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Aug 18, 2019, 10:30 AM
Sure, I can fly after sunset!?
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Discussion

HELP! CG first balsa build


My grandson and I just finished our first balsa build. It is a Dumas Kit: Curtiss P-6E Hawk 17.5 inch wingspan, rubber powered.

After 6 days, it is done!

So the last step is to balance it. It is very tail heavy. The plane weighs 51g but It will take 26g to balance it!

I can’t even think of a way to get that weight inside. I hate to tape it underneath.

Am I missing something?

Oh, he liked the cowl white, so that’s how we left it.
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Aug 18, 2019, 12:56 PM
'Douglas' to his friends.
Well done on finishing the build; that's not the easiest of 'planes for cutting one's teeth on..! Yes, this type of model is often tail-heavy, but, once built, there are not too many solutions. Is there space inside the cowl for a couple of fishing weights..? Some weights are 'bomb' shaped, and could be fixed to the underside of the nose without looking too disgracious, maybe..? One can also get lead strips, and fix them around the cowl. Any weight added would be most effective as far forward as possible, so some compromise may be necessary. Good luck, and let us know how this progresses, please..?
Aug 18, 2019, 03:09 PM
Sure, I can fly after sunset!?
Thread OP
I bought three of these “walnut” scale planes. This is my first build. NOW I know to check for approx CG before gluing it all up.

The base of the cowl is CA’d to a sort of firewall. That ain’t coming apart. BUT, your suggestion for using sinkers could work. That’s a great idea. Have to ensure it doesn’t interfere with the rubber band. With sinkers, I could lock them in place with monofilament line and some judiciously placed 1/32 “ holes.

I was surprised where the CG is supposed to be. Typically it is at the thickest part of the wing; and that is exactly where it is. But doesn’t the lower wing affect the CG?
Aug 18, 2019, 04:30 PM
'Douglas' to his friends.
Funny things, biplanes. The C of G of the 'real' 'plane would probably be diferent, too, as there's a huge chunk of heavy metal on the nose. Start with whatever the plan says (or slightly more forward, if in doubt...); don't bother trying to work out by theory where it should be. If you could get a half-decent video of your first glides (over longish grass..?) you'll doubtless get some good trimming advice from the Experts here (No, I'm not one of 'em , but I'm trying; very trying...).
Aug 18, 2019, 08:25 PM
B for Bruce
BMatthews's Avatar
First off congratulations on what looks like a very clean and crisp build! ! ! That's a fine looking job you two did of the model.

For a rubber power model of this size 50 to 60 grams is already as much as you want it to be. So anything you can do to reduce the nose weight you need to add would be a HUGE benefit.

If it were me I'd cheat and add a nose extension. Hopefully the plans show the bulkheads for the fuselage? If so I'd cut two more F1 bulkheads and connect them with 3/4" long stringers. Glue this to the current F1 and then the cowl to the new front bulkhead. The extra length will allow the model to be balanced with a lot less nose weight and give you a far better chance of flying acceptably with rubber power at the 60+ grams that it would still be with the nose job.

Also find a field with 6 inch or taller grass where they will allow you to fly and do all your testing there. With the inevitable rough landings you'll have during trimming it really aids with reducing or eliminating damage to the model.
Aug 18, 2019, 11:01 PM
If it flies, I can crash it.
rocketsled666's Avatar
Are you sure you checked the CG at the right place on the wing?

A little weight at the back of the plane needs a lot more weight at the front to compensate. That tail wheel looks like a couple of grams of weight at the back that'd probably need 5x or more to compensate at the nose.

If you really do need weight, the further forward you can get it the less you'll need. What I would do is find a couple of steel washers with an ID that was the same as the OD of the back end of the prop hub. Pull the hub out of the cowl, remove rubber (don't let go), slip the washers over the back end of the hub, reattach the rubber, put the hub back in to the cowl. The washers will be retained between the front flange on the hub, and the cowl.

If you didn't like the way that looked, you'd have to remove the cowl, but you could epoxy the washers on the backside of the cowl coaxially with the prop shaft, and get them further forward than most anything else.

Not the same thing, but by way of example, I have a PT-17 that requires nose weight to balance, no option. To help get me there, I used a big brass prop nut that weighs a couple of ounces, and I poured steel shot and epoxy in to the dummy engine cylinder heads so all the weight I added was as far forward as possible. I managed to balance the plane with 4Oz of ballast. Most require nearly 2x that amount.
Aug 20, 2019, 10:08 AM
Sure, I can fly after sunset!?
Thread OP
Thank you all for the suggestions.

My Grandson didn’t want to alter the looks of the plane, for now.

I often use Web-O-Calc when reviewing power systems. I decided to enter the specs for this biplane, including the added ballast weight to reach CG. I pretended it would run off of a 1S motor. It resulted in a “backyard flyer” suitable for a beginner. The stall speed would be 11 mph.

So, I used Dad3353’s suggestion. I attached several split sinkers to monofilament line, like beads on a string, run through 1/32 inch holes in the base of the cowl. This put the weight as far forward as possible, within the cowl, suspended out of the way of the prop shaft and rubber band.

I took it out for a glide test in high weeds and it glided great, albeit a bit nose heavy. Just need to remove a split sinker or two.

My grandson is visiting again this coming weekend, so I didn’t want to try to fly it without him. However, I did give the rubber band about 150 turns to try an “extended” glide test. I was surprised at how little “power” there was from the spinning prop.

I’m in the dark here. How many turns should it take? I read somewhere 100 turns per inch of rubber band. Oh, it took about 50 turns just to get out the slack. Here is what I mean.

I’ve also read to soak the rubber band in castor oil, glycerin, etc.
Aug 20, 2019, 10:15 AM
Sure, I can fly after sunset!?
Thread OP
More pictures
Aug 20, 2019, 10:40 AM
B for Bruce
BMatthews's Avatar
The rubber that they give with the kits is always a joke. Sorry to say but the most you'll get out of that one solitary small section motor is going to be a short burst of flying level or at most a few feet of climb and then an extended powered glide. And that would be a best case for the little loop of 1/8 wide rubber it comes with. You really need a loop of 3/16 rubber or a double loop of 3/32 (as in one very long loop that you fold in half). The rubber needs to be fresh or it will tend to break easily during winding. And that can ruin your day in a blink. So I'd recommend that you buy a little bag of it via mail order. And to make the shipping worthwhile I'd suggest a 5:1 ratio winder at the same time. You can buy these things from outfits like Easybuilt or FAI Model Supply or Volare Products

And yes, the rubber does need to be lubricated or it'll tear itself to shreds pretty quickly and break once you start packing in the turns. You can use any number of options. But they need to be something that stays "wet". There's any number of automotive rubber and plastic preservatives that are slippery when applied but dry. I do know that a lot of folks like a spray on/wipe off product called "Son of a Gun" to use as a rubber lube. Spray it on lightly and rub it in and only blot away the worst of the excess.

The old school lube is a mix of half and half thick liquid hand soap and USP glycerine. Then add about 10% more castor oil. You likely have the hand soap already so you would only need the glycerine and castor. Both of which should be available from a better stocked pharmacy.
Last edited by BMatthews; Aug 20, 2019 at 10:45 AM.
Aug 20, 2019, 01:16 PM
Sure, I can fly after sunset!?
Thread OP
Should I shorten the rubber band? I used the entire length they gave me.

Same question if I buy some decent rubber.

There are two tall hills within ten minutes of here. Maybe we’ll fly/glide it there this weekend.

I started using thin CA for the build but it made such a mess that I switched to medium CA. Is the CA the reason the plane is so heavy? 80g overall.

Another question. How do you keep CA bottle nozzles from congealing into a brittle mess? These two CA were brand new a week ago but cannot even be capped now. That’s my long experience with CA. I now have pins in each bottle, but they get stuck quickly. I often have to remove them with pliers. I hate using CA.

I bought a package of those CA tips but it turns out they didn’t fit.
Aug 20, 2019, 02:31 PM
B for Bruce
BMatthews's Avatar
Used sparingly CA isn't any worse than any other glue. But if you used fairly big drops and there's fillets and blobs on the majority of the joints it would certainly be a contributor to the issue.

Leave the motor the length it is. Just tape or find an easy way to keep the nose block from falling off. Wind the motor up until you get a double row of knots as seen through the cockpit. That's about the best you can do without more stuff to learn more about rubber flying at this point.

If you are fascinated by rubber flying at this point and want to try an easily built and likely better performing model have a look at Peter Chinn's Project 3.

This would be a good design to fly with either two or three loops of 3/32 rubber and as you can see from the plan in the link it's a pretty easy build and very simple to cover. And to make things easier leave off the landing gear.
Aug 20, 2019, 04:20 PM
Sure, I can fly after sunset!?
Thread OP
Quote:
Originally Posted by BMatthews
Used sparingly CA isn't any worse than any other glue. But if you used fairly big drops and there's fillets and blobs on the majority of the joints it would certainly be a contributor to the issue.

Leave the motor the length it is. Just tape or find an easy way to keep the nose block from falling off. Wind the motor up until you get a double row of knots as seen through the cockpit. That's about the best you can do without more stuff to learn more about rubber flying at this point.

If you are fascinated by rubber flying at this point and want to try an easily built and likely better performing model have a look at Peter Chinn's Project 3.

This would be a good design to fly with either two or three loops of 3/32 rubber and as you can see from the plan in the link it's a pretty easy build and very simple to cover. And to make things easier leave off the landing gear.
Yup. Lots of excess CA. Virtually impossible to remove without breaking ridiculously fragile 1/16 sq balsa strips.

The cowl is actually securely held in place with 4 neodymium magnets.

That looks like an easy plane to build; however I’ve had it with 1/16 inch sq balsa.

The good news is that I won a Goldberg Anniversary Cub kit several years ago and never had the confidence to build it. I think this build has given me enough experience to give it a try. Best thing is that there are no 1/32 balsa strips in the box!
Aug 20, 2019, 09:25 PM
B for Bruce
BMatthews's Avatar
When I use CA for small craft like this I first off make sure the joints fit well with little or no gaps. Then I dribble two or three drops of thin CA onto a little bit of wax paper. I think food wrap plastic would work too. It won't kick and cure on you. I then use a bit of wire to pick up and transfer a little micro drop of the glue. This gives me clean gob free joints and hopefully avoids any excess weight. Being thin CA it'll kick really fast. So the glue is applied to the dry fitted joints and the micro drop on the wire wicks into the joint like a hungry dog snapping a treat out of mid air.

The wire I found worked well for this is simply a paper clip with the one leg bent out straight. I then file it (you could use a grinding wheel in a Dremel to do this too) to form a tip on the wire that looks like a triangle file. The flats hold the amount of glue needed for a small joint of this sort.

I hope that gives you a good idea for the next one.

I actually got away from using CA for such small stuff as I did find that overall it was rather brittle and I was constantly re-gluing joints. I've since switched to using yellow wood glue SLIGHTLY thinned down with a little water. Like one part water to 8 parts glue. And then I use a tooth pick to move the glue from the little puddle to the small joints of this size of model. In this case I don't apply it to the dry fitted joint. I actually wet both side of the joint and then push the part in place. But I do so sparingly since a little goes a long way when used like this.

The wood glue is actually tougher as it's slightly flexible. And I don't think it's any heavier when used in the proper amounts. After all it sure doesn't take much to be stronger than a piece of 1/16 square balsa, right?

Again I hope this gives you some ideas and encouragement to try another small free flight model.
Aug 21, 2019, 06:00 AM
Sure, I can fly after sunset!?
Thread OP
Thanks for the tips.

I originally bought three of these kits. One went to my brother so I still have one left. It is another 17.5 inch biplane. Maybe I’ll try building it myself
Aug 21, 2019, 06:02 AM
Sure, I can fly after sunset!?
Thread OP
I would imagine that not using CA would greatly increase the build time.


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