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Adding weight to planes

Old 08-04-2006, 09:00 PM
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SIREX
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Default Adding weight to planes

Why is it bad to add weight to CG plane. If the motor has enough thrust and the plane is balanced what could go wrong? This is a serious question I just had to add over a pound to my plane!! Giant big stick with BCMA SPE 40 gas. I have moved everything I could but do to EI things have to stay where they are.

Thanks
Old 08-04-2006, 09:30 PM
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gjeffers
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Default RE: Adding weight to planes

its not that "anything is gonna go wrong" but planes just fly better the lighter they are

Lighter is better is all!
Old 08-04-2006, 09:33 PM
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RCKen
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Default RE: Adding weight to planes

Sirex,
Well there are actually many factors to take into account. Here are just a few things to think about.

Square inches - You're wings are only so big, you can't increase the amount of wing you have. So when you increase weight of the plane you increase the wing loading. When you increase the wing loading you change several critical flying characteristics like higher landing speeds, longer take off runs, and overall more sluggish plane. You should always strive to have the lowest wing loading you can possibly get.

Structural stress - As you increase the weight of the plane you increase the stress on all of the major structural components of the plane. Higher weight means that even simple aerobatics can stress wing components to the breaking point. Higher weight means that every landing puts more stress on the fuselage, and a rough landing has more chance of breaking something. Unless you have taken measures to strengthen the structural components of the plane you should really reconsider adding as much weight as you have.

Centrifugal force - When you add dead weight to one end of the plane you will change the way the plane behaves in circular movements. This can have dramatic effects on a plane, and usually not in good ways. Here's a simple experiment to demonstrate what I mean. Take a yard stick and put it in your hand. Now swing your arm down in a circular motion. As the stick travels downward stop the motion as quickly as you can. Not so hard right?? Now tape a pound of lead to the end of the stick and repeat the same motion. Now it's just a bit harder to get it stopped!!!!! The same thing happens with your plane in the air with dead weight in it.

Radio components - Now if you take all of the above effects and add them up they all put added stress on your radio. So now you have to either put in higher capacity servos, or face the very real possibility of blowing servos out while in flight.

I know that others will add to this list, but this what I can think of right off the top of my head. A pilot should always strive to make their plane as light as possible. Remember, gravity is THE BIGGEST ENEMY of airplanes. The more weight you have, the more that gravity is going to affect you. A case in point. Last year I built a GP Slow Poke 40. I had read that most pilots had to add 8-12 oz of weight to the nose of the plane to get it to balance the plane. During construction I did everything I could to lighten the tail of the plane. Between that and careful placement of the battery, receiver, and servos I only had to add 2 ounces to get the plane to balance. I was rewarded with a great flying plane that is a true joy to fly. That's really what all this boils down to. Whether you add a bunch of weight or not will determine if you have an overweight plane that is a pig in the air, or if you have a lightweight plane that will fly circles around others!!!

Hope this helps

Ken
Old 08-04-2006, 09:38 PM
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Default RE: Adding weight to planes

Weight hurts everything. You mentioned thrust. Thrust is turned into forward movement which is acceleration. This is affected by weight. Also affected are take off and landing speed,cornering and overall manuverability. If you have to add weight then you have to. But it should be the last resort. I would try and slide the engine forward or back before I would add lead.

David
Old 08-04-2006, 10:18 PM
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Missileman
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Default RE: Adding weight to planes

one thing not yet mentioned about adding weight. The farther forward or aft, depending on desired movement of CG, the less weight you will need to add. ie.. a pound of weight near the CG can be taken care of with a heavy (2 oz. or so) spinner at the nose instead.
Old 08-05-2006, 07:28 AM
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Default RE: Adding weight to planes

Also worth mentioning is that there is NOTHING wrong with adding weight (within reason).

If weight were a bad thing, 747's would be covered with fabric and dope.

Depending on the airplane and power, a little extra weight can be a good thing - it helps to keep the plane stable and lets it cut through the air better.

many years ago, weight was a big consideration due to the lack of engine performance - And nowadays, with the 3-D craze, people are again going weight-crazy.

I recently added 22oz of lead to the nose of a GP Seawind which, from the factory has about a pound of BB's already in the nose.

Bottom line: it is better to add weight than try to fly an unbalanced airplane.
Old 08-05-2006, 08:41 AM
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Default RE: Adding weight to planes

ORIGINAL: MinnFlyer

Bottom line: it is better to add weight than try to fly an unbalanced airplane.
True.. However I will interject this comment which is nothing more than my opinion,
if there is any way possible to move the cg by simply moving equipment that is already in the plane instead of adding weight, that is the route I would take provided it can be done without compromising the airframe. Examples, moving the battery pack, servos, or receiver forward or back. Once you have optimally positioned the gear and you still cannot balance THEN add weight if you need to. As has already been mentioned, add the weight as far forward or back as you POSSIBLY can, again it will require less weight if you do.

As Minn said, some weight might help the plane fly a little more stable if you are on the low end of the recommended wing loading. But be cautious in doing so, because as Ken stated the wings can only lift so much weight for their area and airfoil design. Once the plane gets too heavy it will be sluggish and difficult to fly unless it is going faster. This means faster landings and a faster minimum flight speed, because the plane will stall at a propotionally higher speed. But up to a point (maybe a few ounces), additional weight will not be too noticeable.

Minn I am surprised that your Seawind flew well with that much weight added. That Seawind is pretty small, how did it perform after you increased the weight? What changes did you see before and after from the increased weight? I am curious!
Old 08-05-2006, 11:57 AM
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Default RE: Adding weight to planes

Knowing nothing about the Giant Big Stick I did a search for the specs and it seems the weight range is 13-15 pounds. Adding 1 pound to the minimum weight means the weight has increased by 7.7% so the wing has to make an extra 7.7% lift which means it has to fly faster. But lift increases by the square of airspeed so to make an extra 7.7% lift it only needs to fly 2.8% faster. In other words, if the stall speed at 13 pounds was 20mph then at 14 pounds it'd be 20.56mph so basically you'd never notice the difference in stall, take off or landing speed.

In flight, if the 13 pound model could pull a maximum 10G turn then the wing is capable of producing a maximum possible lift of 130 pounds at whatever it's maximum flying speed is. No matter how light or heavy the model is, that's all the wing can do at that speed. So if it weighs 14 pounds then the most G's it can pull would be 9.3 (130 pounds lift/14 pounds weight).

Old 08-05-2006, 12:27 PM
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Default RE: Adding weight to planes

A balanced plane flys well. A tail heavy plane crashes well.
Old 08-05-2006, 12:52 PM
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SIREX
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Default RE: Adding weight to planes

Thanks Guys

I balanced it out with a little over a pound. I saw some of the other engines other people wer putting on these planes and I am still under on weight. I had to add some tail weight on my H9 ultra stick and it flys fine. I can see where doing everthing possible to get the plane to balence with just the components would be desirable but in this case I had no choice. I think the BCMA SPE 40 will have enough power to do anything I want. I plan on flying it on Sunday I will post a report. Thanks again.

Shayne
Old 08-05-2006, 01:19 PM
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Default RE: Adding weight to planes

The exception to the "lighter is better" rule is sailplanes or gliders. They often intentionally carry ballast to increase the weight of the plane for better penetration into headwinds. Full scale sailplanes often have water ballast tanks. The water can be dumped in flight so the plane can land light.

I once rode in an almost empty Boeing 737 for a short hop from Tampa to Jacksonville FLA, meaning that they probably had a light fuel load as well. It must have been off the ground after only a quarter mile or so. What a contrast to the same plane full of passengers and with full fuel tanks!
Old 08-05-2006, 03:46 PM
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Default RE: Adding weight to planes

ORIGINAL: MinnFlyer

If weight were a bad thing, 747's would be covered with fabric and dope.
Will forward your comment to Boeing's engineering department - maybe you'll get a suggestion award...

I suspect your comment was slightly tongue in cheek, but rest assured that there are folks in the Boeing design department whose sole job is to find ways to remove weight, supporting my belief that weight on an airplane is a bad thing...

On FS airplanes that carry passengers and cargo for a living (say, an MD80), if you increase its empty weight by 200 lb (0.24%), that's one fewer passenger you can carry under max weight conditions... In today's highly competitive airline market, the profit margin on some routes is the incremental revenue generated by a half dozen passengers... Take one pax away and you've cut your profit by 17%... Adding 200 lb to the airplane just for giggles might be a tough sell to the CFO...

In model scales, with our high power to weight and structural margins, additional weight's major contribution is manifested in degraded performance and increased stress on the airframe, and IMO the lightest model that remains within CG limits is best.

Cheers!

Jim
Old 08-05-2006, 04:49 PM
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Barry Cazier
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Default RE: Adding weight to planes

You don't see a lot of 747s flying 3D. At least not successfully.

I keep mine as light as physically possible. But...if I can't balance it, I'd sell the plane.

Thanks
Barry
Old 08-05-2006, 05:50 PM
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Default RE: Adding weight to planes

Mike, The Seawind does not have a "CG Range" it has to be balanced exactly where GP specifies (Dry)

I had to add 22oz of lead to the nose and that was WITH the battery pack in the nose as well (See attached pic)

I should mention that I omitted the nose wheel assembly which accounted for some of the additional weight needed.

How did it fly? It's the best seaplane I ever flew!

Here's a link to the review, check out the video:

http://www.rcuniverse.com/magazine/a...article_id=723
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Old 08-05-2006, 06:22 PM
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SIREX
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Default RE: Adding weight to planes

That sea plane is to cool. I have never seen one fly!! WOW!

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