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Author Topic: Korda Class C Tractor  (Read 2922 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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OZPAF
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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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calgoddard
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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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Re: Korda Class C Tractor
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Starduster
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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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calgoddard
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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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calgoddard
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« Reply #84 on: Today at 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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Re: Korda Class C Tractor
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