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Author Topic: Speed Merchant - RTP Rubber Speed Model  (Read 904 times)
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kittyfritters
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« on: January 13, 2019, 04:56:02 PM »

Here is the preliminary drawing for The Speed Merchant, my Round-The-Pole rubber powered speed model, for the contest at Duarte on February 16th.  Before I cut wood I'm considering moving the wing to the bottom of the fuselage.  the tether wire will go from the right side of the fuselage, embedded in bottom of the wing about half way between the leading edge and the spar, to the left wing tip where it will have a loop.  Will test both two and four blade prop.  If other obligations permit I will have both the built up fuselage and rolled tube versions ready for test flying at Duarte on January 19.
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Speed Merchant - RTP Rubber Speed Model
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kittyfritters
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« Reply #1 on: January 22, 2019, 07:27:42 PM »

Went to the Duarte High indoor flying session Saturday night.  (Even got my wife to go with me.)  Since we had not really done this before the intent was to test the indoor RTP setup for the Rubber RTP Speed contest next month.  Boy! Is it different from straight line rubber speed!  Dave Gee and I got it down to a science but it was a lot of work.  At first we couldn't get the models unstuck in the first lap as required by our rules.  We also discovered that these models need more wing than we anticipated. (Some people speculated that they wouldn't need wings at all.)  The also need small props because the torque effects at the end of the tether are magnified.  All of this had not shown up in our previous RTP flying with scale models and slip-togethers since we weren't trying for high speeds. RTP flight dynamics seem to become more critical at about 12 mph.  We'll go over the results at the OFFC meeting this Wednesday and the Black Sheep Meeting on the 02/06.  Be interesting to see what changes will be made in the models being built for the contest with the new information

KF
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kittyfritters
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« Reply #2 on: January 24, 2019, 04:11:52 PM »

Here's a video of my test hack at OFFC Wednesday morning. Couldn't get it over 13.5mph at Duarte last Saturday, Changed to a smaller prop and cut the wings back down, in stages, to the original span and got a 24.43 mph flight out of it, probably the limit for that airframe, The vid was taken at the end of the session when the rubber was getting tired. It's only doing 16.34 mph in the video, Has too much incidence, jumps off the ground too soon and flies too high, but I think it give me the parameters for my real racer. Oddly enough, I think a scale D-12 Bonzo would do quite well.

https://www.youtube.com/watch?v=1MoqCCBgFmc

More Later!

KF
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kittyfritters
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« Reply #3 on: February 08, 2019, 01:11:41 AM »

At yesterday's O.F.F.C. meeting we did more testing for the RTP speed event.  I brought another test hack, an ROG type stick model with the wing and tail surfaces of my Speed Merchant design.   We had the three foot pole called for in the rules. 

My test hack had the wing held on by a rather strong rubber band so I could easily adjust the incidence.  As I had thought the airfoil I am using needs a degree and a half of incidence to get the model off the ground within the first lap.  Although with the three foot pole everything seems to get off the ground in the first lap.

I was using a five inch, Guillow's red prop and easily getting up in the twenty mile per hour range with four strands of 1/8" flat, tan, sport rubber.  One thing I did find is that winding a thousand turns, or more, into a motor with four to six strands of rubber will rip the free wheel dog right off the front of the prop.  Since we don't need a freewheel for this kind of flying just bend the shaft over the front of the prop.  A may actually use the test hack as one of my entries in the contest with a more aggressive prop, maybe a four bladed prop.  Another thing I found was that if you forget to aim one of these short winged monsters out of the circle at launch the line will go slack and the torque will wind the model into the line.  With these motor stick types the line gets wound into the motor.  We will need to have spare lines at the contest.
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Re: Speed Merchant - RTP Rubber Speed Model
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« Reply #4 on: February 15, 2019, 10:33:45 PM »

Finally, I got to work on my model, the Speed Merchant.

I decided to go with the rolled tube version since Bill Watson gifted me with a rolled tube he made for his model that apparently did not meet his specs.  I found it to be a very nice tube.  He made it the way I do by rolling the wet balsa and a layer of wet silkspan around a mandrel.  He lets his dry overnight.  The silkspan drys first and wicks the moisture out of the balsa.  I'm not that patient so I bake them in the oven.  Since my wife is not retired and I am a house husband I can get away with it.

Now to make the nose cone and tail cone.  As it turned out I had the nose cone already made, I found it in my junk box.  It was the nose cone from my "Unintended" P30 that was destroyed in the opossum invasion last year.  It was for the same diameter rolled tube fuselage.  All I had to do was replace the prop with the aggressive Ikara prop that I have been testing and install a new prop shaft. 
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Re: Speed Merchant - RTP Rubber Speed Model
Re: Speed Merchant - RTP Rubber Speed Model
Re: Speed Merchant - RTP Rubber Speed Model
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« Reply #5 on: February 15, 2019, 10:35:31 PM »

The tail cone I had to make.  I cut a 1/64" plywood base disk using a compass then rough cut out six 1/8" thick disks also with the compass.  Using the compass left a hole in the center of each disk so I could put a pin through it to line them up for gluing with CA.  When the CA had set, about a minute later, I put the stack on a Dremel tool mandrel and chucked it into the tool.  

I put on my mask, turned on the tool, grabbed my  sanding block and 90 seconds later had a tail cone.
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Re: Speed Merchant - RTP Rubber Speed Model
Re: Speed Merchant - RTP Rubber Speed Model
Re: Speed Merchant - RTP Rubber Speed Model
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« Reply #6 on: February 15, 2019, 10:37:43 PM »

Now, all I had to do was key the tail cone, run a prop shaft through it as a rear hook, and I had a fuselage.

KF
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Re: Speed Merchant - RTP Rubber Speed Model
Re: Speed Merchant - RTP Rubber Speed Model
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« Reply #7 on: February 17, 2019, 05:23:01 PM »

After the flying my test hack (henceforth known as "The Hack") I decided that the wing needed two inches more span to handle the torque on takeoff.   I simply enlarged the drawing spanwise.  The original drawing had too many ribs so I decided to make the wing and the tail surfaces out of 3/32" stock and simply sand to a symmetrical airfoil.  I flexed the structure and added gussets where needed.  The covering was red Esaki tissue applied with glue stick.  It was shrunk with rubbing alcohol but not fixed.

The tail section was simply glued to the tube.  To attach the wing I glued two lengths of 1/8" square stock to the bottom of the wing a quarter inch to either side of the center line. I wrapped a piece of fine sandpaper around the tube, set the wing on it and moved it back and forth until the wing sat flush on the tube.  The 1/8" square stock was now contoured as a wing saddle.

The tether wire was wrapped around a dowel the same diameter as the tube and then slipped over the tube.  The landing gear wire was bent like it was made for an ROG model with a 1/8" stick, then the top section was bent back about 85 degrees and bound to the bottom of the tube with thread and CA.  The rules said it had to have a canopy so I glued an old Enterprise bubble canopy on top of the tube.

Now I put rubber in the fuselage, wound the motor and let it unwind, and found the CG.  I slid the tether wire to the CG and tacked it in place with CA, then positioned the wing and glued it in place.

Ready for the contest!

KF
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Re: Speed Merchant - RTP Rubber Speed Model
Re: Speed Merchant - RTP Rubber Speed Model
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« Reply #8 on: February 20, 2019, 02:33:59 PM »

Thanks for these Speed posts KF - very interesting.

Stephen.
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« Reply #9 on: February 21, 2019, 12:37:21 AM »

More fascinating stuff KF. You may be able to trim these to fly a little faster by adjusting the tether hook either fore or aft relative to the CG.
Moving it aft will increase yaw outwards for better line tension while moving forward slightly can reduce drag by aligning the fuselage closer to 90deg to the line and thus tangential to the flight circle.
However you fellows no doubt have already tried this I imagine, as it is old hat CL stuff.
Keep up the interesting development.

John
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