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Old Aug 27, 2009, 12:24 PM
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Mcdyvie's Avatar
Germany
Joined Jul 2009
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how heavy is too heavy?

Hi guys and gals,

I am in the process of building my first foamy, and am just about done with the electronics. The motor should arrive on the weekend, or Monday at the latest. The problem is, he states AUW at 275g and I will be coming in at 375-400g. HOWEVER, I will be using a much stronger motor (200W 2830-8T, 1400Kv) which is designed for 550g-800g warbirds. Will this difference in motor power offset the extra weight on the plane, or is the inherent design of the plane not made for all that extra weight? i.e. will it drop like a brick when I let off on the throttle? I wanted to make it beefy, so it is less fragile, and that has added a bunch of weight. I could probably trim about 50-60g off of it if I had to. Any ideas for a first-time scratch builder?
Here is the link to the plans:
Bi Night Bipe

Thanks in advance!
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Old Aug 27, 2009, 02:25 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,246 Posts
"They say a brick will fly with enough power", but then who wants to fly a brick ?

Weight usually converts into speed, especially downwards, and not helpful trying to load slowly. It also increases the stall speed.

Speed can often convert into structural stresses.

That said, depending on the model and it's wing loading, (weight/wing area), it may not make too much of a difference.

The thing to remember with foamies is, the best thing to add is more lightness. Lighter models don't hit the ground so hard when they forget to fly, or when the ground jumps up at them.
Strengthening a model to withstand crashing is a bit of a 'catch-22'.
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Old Aug 27, 2009, 03:02 PM
SG Talon... Super Genius.
Lenox, Michigan
Joined Nov 2005
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That plane will probably fly good up to about 500g (17-18 oz).

Obviously the lighter you make it the floatier it will be. It all depends on what you want the plane to do. Are you looking for an insane 3D monster that will do every stunt like it was made for it? Or do you want a plane that will fly nice outside, and you just kinda swoop around doing basic aerobatics (Loops, rolls, snaps, etc)

At the 400-500g range it will probably be a nice outside parkflyer that can handle some wind. What size batteries are you using?

I would say for that plane you could probably go with a smaller motor that is also lighter. But a 1000 mah 3s is probably ideal for it.
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Old Aug 27, 2009, 03:37 PM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
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Quote:
Originally Posted by eflightray
The thing to remember with foamies is, the best thing to add is more lightness. Lighter models don't hit the ground so hard when they forget to fly, or when the ground jumps up at them.
Strengthening a model to withstand crashing is a bit of a 'catch-22'.
Agreed. When in doubt, I've gone with lighter over strength.

F = m*a

If "m" is the mass of your plane, and "a" is it's acceleration towards an object you'd rather it not hit (but is going to anyway), the number for "F" is going to be higher (meaning more breakage, due to more mass) if the plane is heavier/stronger. Lighter means the force is lower, and more likely to bounce, or not break as much.

That said, you can't make a plane TOO light, especially if it's going to be used for aerobatics. There's a happy medium, and that middle ground varies depending on the pilot, too. I've built some planes that I wouldn't be happy flying at more than 10 ounces, and others have had wonderful success with them at 17 ounces.

To each his own, I guess.

The question becomes: How fast are you comfortable flying? What is your tolerance for fixing damaged planes if/when you crash them? And of course, what equipment do you have available, and if it's larger than what the plane really can support, should you consider scaling up the plane to account for that?

It really becomes a lot of relative answers, and the more you build/fly, the more you know how you prefer to build your planes for the way you like to fly them.
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Old Aug 27, 2009, 08:54 PM
Lost but making good time
Joined Nov 2008
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mcdyvie,

Looks like a beautiful biplane. I don't work a lot in metrics so bear with me. I first determine my wing area in square feet. If you want to have a sloflyer, i.e. something you can almost walk next to on a calm day, you don't want anymore than 5oz of all up weight per square ft of wing area. When you get between 6 and 10 oz. you really have to start concentrating on "flying" the plane", i.e. it has to go faster to keep it airborne and you have to make corrections faster so it doesn't crash. Anything above that and you had better really know what your doing.

This plane would probably fly well with a 22mm x 5mm 1500 Kv outrunner (30 grams). You will have more than enough power but have added to the weight/square foot of wing area which will mean flying it faster to keep it airborne. Make sure your center of gravity is between 1' - 2" in from the leading edge of the top wing. Adjust the battery accordingly.

Best regards,

Hankg
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Old Aug 27, 2009, 10:11 PM
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I grapple with this still but have mostly broken the 'habit' of building for crashing.

It has amazed me the difference in my building when building for flying.

Of course I think this mindset change has also come with my flying ability improvement over the last year.

Flight loading is nothing compared to ground loading
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Old Aug 28, 2009, 02:02 AM
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Mcdyvie's Avatar
Germany
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Thanks for the replies, all very helpful information! Most of all you have given me some peace of mind about my first SB foamy. I am not looking for an insane 3D monster yet. First and foremost, I want a parkflyer that can handle some wind (I fly at a sailplane airstrip field, thus it is usually windy) has great vertical and speed, and can do some mild 3D. This is my first 3D plane, and as I progress in my 3D skills, I may build a YAK or another much lighter version of this one since I have always wanted a Bipe and this one just looks good.

As for batteries, I have several options from the hangar (the first ones from a heli):

950Mah 2S 15C x 2
800Mah 2s 15C x 1
1000Mah 3s 30C (x2 on order from HC) woot!!
1300Mah 3s 15C x2 from the trusty old Super Cub

I will keep you guys posted as to how it flies with video/photos, I hope to get her flying by Monday/Tuesday at the latest.

Cheers,
McDyvie
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Old Aug 28, 2009, 10:54 AM
Lost but making good time
Joined Nov 2008
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Going4speed,

Reading your comments brought back some memories. When I started in this hobby about a year and a half ago, I built my foamies so they could fly through brick walls.
I didn't realize the extra weight forced me to fly faster and my reflexes were'nt ready for the abrubt changes that had to be dealt with at faster speeds. I broke a few motor shafts augering in.

Miyamoto Musashi, circa 1600s, reputed to be Japans greatest swordsman, once said " if you have to think in a sword fight, your dead". If you have to think " Oh my gosh, what do I do to keep from augering in", you may not be dead but your plane may be.

Looking forward to how the first flight goes, Mcdyvie.

Regards, Hankg
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Old Aug 28, 2009, 12:12 PM
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I had went back to rudder elevator after a spectacular loss just to get a build up of confidence.

Then when I went back to aileron my reflexes were much improved. My yak was getting close to the ground and it was almost like magic that I recovered multiple times from trying new things.

The 'thinking' part is now at the thumb level and not the head level

Quote:
Originally Posted by hankg
Going4speed,

Reading your comments brought back some memories. When I started in this hobby about a year and a half ago, I built my foamies so they could fly through brick walls.
I didn't realize the extra weight forced me to fly faster and my reflexes were'nt ready for the abrubt changes that had to be dealt with at faster speeds. I broke a few motor shafts augering in.

Miyamoto Musashi, circa 1600s, reputed to be Japans greatest swordsman, once said " if you have to think in a sword fight, your dead". If you have to think " Oh my gosh, what do I do to keep from augering in", you may not be dead but your plane may be.

Looking forward to how the first flight goes, Mcdyvie.

Regards, Hankg
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Old Aug 28, 2009, 12:28 PM
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Mcdyvie's Avatar
Germany
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Quote:
Originally Posted by going4speed
I had went back to rudder elevator after a spectacular loss just to get a build up of confidence.

Then when I went back to aileron my reflexes were much improved. My yak was getting close to the ground and it was almost like magic that I recovered multiple times from trying new things.

The 'thinking' part is now at the thumb level and not the head level

So true! On my T-28 while ROG'ing yesterday, I got hit out of the blue by a hard crosswind right after it took off. It pushed the right wing right up and pushed the T-28 to 90 off the runway and the left wing was about to kiss the ground. It would surely have cartwheeled and broken the wing off at least. Only some reflexive aileron thumb movement saved it and I got away without a scratch, or a wreck. Even a few months ago, that plane would have bit the dust. If I had had to think about how to save it, it would have been too late. The swordsman spoke true!

Also, here is a very short video of the Bipe with all electronics hooked up and some servo testing. Just waiting for the motor! Those control surfaces MOVE man! I have TA'd them to 50% for the maiden since I have never flown a 3D bird before. I can't imagine just how nimble that thing will be with full throws.

Bipe servo testing
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Old Aug 28, 2009, 12:46 PM
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United States, UT, St George
Joined Jun 2009
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yea that will have some crazy roll with dual rates on
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Old Aug 28, 2009, 08:16 PM
Lost but making good time
Joined Nov 2008
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Mcdyvie,

Plane looks great, started to salivate when I saw the control surfaces move! Get some altitude on it before you start to experiment. Biplanes fly differently than monoplanes. I was greatly surprised when I put my 30" Epp Ultimate bipe in the air the first time.

Watch the c.g. location.

Regards,

hankg
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Old Aug 28, 2009, 10:54 PM
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maybe its just that angle in the video but that elevator looks smallish.
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Old Aug 29, 2009, 01:09 AM
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Mcdyvie's Avatar
Germany
Joined Jul 2009
994 Posts
Quote:
Originally Posted by hankg
Mcdyvie,

Plane looks great, started to salivate when I saw the control surfaces move! Get some altitude on it before you start to experiment. Biplanes fly differently than monoplanes. I was greatly surprised when I put my 30" Epp Ultimate bipe in the air the first time.

Watch the c.g. location.

Regards,

hankg
Thanks Hank, will do! I plan on taking her a few mistakes high before I start messing with the controls.

@going4speed- Built according to the plans, but is there a standard formula for that? In the video the plan maker posted, it seemed to have sufficient elevator response.
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Old Aug 29, 2009, 05:15 AM
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Toronto,Canada
Joined Jul 2002
435 Posts
Quote:
Originally Posted by Mcdyvie
The motor should arrive on the weekend, or Monday at the latest. The problem is, he states AUW at 275g and I will be coming in at 375-400g. HOWEVER, I will be using a much stronger motor (200W 2830-8T, 1400Kv) which is designed for 550g-800g warbirds. Will this difference in motor power offset the extra weight on the plane, or is the inherent design of the plane not made for all that extra weight? i.e. will it drop like a brick when I let off on the throttle?
Motocalc is a great program for "what if" power calculations

You can download a 30 day trial here :

http://www.motocalc.com/motodown.htm

I guessed the wing area for your plane at ~500 in^2, at ~14 oz, according to MotoCalc, you'll probably want to run 3s, and maybe a 10in or larger prop (if it will fit), (It predicts only ~176 fpm climb rate on 2s, 8x4 prop).

In contrast, Motocalc predicts that the ~10oz plane will fly very well on 2s with the 8x4 prop. (~1200 ft/min climb)

How did your plane get so heavy ?

Some great intro info on electric power systems :

http://www.stefanv.com/rcstuff/tutorial.html
http://www.stefanv.com/rcstuff/qf200103.html
http://www.wattflyer.com/forums/showthread.php?t=24238
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Last edited by Mike Stramba; Aug 29, 2009 at 05:36 AM.
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