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Author Topic: How do you control the high power at launch?  (Read 2614 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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torqueburner
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« Reply #40 on: March 17, 2018, 10:08:48 AM »

5.5" diameter with wide blades. I wouldn't be reporting on anything that wasn't legal.

Bill, my students are using a similar prop, but it requires a much shorter 1.5 g loop.  They are wondering about the pitch of your prop.   

Thanks.

Dave D.
Kutztown, PA
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bjt4888
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« Reply #41 on: March 26, 2018, 10:24:21 AM »

Bill and CoachChuck,

Wanted to thank you both once again for the helpful data. Sorry to not be communicating much this year.  I have 20 HS students doing helicopter this year that are keeping me crazy busy.

Bill,

Did you happen to measure the prop pitch for your WS design?

Thanks,
Brian T
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bjt4888
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« Reply #42 on: March 26, 2018, 06:27:11 PM »

Bill,

Sorry, I didn’t notice that the previous post was already asking about prop pitch.

Thanks,

Brian T
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Olbill
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« Reply #43 on: March 26, 2018, 10:46:07 PM »

I keep forgetting to measure it. I'll try again tomorrow.
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Crtomir
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« Reply #44 on: April 24, 2018, 09:33:24 AM »

We are having the same problem.  Up until now, we have been flying in gyms that are 22-30 ft high, so we never had to do maximum torque.  Now, for our State competition, the ceiling is 48 ft., so we would like to max out our torque.  However, when we increase our torque too much, the plan goes up first at a fairly steep angle (40 degrees to the horizontal), and then levels off, dives down, and then eventually recovers and starts to climb again.  If the torque is way too much, the plane dives into the ground. 

At first, we thought this was because it was bending the motorstick, so we added a bracing to the motor stick.  Under full torque, it's not bending, but we still get a dive.  We can't figure out why. 

We may just have to live with winding to max torque and then unwinding to a torque level that does not cause the plane to dive.  However, with lower torque, it won't fly as high. 

I'd really like to figure out what is going on with our planes at high torque.  As I said, we never had to do this before.  We started flying with half motors so we could wind to full torque and still fly in a 24 ft high gym without hitting the ceiling.  The half motor spacer does not stretch like the half motor does.  Would this affect the pull on the motorstick differently than using the full motor both wound to the same max torque?

Would it help to move the wing back on the plane and increase the angle of incidence of the wing?
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ceandra
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« Reply #45 on: April 24, 2018, 09:41:59 AM »

I would try increasing the SSM first (move CG forward, or wing back, increase incidence). If that is not simple, add some clay to the front, even if it sends you over 7g, just to test it out. With the small tail this year, you do not want to get the wing too far back. Ol' Bill reported substantial improvement moving wing forward and adding ballast up front despite the increased weight. Try the ballast first, its a quick way to test for improvement.

You may also want to try a little lower pitch on the prop so that it continues to climb after the initial burst (or slightly thicker rubber). You will probably still want to unwind a little. You may be winding to 0.6-1.0 oz-in, probably don't want to launch above 0.4 or so. Remember, launch torque is not the only lever you have to adjust for more altitude.

Chuck
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Crtomir
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« Reply #46 on: April 24, 2018, 09:57:57 AM »

We have a homebuilt torque meter, but I think the numbers are close to accurate because a torque of 0.4 seems to be about the limit, where we dive down slightly, but then recover to a good flight.  A torque of 0.5 seems to cause us to dive into the ground. 

We should try to increase the wing incidence angle and move the CG forward. The stabilizer boom is pretty long on this plane, so moving the wing back a little might be okay.

We are flying with flared props that we made from Freedom Flight.  The pitch is 22 degrees as measured near the tip using the pitch gauge from Freedom Flight.  The plane is our own design.  I'll see if I can post some pictures.

I'm not sure if we are getting the full benefit of a flared prop if the pitch is too low, but higher pitch did not work as well (lower altitude and lower flight time).
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calgoddard
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« Reply #47 on: April 24, 2018, 10:17:53 AM »

Crtomir -

You stated:

"We are flying with flared props that we made from Freedom Flight."  

In general, flaring props are only used when flying in a low ceiling venue. They are a way to legally vary the pitch of the prop during its flight.  The WS rules allow a flaring prop, but not a prop that has mechanical devices, like springs, that actively vary the pitch or diameter of the prop.

At launch, the high torque of the rubber motor causes the blades to flare, which increases the pitch. This acts to suppress the climb until the torque bleeds off a bit.  This is not what you want to happen when flying a WS model in a high ceiling venue.

I think the 5 1/2 inch red SIG plastic prop is legal in 2018 WS. Its blades wont flare a noticeable amount, even at a very high launch torque.  The blades of the Ikara prop may need sanding to flare, so a stock Ikara prop may be OK for flying in a high ceiling venue, provided it is trimmed to meet the max diameter limitation set forth in the 2018 WS rules.

If you want the best trimming advice from experts who frequent HPA, post a link to a YouTube video of a flight of the model which shows the problems your team is experiencing. It is difficult to accurately describe in words every nuance of a particular flight pattern.

Ceandra has given you some very good advice.

Trimming, prop selection, rubber sizing and winding are different for high ceiling venues versus low ceiling venues.

The vast majority of WS competitors at the National competition will simply fly the same model they flew at regionals or State. It won't climb high enough and/or it will run out of turns while still many feet above the ground, at which point it will head straight into a wall. Some teams, wanting their models to climb much higher in a a high ceiling venue, will launch at an excessive torque, the MS will bend, and the flight will be over rather quickly.

When the National competition is held in a high ceiling venue, a handful of the best WS teams will have switched to a lower pitch prop and a longer rubber motor, and practiced with partial rubber motors, winding with a torque meter. Those are your team's true competition.

I want to pass on one final piece of advice I was given many years ago.  It may have been Olbill who gave it to me. It went something like this.  "Your plane should be at the minimum legal weight.  You would not want to run the Boston Marathon with two bricks in your pockets."  So if you end up adding ballast to fix your current problem, figure out a way to get your model back down to .01 -.02 grams above the minimum legal weight.  
« Last Edit: April 24, 2018, 10:50:27 AM by calgoddard » Logged
Olbill
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« Reply #48 on: April 24, 2018, 10:42:16 AM »

We have a homebuilt torque meter, but I think the numbers are close to accurate because a torque of 0.4 seems to be about the limit, where we dive down slightly, but then recover to a good flight.  A torque of 0.5 seems to cause us to dive into the ground. 

We should try to increase the wing incidence angle and move the CG forward. The stabilizer boom is pretty long on this plane, so moving the wing back a little might be okay.

We are flying with flared props that we made from Freedom Flight.  The pitch is 22 degrees as measured near the tip using the pitch gauge from Freedom Flight.  The plane is our own design.  I'll see if I can post some pictures.

I'm not sure if we are getting the full benefit of a flared prop if the pitch is too low, but higher pitch did not work as well (lower altitude and lower flight time).

Can you compare the dimensions of your model to the Finny 18? Also where is the CG on your model?

Any change of CG whether by moving the wing back or adding weight to the nose will probably require an incidence change. Hopefully that is an easy matter with your model.

A better plan than moving anything would be to reduce the weight of the tail, tail boom and the rear of the motorstick like I did on the Finny 18. Then if you're under weight you can add ballast to the nose which will also help your problem.

Using a lower pitch is okay with a flaring prop. The flare should slow the RPM's regardless of the starting pitch. Starting out at a lower pitch will let the model continue to climb even with the prop flared. One thing to consider is that unmodified Ikara props are too stiff to do much flaring. Making the blades flare requires a lot of careful sanding or scraping. Making a flaring hub is pretty simple. There is info about that in other places on HPA.
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mkirda
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« Reply #49 on: April 24, 2018, 02:16:33 PM »

So I want to point out something here.

This weekend, the Illinois SO champs were held in the Armory at UofI's Urbana campus.
This is probably going to be the highest and likely best site for any SO competition anywhere in the country.
(I know, we are lucky to get in there once a year.)

With the simple model I made and published in INAV, basically an enlarged Denny Dart, I managed to get to fly a 3:03 and it never got higher than 16m, plus my 1.8m roughly height.
This was with a simple Sig red prop and hanger and crappy 4/04 TanSS.
The only modification to the prop was scraping one side down to get it balanced, then twisting the blades to be equal pitches.
That flight - I checked the pitches afterward and they were unequal again, at 8"P and 7.5"P using a Retro-RC 5.5" pitch gauge.
I was flying with a 21.5" loop and launching at 0.5 or 0.55 max in-oz of torque.

The point being, you don't need any fancy plane or prop or whatnot to fly in higher places. You need to match the rubber to the prop. That's the entire ball of wax.
I probably could have gotten a better time with a slightly shorter loop of rubber, coming down consistently with 100-200 turns.
But I hit the time I wanted, 3 minutes, given the constraints - stock Sig prop hanger assembly and crappy rubber - that I made for myself.

Anyone else could have built the same plane and gotten 2:30 consistently with an 18.5" loop that would run out of turns 30-40 feet up.

I was psyched that some kids beat this time unofficially.

Regards.
Mike Kirda

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