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Author Topic: How do you control the high power at launch?  (Read 1183 times)
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ceandra
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« Reply #25 on: January 03, 2018, 10:37:27 AM »

Bill:

Thanks for your numbers and experience. Your results are very similar to ours on our version of Finney-18, which has a flat wing, carbon spars, tip plates. We found with the small stab that the CG had to be substantially further forward than prior years. If I recall we are at about 5.5cm forward of the TE of the wing, whereas last year we were within perhaps a cm (don't have the numbers in front of me). We set the decalage at about 1/4" wing incidence, 0 degrees tail, and adjusted the CG until it was just off of stall. Later moved wing forward a bit to get greater tail moment, putting ballast on the nose. With built-up tip plates, we came in at under 4g without rubber or ballast! The plane does 1:20 on half rubber with 15' rise, so we are hopeful for 2:30-2:40 with 30' ceiling at Regionals this month. The carbon completely eliminated issues we had with warping in weather changes.

Chuck
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Olbill
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« Reply #26 on: January 03, 2018, 11:13:27 AM »

Excellent Chuck! I hope it continues treating you well.
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Little-Acorn
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« Reply #27 on: January 04, 2018, 12:37:50 AM »

Bill, that's quite a propeller in the picture. Looks like ALL the blade area is ahead of the (comparatively rigid?) axis the blade can twist around under heavy loads. I've heard of such props, designed to coarsen their pitch under heavy loads (such as when first launched as I described in the OP) so that they turn slower and/or become less efficient, so less power is dedicated to pulling the plane forward when the rubber is wound to its highest torque.

Can you give us any details of that prop? Did you make it yourself? Or get it as a kit or a finished product from somewhere?
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Olbill
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« Reply #28 on: January 04, 2018, 02:15:35 AM »

I think that prop started out life as an Ikara SO prop - the big flaring one that Ray Harlan sells. One of the first mods was to remove all the blade area behind the spar. Then it was cut down to whatever specs were in force at that time. I always glue a new .018" wire shaft into the hub and use a Harlan PP or SO bearing. You might find a description of that prop in one of my posts from previous years.

When I finished the model I wanted to test fly it in the house and that was the only prop I could find. I just cut off the blades to make it 5.5" and let it rip. It might do okay or I might need something a little more conventional. I found several other old SO props so will try some of those as well.
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SyLa-20871
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« Reply #29 on: January 11, 2018, 02:37:37 PM »

All:

I am starting out with my son flying Wright Stuff this year.  All the questions asked by Little-acorn reflected mine as well.  Thank you all for sharing your expertise and knowledge.  I feel this is turning into a fun hobby!

Best,
Lee from Maryland
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Olbill
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« Reply #30 on: January 13, 2018, 10:20:31 AM »

2:06 first flight no touch,  max height 22', 5/15 SS rubber 17.5" long,  launch torque .2 in-oz.

2:20 second flight no touch.

2:37 4th flight, 15.25" motor with some ceiling banging. 60 turns remaining. Will try once more with motor about 16".
« Last Edit: January 13, 2018, 12:05:44 PM by Olbill » Logged
Olbill
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« Reply #31 on: January 13, 2018, 12:59:35 PM »

2:53 5th flight on 15.5" motor.

Wind data:
2100 turns in to .82 in-oz. 190 turns back off to .28 in-oz launch torque. 120 turns remaining, 621 rpm.
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Olbill
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« Reply #32 on: January 13, 2018, 02:14:08 PM »

3:00 on last flight on 16.5" motor.
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Little-Acorn
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« Reply #33 on: January 13, 2018, 04:47:56 PM »

OlBill, are these flights on the Finny 18 you posted a picture of (in Reply #17) and a plan for (in Reply #20)?

You have mentioned a number of mods, are they shown in that plan? If not, any chance you could post an updated version, either photos of the plane of an updated plan?

Good stuff!

A three-minute flight, holy cow....
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Olbill
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« Reply #34 on: January 13, 2018, 06:14:59 PM »

The plan shows the current model. All of the mods were to move the CG forward. The tail tip plates were cut down, The tailboom was thinned and the motorstick was tapered on the top and both sides (that's not shown on the plan). All of the weight saved was put into the ballast as far forward as possible.

This is a very simple model to build. I think even with the changes that I have less than 4 hours in it.

The weird looking Ikara prop worked great and the large blade area helped to keep the RPM's to a reasonable level. The 3:00 flight had one touch on the ceiling.
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Little-Acorn
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« Reply #35 on: January 14, 2018, 05:47:57 PM »

Olbill, this is great information, thank you! I'm somewhat new to this sport, and there's a lot I need to learn (and probably always will be).

I'm curious about a couple of things:

1.) Why did you bend the rear tail boom down and put the stab 1-1/2" below the level of the motor stick; while putting the wing only 1-1/4" or so above the motor stick? Seems simpler to have the tail boom sticking straight back from the motor stick on the same level, and have the wing maybe 2-2/1" above the motor stick. Most of my planes are like that, but as I said I've got a lot to learn.

2.) Why taper the tail boom? Most of mine are just a 1/8" square lightweight balsa strip. I'd guess that tail forces are very light in a model like this, and even 1/8" square may be stronger than it needs (one of my tail booms is 3/32"x1/8", seems fine so far). In my planes, I saw this as a good place to reduce the dreaded "weight aft of the CG".

3.) What are "tissue tubes" that you mentioned on the overhead view of the wing center and stabilizer center?

4.) What is "SSM"? Static Stability Moment or some such?

I'm considering building something like this, but with the differences I mentioned above. Probably build up the tip plates on wing and stab, and cover them (and the rest of the surfaces) with Ultrafilm to save weight. And all the weight I can save, will go toward putting a weight on the nose as far forward as possible to bring airframe weight up to 7.0 grams, and scoot the wing as far forward toward the propeller as possible while still keeping the model statically stable.

That said, I read your description of 2:30 and even 3:00 flights with my jaw on the floor. The best any of my planes have done, is 1:52 with a .062" wide FAI Sport rubber motor and a ground-adjustable-pitch prop that I modified from one of Ziegler's designs, see https://youtu.be/dhWW1mHXqic .

If I had half a brain, I'd stop asking questions and build one EXACTLY like what you show on your plans, maybe with reduced-height wing tip plates as you mentioned, but no other mods. But I'm afraid that half a brain is all I can claim credit for, and not much experience, so I'll probably keep stumbling thru the woods instead of following the obvious, proven path.

But I'd deeply appreciate any comments, good or bad, that you and others might have about my plans.

Thanx again!
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Olbill
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« Reply #36 on: January 14, 2018, 06:57:57 PM »

Olbill, this is great information, thank you! I'm somewhat new to this sport, and there's a lot I need to learn (and probably always will be).

I'm curious about a couple of things:

1.) Why did you bend the rear tail boom down and put the stab 1-1/2" below the level of the motor stick; while putting the wing only 1-1/4" or so above the motor stick? Seems simpler to have the tail boom sticking straight back from the motor stick on the same level, and have the wing maybe 2-2/1" above the motor stick. Most of my planes are like that, but as I said I've got a lot to learn.

2.) Why taper the tail boom? Most of mine are just a 1/8" square lightweight balsa strip. I'd guess that tail forces are very light in a model like this, and even 1/8" square may be stronger than it needs (one of my tail booms is 3/32"x1/8", seems fine so far). In my planes, I saw this as a good place to reduce the dreaded "weight aft of the CG".

3.) What are "tissue tubes" that you mentioned on the overhead view of the wing center and stabilizer center?

4.) What is "SSM"? Static Stability Moment or some such?

I'm considering building something like this, but with the differences I mentioned above. Probably build up the tip plates on wing and stab, and cover them (and the rest of the surfaces) with Ultrafilm to save weight. And all the weight I can save, will go toward putting a weight on the nose as far forward as possible to bring airframe weight up to 7.0 grams, and scoot the wing as far forward toward the propeller as possible while still keeping the model statically stable.

That said, I read your description of 2:30 and even 3:00 flights with my jaw on the floor. The best any of my planes have done, is 1:52 with a .062" wide FAI Sport rubber motor and a ground-adjustable-pitch prop that I modified from one of Ziegler's designs, see https://youtu.be/dhWW1mHXqic .

If I had half a brain, I'd stop asking questions and build one EXACTLY like what you show on your plans, maybe with reduced-height wing tip plates as you mentioned, but no other mods. But I'm afraid that half a brain is all I can claim credit for, and not much experience, so I'll probably keep stumbling thru the woods instead of following the obvious, proven path.

But I'd deeply appreciate any comments, good or bad, that you and others might have about my plans.

Thanx again!

1. Wing posts are short b/c they're stronger and lighter and I've never felt there was an advantage to long ones. The droop boom is to get more wing/stab separation. You didn't ask about turning the motor stick to the left. I do that instead of turning the prop bearing to the left which I feel puts a friction load on the prop shaft. All of these are features that I use on all of my models. Others probably don't agree with some or all of them.

2. 1/8" sq. seems a little small but it may be fine. Since the tail boom is a cantilever beam the greatest bending is at the joint with the motor stick and it goes to zero at the aft end. That's the reason that most people use tapered booms.

3. That was an oversight and was left on from a previous plan. On most of my recent models I use polyimide tubes. Tissue tubes are made by winding tissue and glue around a suitable mandrel. They work fine but are something I'm happy to not have to make.

4. I know very little about aerodynamics. I seem to remember that some people who do know a lot have complained about the term Static Stability Margin. At any rate its application to what we do is help you to determine whether a model is going to have the problem that yours had and that mine had on my first attempt - either stalling or diving with no stable flight adjustment possible. Or a low SSM could manifest in a model that is very speed sensitive - like flying okay at low speed and diving when the speed increases. There are two things that change when the SSM changes. A higher SSM will usually indicate a more stable model but the sink rate may increase. The lower you can make the SSM in general the more efficient the model will be and flight times will be higher. So the trick is to get the SSM just right.

If you decide to build a copy of my model I would encourage you to not change the wing position. I do a lot of fiddling with the program to optimize the model. Changes may help or they may not help. Of course that's completely up to you as to how to proceed. Built up tip plates are not necessary if your weights are in line with what I'm showing but are okay if that's what you want to do. Moving the CG farther forward probably won't have a major effect on flight times.

Here's a little about motor selection:
I started with a 17.5" motor. This flew well except there was no enough power towards the end of the flight. I decided right away that a 16" motor was going to be about right. The next motor came out at 15.25". It improved the flight time but was almost dead stick at the end of the flight. The next motor was 16.5" and was close to perfect. Note that the big prop was having the effect of keeping the RPM's from going thru the roof like they do with the little stock 5.5" props. If you're using a really small prop the RPM's are going to push you to longer motors just in order to keep the prop turning for the length flight that your model might be capable of. My prop was turning at a little over 600 rpm.
(there's some kind of problem in my record keeping about motor lengths - probably senility setting in - but I'm sure the first and last motor lengths are correct.)

On the 3:00 flight the model hit once and lost a little altitude. The flight would probably have been no-touch in a 24' or 25' site.
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Little-Acorn
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« Reply #37 on: January 15, 2018, 01:32:46 AM »

You called your prop a "big prop". Was that due to blade width, or prop diameter, or...? What diameter is it?

I just got finished designing one (5.5" diameter) with fairly wide blades, and with all the prop area ahead of the blade axis, something like the one in your picture. Built it for one of Ziegler's ground-adjustable-pitch prop hubs. Hopefully it won't start adjusting its pitch in the air!

Will hopefully find out if it improves anything, tomorrow.
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Olbill
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« Reply #38 on: January 15, 2018, 10:54:57 AM »

5.5" diameter with wide blades. I wouldn't be reporting on anything that wasn't legal.
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bjt4888
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« Reply #39 on: January 20, 2018, 10:07:22 AM »

Bill and Coach Chuck,

I just wanted to thank you both for the excellent reports, data and recommendations. I am primarily helping my four high schools with Helicopter this year, but one of my outstanding high school science teachers has a daughter in middle school doing Wright Stuff and I definitely have to help her.

In order to save a little time, I had them buy the Laser Cut Planes kit. We are using the wing, stab and fin from the kit and a longer, custom fuselage and tail boom along the lines of Bill’s design.

Thanks again,

Brian
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