Hobby Shop EZB, Part 5
An article by Larry Coslick
Illustrated by Steve Gardner,
published in Indoor News and Views (INAV)
The thin blades need camber to help retain their shape
The thin blades need camber to help retain their shape. To get camber into the prop blades a camber form is made from 3/32" soft balsa. The camber form is made by taking the prop blade template and cutting it 1/8" larger than the template. From the hub to about 2/3rds the length of the form the thickness is 3/32". Taper the last 1/3rd to .045" at the tip. Sand an airfoil into the form leaving the leading and trailing edges .020" thick. From about one inch from the hub up to the hub the camber fades to nothing. The edges will get thicker than the .020" from the one inch point to the hub, where they will he 3/32" thick. Hold the form at different angles to the light and check for depressions or flat spots and use sandpaper to adjust as necessary. Soak the form in cool water for 30 minutes and then place the tip of the form 7" from the center of a 26" pitch block.
Wrap with an Ace bandage to hold the form to the block and allow to dry. After the form has dried soak the blades in cool water for about 15 minutes. Float one blade over the other while they are still in the water and line up one edge. Remove from the water and stack the wet blades on the camber form, and again place the tip end of the form 7" from the end of the pitch block. Use the prop template to cut a cap from 1/32" balsa to protect the blades from the Ace bandage. Run water over cap for a few seconds, and place over the blades on the camber form. Wrap the pitch block, form, blades, and cap with the Ace bandage. Let the blades air dry for two days. To separate the blades once they are dry, place a single edge razor blade between the two blades and run the blunt edge of the razor blade carefully around perimeter of the prop blades.
Prop Assembly-Take the prop spar and place it on the pitch gauge. Make a prop stop from scrap balsa and tape it to the top of the gauge at the 7" mark.
Move a swing arm lamp directly behind the gauge next to the base. When the blade is placed close to the spar the light will show the exact position of the spar through the blade. Do not use Ambroid or other cellulose cements. The pitch will change as the glue cures because cellulose glues shrink too much. Use carpenters glue. The 45 deg. protractor at 4" will give a pitch of 25". Have a blade ready and place a small amount of glue at the hub, the center, and the tip of the prop spar. Immediately move the blade to the spar and attach the hub first, then attach the tip. The tip should be next to the stop. Reach behind the blade and press the blade to the center section of the spar. Check to see if the spar is on the reference line drawn on the blade. Adjust now if necessary. After 10 minutes, remove the spar and place two dots of glue between the hub and center of the blade. Two more between the center and the tip. Place the spar back on the gauge and make sure that both edges of the blade touch the protractor at the 4" mark. If one of the edges is higher than the other, the spar can be tweaked, gently twisted to get the blade to touch front and back. Wet the spar by the hub and tweak it past the desired pitch After a few adjustments it should hold the position. Attach the other blade the same way. The prop is now complete.
Final Assembly
Fin- The motor stick and boom should be attached and straight in line with each other. Glue the fin to the left side of the boom, 1/16" in front of the stab. The stab is installed later.
Wing Posts - Before the wing posts are installed cut a step at the top of each post. Cut the step 1/32" deep and the depth of the wing spar. Bevel all four faces at the other end of the wing posts. Place the paper tubes on the posts and make sure that they fit snugly. This is important!
Wing Assembly Jig - The wing assembly jig is used to correctly position the wing posts while they are glued to the wing spars. The post guide holds the wing post square to the spars while the wing supports hold the wing square to the face of the jig. (see drawing)
After the glue has set on the wing posts and paper tubes, install the wing on the motor stick. Place 1/32" positive incidence in the wing. One final adjustment needs to be made to the wing Loosen the glue joint at the rear-wing post where it meets the rear spar with acetone. Put. downward pressure on top of the right rear spar several inches from the center rib. You want 3/32" wash in ( rear spar down) on the right wing panel. This will slightly wash out in the left panel. Place the model in a stooge and support the wing until the glue has set. This model will not fly properly unless this adjustment is made.
Stab - Glue the stab to the boom with thinned carpenters glue. The stab is glued onto the boom with the left tip about 3/8" high. This is called "stab tilt" and is used to make the model turn to the left. The stab should be flat , or with a slight amount of wash in on the left panel. Warps can easily be removed during assembly by placing downward pressure on top of the L/E spar by the center rib while supporting the boom with your thumb. This adjustment is done on whichever side of the stab that needs it. Hold or support the stab until the glue sets.
Set up & Trim
Final check- Before the model makes its first flight you need to make sure all the components fit together properly. Make certain that the wing posts fit snugly in the paper tubes. The side walls of the paper tubes must be stiff. If they are not the models flight pattern will be erratic. To fix loose or weak tubes use a bit of ambroid on the outside of the tubes. If this doesn't tighten the tubes enough then use a very small amount of glue to coat the inside of the tubes. Check the thrust bearing for 2 degree of left thrust as per plans. The wing must be washed in on the outboard panel, with 1/32" positive wing incidence. Make sure that the wing is less than 18" in span and the chord is slightly less than 3" wide. Re-check the prop for 25" of pitch The stab should be 3/8" higher on the
left side. Finally, the motor stick and tail boom should be straight in line with each other.
I am going to assume that you have no experience in trimming an indoor free flight model. Duration models fly to the left in a nose high flight attitude. We help the model turn left by tilting the stab so that it is higher on the left side. The prop thrust bearing is offset about 2 degrees to the left. Offsetting the rudder is not very effective and so it is not used on this model. Stab tilt and thrust offset are more effective. Next, the model must fly nose high, just under the stall, for maximum duration. This slows the model and also slows the rotation of the prop. Negative incidence in the stab is what causes the model to fly nose high. A really good tail boom will naturally flex to give the needed negative incidence. Here is an easy way to test the stiffness of your models tail boom. Hold the assembled model by the front of the motor stick. The prop does not have to be on the model. Lift the model vertically about 3" and then push it back to its original position. Repeat this procedure several times. This will load the stab and boom. A fairly stiff boom will flex up and down about 2 inches and a floppy boom will flex 5 to 6 inches. Now rotate the model gently on its roll axes from side to side. The wing and stab will follow each other on a stiffer boom. On a floppy boom the stab will twist one way while the wing twists another. In my opinion the tail boom is one of the most important components of an EZB. Its importance doesn't usually show up until the motor is really torqued up. My design has the wing mounted very close to the front of the motor stick. This makes for a longer tail moment arm and moves the center of gravity behind the trailing edge of the wing. This makes the stab carry a larger portion of the load. This is evident by the upward flex induced in the stab during flight. When the stab is loaded, the boom also bends upwards. The more power that is loaded into a motor the greater the boom will bend. If the model has a floppy boom it will stall or flounder around until the torque drops off. When the motor stick and boom match, the model will perform smoothly throughout the entire usable torque range.
First flight - Set the model up with 1/32" positive wing incidence. Tie up a loop of robber .033" X 10". Wind in 300 turns and place the motor on your model. Go to the center of the floor. Hold the model about eye level, with the nose of the model slightly elevated. Release the prop and gently push the model forward. The model should circle left in a 20' to 25' circle. If it stalls, move the front wing post down slightly. If it dives, relaunch and make sure you launch with the nose raised, if it still dives make sure that you still have 1/32" incidence in the wing and check to see if the model has too much down thrust in the bearing. Increase the wing incidence another 1/32", but no more than 1/16" over all. If the model needs more than this you should tweak the tail boom to help get the nose up. This should correct any diving.
With 300 turns in the motor a .6 gram model should maintain level flight. A slightly heavier model (.75 g) will probably not maintain its height, but it should come close. When the model flies without stalling, check the circle. If the circle is greater than 25', twist the tail boom so that you have more stab tilt. Do the opposite if you need a wider circle. Hopefully your model will be flying with a nose high attitude.. If not, an adjustment has to be made to the tail boom. If you had more experience I would suggest sanding the boom slightly so that it would flair. Lets do it an easier way for now. Starting about 3" behind the rear hook, bend the boom upward about 1 degree. 1 degree puts about 1" negative incidence in the stab. Wet the area where the bend is to be with saliva and be careful. Don't apply too much pressure as the boom may break. Rewind the motor and check for the 25' circle and a nose high attitude. If the model is doing both, start adding turns in the motor in multiples of 100. Do this until the model starts bumping the ceiling.
You could continue adding turns, but there is a possibility of damaging your model. Depending on your flying site, you now have two choices. Experiment with different rubber sizes and launch torque, to get the most out of your model, or start flying on quarter motors.
If done properly, quarter motor flying under a low (25') ceiling can accurately predict the time your model with do in a high ceiling A 22' to 26' site is a perfect place to get ready for contests with ceiling heights of around 120'. If you decide to use quarter motors measure the distance from the rear hook to the back of the prop hook. Make a dummy motor 3/4 the length of your measurement from .015" wire. Wrap thread 1.5" on each side of center and apply a light coat of CA. This give a place to add ballast and to hold on to when the motor is torqued up. The prototype performed well on a 3" loop of .033" tan II. To get the motor off the hook on the winder without loosing turns an "O" ring is used. This is a very small plastic ring through which the motor is threaded before it is tied. These things are made from thin slices (.025" to .030") of the plastic stick found on the cheapest Q-tip copies. Use one 0 ring on the front end of the 3" loop.
You need a reliable way of balancing the quarter motor and dummy motor. The dummy motor must weigh three times what the rubber weighs. This is important. You can use a portable scale or build a quarter motor balance beam See plans for my balance beam. Each time there is a change to the weight of the motor, you need to add or remove weight from the center of the dummy motor. Non-drying clay sold at toy and art supply stores is good for adding weight to the dummy motor.
When flying on quarter motors the model and the prop need to be released at the same time. The torque drops off quickly on a quarter motor once the prop stars to turn. You can't tell if the model will handle the torque that is loaded on the model if turns are allowed to spin off before the launch. If your model stalls on a quarter motor it will certainly stall on a full motor.
I'll give you an idea of what the prototype looked like when loaded with .13 inches oz. of torque.. Hold the wound model in front of you, and sight down the motor stick to get the proper view. The wing was fiat with no warps in either wing panel. The motor stick and boom were bent downward in a slight arc. The stab had lost some of its tilt but was still high on the left side. This torque was more than enough to get to the 116 foot ceiling at Johnson City.
One last bit of information on motor sticks. If your model stalls at a high launch torque and you think the boom is OK the problem could be with the motor stick. It might be too strong. The model will fly great on low to moderate torque, but stalls when released at the desired launch torque. Try this. If the model stalls at .12 inches oz, wind to .15 inch oz. and relaunch. If it climbs 4 to 5 feet higher then stalls, the motor stick is probably too stiff. To make certain wind and launch at .18. If the model climbs to around 20 feet before stalling the motor stick is definitely too strong. Take a sanding block and sand the bottom of the motorstick from the rear post tube to one inch in front of the rear hook. Be careful and only make a few strokes with the paper and make another flight. Its extremely easy to remove too much wood and ruin the motor stick. Relaunch at .12 in oz of torque to check if you have removed enough wood. When the stalling at this torque goes away stop sanding the motor stick.
Good Luck !! Larry Coslick
