels are almost the same). When you get the strip sanded into triangular profile, cut three identical pieces from it. The pieces should be big at least as one wing of the hinge you plan to use. One piece per blade, glue them to the leading halfblades (check drawing: ANGLE OF ATTACK for correct position and orientation of the piece (you should end up with leading edge of the blade being on the low side of the blade)).

Blades will be deployed by rubber bands. One piece of rubber band is held by the hook on the hub, the other end is fixed to the hook on the blade. Drawing: BLADE CONSTRUCTION shows you approximate shape of such hook and its position. Using pliers you create the hook from sewing pin. Once you have it, run it through the blade (it will come through easily) and glue on both sides with CA glue. (To understand better how these hooks work you may want to check drawing: DIHEDRAL).

When blades are deployed, something has to stop them at the desired angle of dihedral (do not confuse with the angle of attack). This is what the bladestops are for (do not confuse with the bladetabs). Again, the drawing: DIHEDRAL shows you how it works. Bladestops are cut from balsa (thickness 1/8") and glued to the blades with Elmers glue.

That's all you need to do for the blades. Put them to the safe storage where neither pets nor vacuum cleaner can get and destroy them. The construction will continue with the booster now.
 

Booster

DRAWING: BOOSTER

I recommend following procedure for mounting of fins:

1. Mark the booster tube using provided drawing: FIN MOUNT GUIDE. Draw the lines along the tube and mark 1/2" distance from the booster bottom on each line.
2. On the body tube sand (lightly) each area where a fin will be glued. Sanding will remove gloss finish from the tube surface and glue can penetrate better, thus forming a stronger bond.
3. Put a small amount of an Elmers glue on the root edge of one fin (root edge is the side of a fin that will get glued to the body tube). Smear the glue evenly along the root edge.
4. Press fin again the body tube as if you want to glue it to tube now. Hold a few seconds. Remove fin and put it aside.
5. Repeat with all fins.
6. Once you did this with all fins, wait two or three minutes and then take the first fin and put it into position on the tube and leave there. Because the glue is already 'predried' the bond will tighten soon. Should you find that glue dried completely, add a small amount of the glue to glued surfaces.
7. Repeat with all remaining fins.
8. Use the fin alignment guide (drawing: FIN MOUNT GUIDE) and check that fins are mounted and spaced correctly.
9. Let the glue dry completely.
10. Put fillets on both sides of each fin and let dry. Repeat. Use a reasonable amount of Elmers glue for fillets. Too much glue will add unnecessary weight.

DRAWING: FIN MOUNT GUIDE

DRAWING: BLADE TABS MOUNT GUIDE

Body

Much like John DeMar's Whirl-A-While helicopter, ChiChop have the body consisting from a wood dowel instead of a paper tube. Recommended material is 3/16" birch wood dowel. Cut a 16" piece (try to find very straight and even part of dowel). Final length of the body dowel will depend on the type of nosecone you'll use and you'll have to adjust it for yourself. But 16" is safe length to start with and you do not need to fight with a 36" dowel you get from hobby shop.

Create an eyelet from a music wire (see wire eyelet on drawing: BODY DOWEL). Note that this eyelet has two parallel ends. Using a large pin "drill" a hole exactly in the center of the dowel (from the bottom end). Put a drop of CA glue to the hole and immediately insert one of the ends of the eyelet. You should push as much of it as you can into the dowel. Use CA glue to glue the other end end of eyelet to the outer surface of dowel. Now take a thread and secure the eyelet on the dowel. Once the thread is wound, affix it with a CA glue. This device should withstand the forces delivered at the ejection without breaking the dowel or separation of eyelet.

Take a 13 mm balsa nose cone. Carefully drill the hole through the nosecone. I recommend first to run the wire through the nosecone, then larger wire, then drill small hole and then use bigger and bigger drillers until you can tightly run the dowel through the hole. (You can use a plastic nosecone too, but dowel centering may be more tricky.)

Pull the dowel through hole and adjust as indicated on drawing: BODY DOWEL. Make sure that everything is perfectly straight and then use CA glue to glue dowel to nose cone (put a few drops of CA glue around the tip of nose cone). Kick or let dry.

The last thing is a hub stop, which will hold rotating hub at desired position (check drawing: BODY DOWEL). There is number of things you can use to make this stop, probably the easiest is spare piece of 3/16" launch lug which you affix with CA glue.

To decrease the friction between rotating hub and body dowel you should lubricate the area under hub. Take a No. 2 pencil and "color" that area as dark as you can. Graphite powder will work too.
 

Hub

DRAWING: HUB

Hooks holding the rubber loops (added later, used for blade deployment) are made from pins (you can use thin music wire too). Create 3 identical (or at least similar hooks). Then mark the hub tube (draw lines 120 degrees apart along the whole tube). The hooks are first glued to their positions (see the drawing) with CA glue and then the thread is wound around them and the whole thing is fastened with CA glue again. See the drawing: HUB.

CAUTION: When winding the thread do not use much of force. Be sure that you are not deforming the hub. Always check that hub spins freely on body dowel before you CA glue the thread.

Now you need to create the hinges. You start from standard airplane hinges, but because they are too big for the hub of this size, you need to do some cutting (see drawing: HINGES for detailed instructions on how to modify the hinges). Once you

DRAWING: HINGES

have hinges cut to the correct size, you mount the upper part to the hub body (see drawing: HUB and drawing: DIHEDRAL) with CA glue. When the glue is dry, use a thread and fasten the upper part of hinges the same way you did with the hooks for rubber bands. Again, when thread is wound, affix it with CA glue. Check that you can install lower parts of hinges (using the hinge pins). The construction will continue with rotor assembly.

Rotor

Now you can assembly the hinges and thus attach the blades. Install completed rotor on body dowel and adjust the dihedral on blades (sand the bladestops for this adjustment). See drawing: DIHEDRAL for the estimate of amount of dihedral needed (basically the far side of the leading edge should be at the same height as the top of hub). The amount of dihedral is a parameter you may want to experiment with. Is you make dihedral smaller, the rotor will be less stable but effective length of blades will be bigger (making for slower descend). ChiChop's rotor stability comes also from its increased length due to booster hanging on long kevlar string on the descend. I had no chance (yet) to experiment with the really necessary amount of a dihedral but I suspect that smaller amount of it can be still sufficient.

DRAWING: DIHEDRAL

Install the rubber bands (attach them to the hooks on blades). You need to find an optimal length for these rubber bands (they should be strong enough to deploy and hold the blades open, but not so strong that would damage or destroy them. I use 1 mm x 1 mm rubber bands of 6 cm of total length (per one rubber band).

Nose Cone

You can use 13 mm or 10.5 mm nose cone (balsa nosecone is better, both for its low weight and easy way to mount (just drill the hole in its center and glue it to the upper end of body dowel (with the hub already in its place (check drawing: DIHEDRAL)). I'm not sure if the 10.5 mm nose cone will be superior (performance wise) to 13 mm one, because 13 mm nose cone can cause more uniform flow of the air around the rocket during the boost and coast phase. I leave this question as the exercise to the reader.

Launch Lug

I usually use a CA glue and put 1.5" long launch lug (1/8" size) on one of the blade tabs (make sure that it is perfectly straight). This is far from the ideal position but still works reasonably well.

Flying Chicago Chopper

Preflight Test

If rotor does not spin well, check the similarity of the blades. They should be as much the same as possible. If one is heavier that others, sand it as much as needed. Also check the dihedral for every blade and the angle of attack.

Heading for a Contest?

Get Ready for Flight

1. Take about 2 feet of KEVLAR string (100lb strength is recommended) and tie it using double loop to the eyelet at the bottom of body dowel.
2. Pull both free ends through the empty booster.
3. Make a sliding knot loops on both ends of the KEVLAR string. (See drawing: BOOSTER PREFLIGHT PREPARATIONS)
4. Take a motor (rocket was designed for Estes 13 mm A3-4T) and install it from the bottom of the booster. Leave about 1/4" of motor to stick out.
5. Pull both sliding loops over the part of motor which was left out and tighten the loops (they'll prevent the motor from being spitted out at ejection).
6. Pull KEVLAR string that still remains on the bottom end of the booster into the booster (just pull from the top side of booster).
7. From the top side of booster install some wadding.
8. Pack the rest of KEVLAR string above the wadding. (This may take some patience).
9. Now we are getting to the part where you'll need about 4 hands. First make sure that deployment rubber bands are not hooked at the hub hooks.
10. Fold and move the blades to the boost position (along the body dowel).
11. Hold the rotor in one hand and the booster in the other one. Put the lower nose cone into the booster unit while simultaneously neatly packing the rest of KEVLAR string into the same booster and directing the folded blades (yes, they like to unfold) to the blade tabs.
12. Take a rest.
13. Check the position of blades in the blade tabs area (improve if necessary).
14. Take a piece of masking tape and wind it around the outer part of motor and the bottom of the booster, covering both KEVLAR loops. Press the tape with your nail.
15. Hook up the deployment rubber bands. Now you are ready to fly (well, not you, ChiChop will fly, while you'll worry about the deployment and then chase it downwind).

DRAWING: BOOSTER PREFLIGHT PREPARATIONS

Conclusions

Used Resources:

1. Timothy S. Van Milligan: Model Rocket Design and construction: How to Create and Build Unique and Exciting Model Rockets that Work
2. Gary Miller: personal communication on the topic: Helicopter blades
3. John DeMar: Whirl-A-While(helicopter rocket plan, PDF file)
4. Dan Wolf: Competition Rocketry: Improve Your  Reliability In  Streamer Duration, Sport Rocketry March/April '98

Legal stuff

Disclaimer

Chicago Chopper is a high performance rocket which may easily land on the property belonging to someone else, scaring the herd of stock there and causing a stampede which may result in significant damage on the property and/or lives. If something like this happens, it's not my fault. In other words, you are completely responsible for any consequences of using the information in this article.Reasonable care was taken to ensure that this design is safe to fly but no warranty is given.

Copyright

This design is free to use or reprint unless you want to make any money from it. If you reprint it, do not forget to give me a credit I deserve (omitting my name is a bad idea). If you want to print this article in the newsletter/magazine that is being sold or manufacture this design as a kit, please contact me by E-mail (pavelp@dnastar.com).

Copyright Pavel Pinkas, Ph. D., (c) 2000