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Author Topic: Korda Class C Tractor  (Read 4531 times)
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OZPAF
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« Reply #75 on: August 09, 2018, 06:57:02 PM »

How heavy is your Korda now Cal?

John
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calgoddard
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« Reply #76 on: August 10, 2018, 04:19:19 PM »

OZPAF -

The total weight of my Korda C as shown in the attached photo is 81.26 grams.  This of course does not include the weight of the 40 gram rubber motor.  I still need to add two wing hold down rubber bands and some dowels for retaining them. This will add another 2-3 grams.

Here is the breakdown of the weights of the main components:

tail feathers               9.98 grams

fuselage                   22.57 grams

wing                        23.11 grams

nose block &
prop                        25.57 grams

TOTAL                     81.26 grams


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vintagemike
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« Reply #77 on: August 10, 2018, 06:16:38 PM »

What a nice model! reminds me of its older, bigger cousin which has recently been published on Outerzone
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« Reply #78 on: August 11, 2018, 01:28:51 AM »

Thanks Cal. Checking back at your quoted wing area of 143 ins2 would indicate that even with it's heavy prop the wing loading is still good at around 0.6gms/sq inch.
 
However a lighter prop would certainly help IMHO by reducing the inertia effect of the heavy nose - making it easier to trim.

John
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calgoddard
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« Reply #79 on: August 11, 2018, 10:28:49 AM »

OZPAF -

Thanks for your helpful comments about wing loading.

In his book entitled Rubber Powered Model Airplanes, Don Ross says that a wing loading of 0.5 grams per square inch for a mid-size model with a wingspan of 24-30-inches is "OK".  My Korda C has a wing span that is significantly bigger than a mid-size model. It appears, however, that Ross includes the weight of the rubber motor in determining wing loading. This makes sense.  Unfortunately, if I include the weight of the 40 gram rubber motor the wing loading of my Korda C is way above 0.5 grams per square inch.

In addition to the component weights listed in Reply #76 my Korda C will carry an RF transmitter and batteries weighing a combined 3 grams.

For reference purposes I note that the Korda C built by Karl Gies weighed 71.7 grams.
« Last Edit: August 11, 2018, 11:24:56 AM by calgoddard » Logged
Red Buzzard
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« Reply #80 on: August 11, 2018, 05:07:16 PM »

Hi Cal,

Let's see, at 40 grams of rubber and 81 grams of airframe, you'll still be tough to beat. Then that 17" prop and long motor base gives you a long prop run and you'll be even tougher. And a few grams will mean your Korda will take a few tumbles from the wind, lousy DTs and regrettable cross wind launches and will still be flying when it's covered with glue patches. You'll go crazy obsessing about weight when it will be more fun obsessing about reading air. As Dan Berry has said, "let's not overthink this..." Go fly your pants off!

RB
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« Reply #81 on: August 14, 2018, 10:15:36 AM »

Red Buzzard - Thanks for the encouragement.

I just received an email notification that my new Superior Props 17-inch prop blank which I purchased from Volare Products should arrive in two days. Then the carving and sanding starts anew.

I have been playing around with a mock-up of a prop I made with a new free-wheeler clutch based on a clever design by Stan Buddenbohm.  I am sorting out the optimum dimensions. As shown in the attached photo, I used a 17-inch x 5/16-inch x 5/16-inch stick of balsa wood as a prop spar whose middle section roughly simulates the width of the hub of the second Korda C prop I will make.  The paddles glued at an angle on the ends of the spar help it free wheel in the oncoming breeze to allow me to observe the operation of the clutch.  I want to have all the details of the new clutch worked out so I don't mess up the installation on a newly carved and sanded prop. Once I do this, I will post a complete write-up of the details.  So far the mock-up performs beautifully, but I have a few tweaks I want to make here and there to ensure that the new clutch on my new prop is fail-safe.  I will mount a second mock-up on my Korda C fuselage and do a low-wind test motor run with the fuselage mounted on my stooge when it is clamped to the tail gate of my SUV that is parked in front of my house.  The on-coming breeze will simulate the air flow during a glide.

Today I plan to work on adding locators to the underside of the stab to ensure that the fin is always in the same exact angular position at launch.
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« Last Edit: August 14, 2018, 11:11:46 AM by calgoddard » Logged
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« Reply #82 on: August 14, 2018, 12:45:05 PM »

Hi Cal,

Let's see, at 40 grams of rubber and 81 grams of airframe, you'll still be tough to beat. Then that 17" prop and long motor base gives you a long prop run and you'll be even tougher. And a few grams will mean your Korda will take a few tumbles from the wind, lousy DTs and regrettable cross wind launches and will still be flying when it's covered with glue patches. You'll go crazy obsessing about weight when it will be more fun obsessing about reading air. As Dan Berry has said, "let's not overthink this..." Go fly your pants off!

RB

This is, by far, probably the best bit of wisdom ever imparted on this forum!

Bravo!
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« Reply #83 on: August 14, 2018, 02:49:27 PM »

I can't do any outdoor flying until September. It is way too hot at our flying field right now.

So I may as well mess around making a new prop!  Or, instead, I could clean out and organize my garage Smiley
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« Reply #84 on: August 18, 2018, 10:29:38 AM »

As an alternative to a Garami clutch, I made a simple clutch for the new lighter 17-inch balsa wood prop that I just made for my Korda C.  The weight of my new front-end assembly for my Korda C (including prop, nose block, clutch, washers and prop shaft) is only 14.71 grams (see first picture). This is substantially less than the 25.57-gram weight of the original front-end assembly that I made for this model.  I am pleased that I have reduced the weight of my Korda C by more than 12% (not counting the weight of the 40-gram rubber motor).  See Reply #76 for a listing of the main component weights.

The clutch incorporated into my new front-end assembly embodies a Stan Buddenbohm design which he created and used successfully for many years with no problems.  Stan always comes up with clever designs for his free flight models. Like all free wheeler clutches, this new clutch allows the prop to free-wheel after the motor run is complete, i.e. nearly all of the turns on the rubber motor have been used and the model is entering its glide phase. Free-wheeling of a fixed, i.e. non-folding, prop is essential to a good glide.  A fixed prop that won’t free-wheel acts as a dethermalizer (DT) and kills the glide, or worse, can cause the model to crash.

As is the case with many relatively simple mechanical devices, it takes a lot of words to accurately describe the construction and operation of the Buddenbohm clutch.  It’s really not that complicated. Hopefully the pictures that are part of this post will help you to better understand the following detailed written description.

The hub of the new Korda C prop has an axial length of ¾-inch.  It is bushed with three segments of tubing (see second picture). An intermediate segment is 1/2-inch long, 1/8-inch OD Aluminum tubing while the segments on each end are 1/8-inch long, 1/8-inch OD Brass tubing. The three segments of tubing are held in place with thin CA that weeps all around the outer surface of the tubing segments and bonds them securely to the balsa wood hub. A ¾-inch long segment of 1/8-inch OD Brass tubing is too heavy.  A ¾-inch long segment of 1/8-inch OD Aluminum might experience excessive wear and produce subsequent prop wobble.
  
An inner segment of 3/32-inch OD Brass tubing slides freely back and forth axially inside the three outer segments of 1/8-inch OD tubing.  The OD of the 3/32-inch tubing is slightly less than the ID of the 1/8-inch OD tubing segments so the inner segment can rotate freely within the outer segments, without any wobble. The 1/8-inch long segments of 1/8-inch OD Brass tubing form journals or bearings. The inner segment of 3/32-inch tubing is approximately 1/16-inch longer than the combined length of the outer segments of 1/8-inch tubing.   The intermediate segment of 1/8-inch OD Aluminum tubing is needed to ensure that the forward end of the 3/32-inch Brass tubing does not hang up on the rear end of the forward 1/8-inch long segment of 1/8-inch OD Brass tubing.  A pair of Brass washers and a Teflon washer sandwiched between the same are positioned on the 1/16-inch prop shaft between the hub of the prop and the thrust bearing in the nose block.  The ID of the 3/32-inch Brass tubing is slightly larger than the .062-inch OD of the 1/16-inch prop shaft. Again, there is no wobble when the prop shaft rotates inside the segment of 3/32-inch Brass tubing.

A small, generally triangular-shaped catch made of .027-inch galvanized steel sheet metal is glued with thin CA in a groove sawed into the hub that is spaced 3/8-inch from the prop shaft. You can see the catch in the third picture.  I may have trimmed it down a bit after taking that picture. The plane of the catch extends perpendicular to the axis of one of the blades. The forward end of the prop shaft is bent at ninety degrees to form a drive dog. The outer end of the drive dog engages the rear straight edge of the catch when the clutch is in its drive mode (see fourth picture). A small flat is ground into the drive dog and the rear straight edge of the catch is shaped with a file to the optimum angle in order to increase their area of contact.  Small strips of 1/64-inch plywood are glued to opposite sides of the hub and straddle the leading and trailing edges of the catch. This strips are wrapped with thread and glued with CA to securely anchor the catch in the groove in the hub. A great deal of force will be exerted on the catch.

After the rubber motor is connected to the prop hook, the nose block is installed into the fuselage while pulling out on the prop to ensure full engagement between the drive dog and the rear edge of the catch.  The torque exerted by the wound rubber motor presses the drive dog tightly against the rear edge of the catch and prevents relative axial movement between the drive dog and the catch.  The rearward thrust produced by the prop during the motor run presses the prop forward on the prop shaft. Along with friction, the thrust helps maintain engagement between the drive dog and the catch during the motor run. Inadvertent decoupling of the drive dog and the catch would result in a very undesirable super high-speed unwind of the rubber motor.
  
Here is how the clutch works during a flight. At the end of the motor run, the torque of the rubber motor is insufficient to drive the prop.  The catch disengages from the drive dog as the prop begins to free-wheel, in part due to its rotational momentum. At the same time the prop slides rearward on the 3/32 Brass tubing segment due to the oncoming airflow.  On the first rotation of the prop after disengaging from the drive dog the sloped upper edge of the catch may engage the drive dog and facilitate rearward axial movement of the prop.  Thereafter the catch no longer engages the drive dog and the prop spins freely during the glide portion of the flight (see fifth picture).  The forward end of the inner segment of 3/32-inch Brass tubing engages the curved inner portion of the drive dog and prevents it from pressing down on the hub and inhibiting free-wheeling of the prop.
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« Last Edit: August 18, 2018, 11:05:32 AM by calgoddard » Logged
calgoddard
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« Reply #85 on: August 19, 2018, 05:44:23 PM »

As shown in the attached picture my Korda C now weighs 71.51 grams.  This includes the weight of the new front-end and a wing saddle.

I did another test motor run on my car-mounted stooge.  I wound a 16 x 1/8-inch - 40-gram rubber motor (mounted in the fuselage) to 400 turns, connected the new prop and let the motor run.  Everything appeared to be working just fine. I then wound to 700 turns. The motor run was nice and smooth. The Buddenbohm clutch worked both times.  I was pretty sure that it would.

The motor run with 700 turns was about 95 seconds.  

I think I can get 900 - 1,000 turns into this rubber motor.  I will have to creep up to that amount of turns during the trimming process.  I hope I can launch at 30 inch-ounces of torque or more.

It's beginning to cool off a bit in SoCal. I can't do any 85% of breaking turns trim flights at our club's flying field until mid-September. I won't try to wind and fly with that many turns until I first fly my Korda C with 50 turns, 100 turns, 200 turns, 250 turns, etc. while making incidence, rudder and/or thrust line adjustments along the way, as needed, one at a time of course. I will initially set the CG in the middle of the optimum range recommended on the plan from Bob Holman Plans. I don't anticipate having to make any CG adjustments during the trimming process, but I won't glue the wing saddle in place, and eliminate the rubber bands that wrap around the fuselage and temporarily secure the wing saddle to the fuselage, until I am satisfied with the location of the CG.

I may do a few test glides and 100 turn trim flights in my local 6-acre park.  
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« Last Edit: August 19, 2018, 06:08:26 PM by calgoddard » Logged
LASTWOODSMAN
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« Reply #86 on: August 19, 2018, 06:30:32 PM »

Good info Cal  -  your Korda C really looks like it means business   Cool  - looking forward to your short test flights.

LASTWOODSMAN
Richard
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« Reply #87 on: August 19, 2018, 09:16:37 PM »

Cal,
Your Korda is up to your usual meticulous standard and I can't say anything more flattering than that.  I can't help being a bit mischievous though and asking what you are going to do with the eleven grams you have saved on the propeller assembly; move the wing back a bit to get the CG correct? <smile>.
John
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« Reply #88 on: August 20, 2018, 09:50:18 AM »

Hepcat -

Thanks for your complement.  Did you notice that the wing hold-down rubber bands on my Korda C do not criss-cross each other?

I decided to make a dummy nose block for test gliding my Korda C in my local park.  Personally, I feel that gliding the model with the prop free wheeling gives a better indication of performance in a real flight.  But the new 17-inch balsa wood prop is very light and I wouldn't want to break it before I have the model trimmed for a decent glide.  The dummy nose block will weigh around 14.71 grams - the same as the light weight front-end.
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« Reply #89 on: August 20, 2018, 12:27:41 PM »

" I feel that gliding the model with the prop free wheeling gives a better indication of performance in a real flight. "

Have to agree with that thought.   Regarding the chance of breaking the prop .... that exists whether landing from a hand glide or a flight;  but unlikely in any circumstance if you've rounded off the top of the noseblock plug.
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« Reply #90 on: August 20, 2018, 04:13:50 PM »

applehoney -

I already rounded off the top of the nose block plug on both the front-end assemblies I built for my Korda C, per your sound advice given to me on HPA several years ago.  It has been my standard practice since then.

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« Reply #91 on: August 20, 2018, 05:12:59 PM »

Hi Cal and Applehoney.   I have a newbie question.   Regarding  "the chance of breaking the prop .... that exists whether landing from a hand glide or a flight;  but unlikely in any circumstance if you've rounded off the top of the noseblock plug."    What part is rounded off, and is that to make the prop block "pop  out" on landing ?     I have my Speedster nose block Scotch - taped on so snug, that the nose block does NOT pop out on landing.   In fact, I ran out the rubber on the last short trimming test flight this am, and the plane ran out the rubber sitting up high, into the wind, and came down hard at a steep angle, and bent the prop shaft.  The prop block did not come off.   Should I be making it so that it pops off upon landing ?   The grass was not too short, and I have a slightly tight motor in the fuse.

LASTWOODSMAN
Richard
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« Reply #92 on: August 20, 2018, 07:10:47 PM »

Richard -

The nose block needs to fit snug in the fuselage.  It should not move around.  You need to maintain accurate thrust lines from flight to flight.

You also don't want the nose block to fall out during a flight. If the rubber motor stays connected to the prop hook the dangling prop will act like a crude DT and bring down your model. If the rubber motor does not stay connected to the prop hook you will most likely lose your prop and nose block and not be able to find them.

If you have a balsa wood prop, the nose block should pop out on landing to reduce the likelihood of breaking the prop or otherwise damaging the prop shaft and/or fuselage.  Injection molded plastic props are pretty durable and so having the nose block pop out on landing is not as important when the model has a plastic prop.  However, if the model has a plastic prop, allowing the nose block to pop out on landing can save your model from getting a bent prop shaft.

The top edge of the nose plug should be rounded, with a sanding block, as shown in the attached photo. I mark the top edge with red ink to remind me to insert this edge up into the fuselage.  The rounded edge allows the nose block to roll forward, out of the model, when the prop blade strikes the ground, more or less in the six o'clock position.  The fit between the nose plug and the forward end of the fuselage should be firm, but not so firm that the nose block cannot roll out.

On non-scale models, the nose block is held on with rubber bands, and these yield to allow the nose block to roll out.

On scale models, tiny magnets help keep the nose block in place, along with a friction fit.

I hope I answered your questions.  Applehoney has a lot more expertise than me and hopefully he will correct and/or supplement my reply where he deems appropriate.

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« Reply #93 on: August 20, 2018, 07:34:16 PM »

No correction needed.   However I do not use rubber bands to retain a noseblock - a spring tensioned shaft does that for me on any model, retaining a few residual turns.

However  ...  " having the nose block pop out on landing is not as important when the model has a plastic prop."       Agreed that a plastic may be more  durable but a 'pop-out' noseblock protects the shaft.

Cal -do you have any finish on the prop  ?  
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« Reply #94 on: August 20, 2018, 07:45:15 PM »

Applehoney -

You are right.  A spring tensioner will keep the nose block in and avoid the hassle of using rubber bands to hold it in place.  A spring tensioner will also help avoid rubber motor bunching and CG shifting.  For some reason, I have not perfected the art of building spring tensioners.  It comes down to the spring, and it can be a tricky business getting the right spring force.  I hate making those "safety pin" type springs, and my coil springs seem to produce either too much tension, or not enough tension.

Switching gears, the new 17-inch balsa wood prop that I made for my Korda C was finished by coating the LEs and TEs with medium viscosity CA.  I probably coated a band about 3/8-inches wide with CA.  After the CA dried, I sprayed on two coats of clear Krylon spray, over the entire surface of the prop.  I allowed the first coat to dry before applying the second coat.  I hate the smell of clear Krylon spray. It consists of some sort of polymer with lots of volatile chemicals. I had to spray the prop outside, wearing goggles and a respirator.  The prop is waterproof and will resist absorbing any rubber motor lube that might be on my fingers.  It is not as durable as my "red tank" fiber glass and epoxy covered prop, but the new prop weighs substantially less than the old one.
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« Reply #95 on: August 20, 2018, 10:42:32 PM »

Applehoney -

I have not perfected the art of building spring tensioners.  It comes down to the spring, and it can be a tricky business getting the right spring force.  I hate making those "safety pin" type springs, and my coil springs seem to produce either too much tension, or not enough tension.

Volare Products has a new version of the K Fags spring winder.  I have an original one, and both tension and compression springs are fairly easy to make.  Also, ACE, True Value, and other hardware stores in the USA have an assortment of springs that I have managed to make work.  The "safety" springs are even more fiddly than the coil springs.
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« Reply #96 on: August 21, 2018, 09:52:51 AM »

A locksmith is a good source for small coil springs
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« Reply #97 on: September 10, 2018, 06:45:13 AM »

Update on you C stick?

Tim
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« Reply #98 on: September 15, 2018, 07:16:02 PM »

I drove 90 miles today to our club's flying field in Perris, Califoria.

My goal was to trim my brand new Bob White coupe for our club's F1G contest tomorrow. It is a locked down coupe. I have not previously flown this model.  After two hours I gave up.  There is something wrong with the fit of the prop assembly in the front end of the fuselage. I can't adjust the thrust line as I need to.

So I switched to my Candy G coupe.  I have flown it for about five years.  It usually flies well but not at the club's last F1G contest. I made one or two thrust line adjustments, and got a nice climb. I got two maxes in a row and put it away.  In Perris, we have to stop flying as noon approaches because the wind builds.

I got out my Korda C and installed a 16 x 1/8-inch 40 gram motor.  

I started at 100 turns, then 150, then 200.  The plane seemed to be flying OK.  When I got to 350 turns (motor will take 900-1,000 turns) the plane spiraled down to the right from about 50 feet in altitude, hit the hard dirt and broke one of the blades off the light weight prop.  I glued it together with CA, added a 1/16 shim to remove some right thrust.  I wound to 300 turns and got a nice flight. I put the model away.  It was getting too hot and there were dust devils.      
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« Reply #99 on: September 18, 2018, 07:11:31 PM »

Looking back, I should have followed the advice of Red Buzzard in Reply #14. He recommended that I add some left rudder.  The huge fin of the Korda C has an airfoil cross section (convex on its left side). I think this airfoil cross section may have induced the right spiral.  Perhaps there was also too much right thrust before I shimmed the nose block with a piece of 1/16-inch balsa wood.  I will move the stab stop so that there is about 1/32-inch left skew of the entire tail feathers as recommended by Red Buzzard.  Live and learn as they say. It's certainly true in free flight! This mistake resulted in a broken light weight prop.
« Last Edit: September 18, 2018, 07:22:48 PM by calgoddard » Logged
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