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Author Topic: First indoor FF build progress, started as an A6 but now not so much.  (Read 313 times)
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haversin
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« on: October 24, 2020, 01:35:25 PM »

Broke A6 rules for the prop. It's 8 inches in diameter and uses 1/16” foam blades.
Wanted to have an adjustable blade angle prop so made a tissue tube hub for the prop spars to fit into.
For the MS I used 1/16 x 1/4  7#. If it starts to buckle under motor load I might try reinforcing it on one side.
The pigtail on the thrust bearing is non-functional in terms of being able to wind the prop shaft on to it.

Weight breakdown so far:

MS, thrust bearing, washer,
prop shaft, rear hook:           0.35 g
Tissue tube hub:                   0.04 g
prop :                                  0.44 g

It's at 0.83 g already! Oh well live and learn.
I want to test this with rubber to see if it works so I guess the next step is making a rubber winding jig.


Russ
First indoor FF build progress, started as an A6 but now not so much.
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lincoln
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« Reply #1 on: October 25, 2020, 04:50:39 AM »

Unless you're trying to fly it in your living room, I suggest you relax about the weight. Depending on what the span is, you may find that the 8 inch prop makes it hard to trim. You might consider bending up the end of the rear hook to keep the rubber from falling off.

It looks like you're doing a good job.

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haversin
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« Reply #2 on: October 25, 2020, 01:43:30 PM »

Thanks lincoln.
When I started this build I thought there would be access to a large indoor flying area, now it looks like that's not the case. The design goal then changed to optimize flight for my formal dinning room. My wife and I never do any fancy dinning so this room is almost empty. It's 14.5' x 25' x 9' The 14.5' is going to be problem so lot's of steering!

I don't have any FAI rubber yet so I cut an ordinary rubber band to get a single strand of 5” x 1/8” x 1/32” rubber that weighted 0.5g. After 100 turns there was about 1/8” of lateral bowing in the MS. Going to glue a 1/16” sq stick to the concave side of the MS to reinforce. Also at 100 turns the front thrust bearing was bending in and down causing a bend in the prop shaft. I'm going to glue a small balsa triangle in back of the front thrust bearing support wire to reinforce that.
Onwards and hopefully upwards.
Russ
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haversin
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« Reply #3 on: October 30, 2020, 11:55:39 AM »

The reinforcements to MS and front thrust bearing support wire increased weight by 0.14 g. I probably over did here.  Tried to sand down prop blades to 1/32” but before getting that thin, small pin holes started to appear in the foam. I don't know how much these pin holes would reduce thrust but I stopped sanding. Weight of the prop was reduced by 0.07 g. Going to try to make a 1/32” square outline and cover prop. Finished wing and covering. It has a 12” span (flat) and 2 7/8” cord with 5.4% camber. The covering is from the lightest produce bag found so far: 6.0 g per square meter. Thinking a simple dihedral for this wing. Working on a wing mount that allows angle of incidence and the forward and back position of the wing on the MS to be adjusted. The latest rubber motor is 1/16” x 1/32” x 8” and weights 0.47 g including string and rubber cement for splice and end loops. The dark coloring on the rubber is graphite lubricant. It sort of works, will try a thicker coat.

Current weight break down:

covered wing:                        0.43 g

MS, thrust bearing,washer,
prop shaft, rear hook and
tissue tube hub:                     0.52 g

prop:                                    0.37 g


Russ
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haversin
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« Reply #4 on: November 12, 2020, 03:18:04 PM »

  Well it's finally finished. Not very pretty but it's all mine. The final weight is 1.54 grams with out the rubber motor. I was hoping to beat the A6 limit of 1.2 grams. A lot of weight can be taken off the wing and motor stick.

This build was fun but I made a lot of mistakes: right thrust instead of left thrust, broke the center wing rib when it stuck to my hand after pre-glue, but the mistake that really pisses me off is gluing the wing posts to far apart on the motor stick. I measured this five or six times! I wanted to be able to lower the wing until it touched the motor stick. My thinking is to have the vertical cg position be just above the thrust line to reduce the nose up torque during higher thrust. I wanted to reduce this nose up torque because of the 9 ft ceiling. Is this thinking wrong?

  It took a long time to decide how big to make the fin and stabilizer. I ended up using 1/3 the projected wing area for the stabilizer and 1/8 for the fin. These values are rough rules of thumb from the old 1941 book “Model Airplane Design and Theory of Flight ” by Charles H. Grant. The length of the boom was a total WAG.
The cg was placed at 33 percent of the wing mean cord based on a recommendation from the book “Model aircraft Aerodynamics” by Martin Simons. The equations in Appendix 1 of this book were used to calculate a static margin (sm) of 5.4 percent. In the example calculation of sm given in the book the author called  7 percent  “Hot” which I guess means my airplane is real hot!

I'm currently making test glides with out the prop. A small blob of silly putty with the same weight as the prop was placed on the hub. So far I'm encouraged, however it's turning right not left.

The last attached photo is cover art from “Model Airplane Design and Theory of Flight ” many of you have probably seen this before, but I really like it.

Russ
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Re: First indoor FF build progress, started as an A6 but now not so much.
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haversin
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« Reply #5 on: December 08, 2020, 08:36:34 PM »

Fight testing has been hard on DRF1 (dining room flyer number one). Broken wing leading edges, posts, ribs and prop spars. About half of these did not occur during the actual flights themselves but instead were caused my clumsy handling of the plane! This build pointed out something I did not expect that is: HOW OLD I'M GETTING. My hand eye coordination and fine motor control is not what it used to be. However I do have much more patience now and that helps.

The need for a winder was quickly realized and I made one from an ocean fishing reel. The modifications to the reel can be undone in case I ever go ocean fishing again. The winding ratio is 4.1 to 1. I made a torque meter using 0.02 diameter music wire. The length of the wire from the solder joint at the end of the brass tube to the solder joint connecting the dial indicator is 9.875 inches. Hard plastic from a swizzle stick was used for the front radial load bearing. The dial diameter is 5 inches and the face was designed in Open Office software. The dial is divided into 100 equal angular parts. I made a calibration jig to apply known torques to the torque meter and then use the dial readings to calculate the calibration factor: torque divided by dial reading. I made three different measurements at each of three different torques. The average calibration factor was found to be 1.766 oz in of torque for one complete revolution of the dial indicator ( a dial reading to 1.0). This compares remarkably well with the theoretical calculation of 1.775 oz in of torque per revolution using 11.1 x 10^6 psi shear modulus (carbon steel).

I just started making test flights at 240 turns. The torque meter read 0.3 meaning 0.3 x 1.77 = 0.53 oz in of torque. When making the motor stick I only considered the vertical and lateral bending and did not think about the torsional twist under motor load. This twist is transmitted to the wing via the wing posts and causes wash in on the left wing tip and wash out on the right. While this is correct for the left turn, its flexing the wing too much. So I'm reinforcing the motor stick again.

Russ
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Re: First indoor FF build progress, started as an A6 but now not so much.
Re: First indoor FF build progress, started as an A6 but now not so much.
Re: First indoor FF build progress, started as an A6 but now not so much.
Re: First indoor FF build progress, started as an A6 but now not so much.
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