I don't know whether this is in the correct forums. I have a kind of a "out of curiosity" question.

What influence does the total weight of the plane have on it's flight ability/chararistics?

In other words, lets say you have exactly the same plane with the same engine size...the one is heavier than the other - lets say 500grams or so over the recommended weight and the other is 500grams or so below the recommended weight. Would the lighter one fly better, but will it tend not to handle wind as well < just as an example.

This is just something I was wondering about today. If there is another thread explaining this which I missed then could someone post a link. Thank you

The heavier airplane if balanced correctly...will not be able to...climb as well.....be a little more sluggish on the controls.......will dive faster...on the flair or on approach the airplane will sink quicker.....it will stall sooner.....probably penetrate the wind better....
the lighter airplane if balanced correctly.....will climb quicker......control response will be normal or require less movement for the same result....on the flair or approach the airplane will tend to float...will burn less fuel.....the wind will have more of an effect......

Oh okay, so basically a lighter airplane is better too fly than a heavier plane. That was short but very explanatory to the question. Thank you Jet.

What Jetmech05 said.

I used to work on the C-5 Galaxy. Just imagine the difference from when it flew empty, to when it had a few M-1 Tanks inside it

*heh*

Boy oh boy is THIS thread gonna release a pile of worms.

There's a couple of pretty basic principles going on here.

#1 : Angle of Attack. It is ALL about AOA.

At any given AOA and airspeed, a wing (and we can presume, from your original question, that both aircraft have identical wings....we'll drop subtle variations due to the build, etc) will produce X amount of lift.

We've got a couple of options to 'solve' this:

If we want to fly both airplanes at the same throttle setting, then the heavier airplane will need a higher AOA...and will thus encounter more drag, and find itself flying slower.

If we want to keep both airplanes at the same AOA, the heavier one will need more airspeed, thus requiring more throttle (and inducing more drag).

In either case, what we wind up with is a "slower" airplane. For any given amount of thrust, the heavier airplane will be slower than the lighter airplane.

AOA's also going to dictate some of the things jetmech mentioned above. Faster sink rate and quicker stalls for example. Again, it's simple a function of drag and lift. "engine off" is still a power setting...so given both airplanes deadsticked, the heavier one will either need to be more nose-down for higher airspeed, OR will need to have a higher AOA, slowing it down more quickly, and bringing it closer to the critical AOA.

Climbing, too, is about AOA. An airplane climbs because of excess power. It has more thrust than is required for level flight. Well...remember...increased AOA (necessary to produce the extra lift, remember) implies more drag too...requiring more power. Thus, if X power is sufficient for level flight in the light airplane, it'll take X+1, for example, for level flight in the heavier aircraft. Thus, at X+1 power, the lighter airplane is climbing, the heavier one is cruising level.

#2 : Wind

Hoooooooo boy.

Regardless of the argument's that'll be made otherwise, given a steady wind...say, wind is blowing N at 10 kts, airplane is flying S...neither airplane will know or care about the wind. Both airplanes will react exactly the same way. The heavier one will behave no differently than the lighter one with regards to the wind.

Now, admittedly, sudden and temporary gusts may be an exception here. There IS the issue of inertia in a lateral sense, and the lighter one's inertia will be overcome a touch quicker...that is to say if, in the scenerio above, a sudden 10 kt gust from the E shows up, the lighter one will change its ground track to the east a BIT quicker than the heavier one.

Where it'll make a definite and notable difference is updrafts, thermals, and what we like to call "chuckholes"...plain ole turbulence. Pretty clear that an updraft of, say, 5 kts will lift lighter things more effectively than heavier things.

So...find yourself out at the field with a pretty steady wind down the runway...they aren't gonna beavhe any differently (due to wind...they'll still have all the differences due to weight). Find yourself at the field on a day with variable and rapidly changing winds...you'll argue more with the lighter airplane.

==================================

Having said all of that....you asked if the lighter airplane will "fly better".

THAT is a pretty subjective thing.

My own personal "short answer" would be "Heck yes..lighter ALWAYS flies better". But then, I happened to have a STRONG preference...both in scale airplanes and RC....for the characteristics dictated by the lower AOA in the light airplane.

Inside a DA-20, for example, my light airplane's gonna fly slower before a stall, it's gonna start and stop maneuvers more crisply, it's not gonna sink as fast, it's gonna cruise at a lower RPM (using less fuel) or fly faster at full throttle (if I'm renting wet (heh*), etc. But there IS a tradeoff. On a hot day, lots of turbulence, I'm gonna get bounced around a bit more, and notice the impact of changing ground color (going from "over the grass" to "over the runway" for example) more.

I happen to think the advantages outweigh the disadvantages...but that's me. It's the same in RC. You want an airplane with more margin for error, capable of slower flight, easier to put where you want it, and more forgiving of mistakes? Fly light. But know that you'll argue with it more in unusual or dynamic conditions.

This is why this is so subjective. Personally, I prefer a heavier plane. They handle wind better and are not so "Floaty". A few years back I build a very light version of the GP Ultra Sport. I flew it twice and haven't touched it since. It just doesn't fly as well as either the original, or the ones that I fully-sheeted the wings on (Read that as MADE THEM HEAVIER).

Okay so it depends on what you like and what type of flying you do...there is not a right or wrong on this one - just aslong as the plane can take off and gain altititude.

Let's put this into a scenario I have at the moment (yes it is my Edge again - I'm so on my toes with this plane since I have only had problems from day 1 with it). Anyway, lets take the first scenario as i got it back after the rebuilding of the firewall. The plane will take off and climb vertically without any problems so I guess, from what have been described, is not an overweight plane. Now I have added some weight to the plane due to installing a pilot (got the plane to balance at the same spot as they balanced it after the rebuild) Perhaps a tad nose heavy but nothing to worry about. Now all and all which I have added comes to around 70grams in total upfront...this will really not have an influence will it? If you should guess

Let me think back to my High School metric conversions... Let's see... 70 grams... Carry the three... Uh, I think that comes out to 1/2 mile. No, that can't be right.

70 grams is less than 2.5oz - nothing to worry about. Now, if you fly 3-d, it might be a concern, but I still wouldn't lose any sleep over it.

LOL 2.47 ounches to be exact Minn - give or take 0.01

Thank you I will go fly the plane on Saturday...aggg dammit! I'm working...okay hopefully on Sunday - but I feel more relaxed now. I should really get myself some anti-stress pills for this hobby :P

Minn's dead on, and it looks like you took the only "right" answer in here...it's completely subjective.

Like so many other things in this hobby, changing some aspect of an airplane (weight, prop, CG, a host of other things) WILL, without a doubt, change the airplane's behaviour in some (usually predictable) way. But none of them are right, wrong, better, or worse.

Bottom line...as it nearly always is...

If the plane does what you want it to do, and you enjoy flying it, then it's right.

I could not have said it better myself.

LOL

Given our mutual source for understanding AOA, I'm not real surprised.

Well the plane flew just fine. Had a heck of a time with it on Saturday and Sunday. Only had 4 flights in total this weekend - had some places I had to be so didn't get much flying time. I needed some up elevator trim though to fly level. I'm going to take a bit (only 10 grams or so) out of the nose for now. But as the plane was it flew great. Had some trouble with knife edging. The plane keeps pulling to it's belly in a knife edge. Don't know if it was my fingers, but anyway.

Before changing weight, there are some other tests you should do first. Do a google search for a "Trim Chart". There are several out there. The behavior you describe (required up elevator to fly level, but pulls to the belly in knife edge) indicates to me you're not dealing with a balance issue, but more likely either an engine thrust angle or wing incidence issue. You previously said you rebuilt the firewall, which is what makes me suspect that. Requiring up elevator to fly level does not by itself mean the plane is nose heavy.

Brad

Is there any formula to determine if a plane is considered light or heavy? For example, I was told the following plane was on the 'heavy side' :

Wing span 72 inches.
Wing Area 960 sq. in.
Fuselage Length 62" (Rudder to front of cowling)
Fuselage length 65" ( Rudder to front of spinner)
Weight 9 to 10 lbs

The best chart is in the building manual. They often will tell you the approximate final weight with the recommended engine. Weight will come down to wing loading and how the plane performs will depend on what that wing loading is for a given plane.

That may not answer your question, but if you look at your build manual for that aircraft, I would think that those numbers are somewhere in your manual.

Then, there is a converson chart or formula (I've seen it somewhere) that can give you the wing loading for specific aircraft wing numbers. You can bet that someone will fill in those blanks.... or go here:

http://www.csd.net/~cgadd/eflight/calcs_wingload.htm

and enter your numbers.

CGr

I flew the plane after the firewall was re-built and trimmed it out. Then added the weight and I had to retrim. It is a bit nose heavy, but not so that it sours the flying experience. The reason I want to remove just a bit of weight is because the plane lands kind of hot - really the only reason. It's not really bothering me that much - must I want to try some 3D soon so then I'm going to have to take out a bit of weight perhaps.

But thanx for the advice - I will have a look at the trim chart to see if I can sort out the problem with the knife edge. It can also perhaps help me with a couple of other things I would like to know.

A heavier airplane will handle rolling and turbulent wind currents better than a lighter airplane......when I say penetrate the wind better that's what i mean.....if for example you have a 8 lbs plane whose wing tip flys through a 5 kts updraft, the same airplane weighing in at 12 lbs in the same updraft won't be rolled as bad

Check your CG first, then if the aircraft is nose heavy, you can carefully remove weight and check the CG each time to make sure you don't shift it to far. When satisfied, fly it. Just make sure you use the same measurement point on the wing when checking your CG.

CGr.

I personally like to use "Wing Loading" as a measure of how a plane can be expected to handle. Its not the only one, but it works for me. For the numbers you provided, at the low end of the weight scale (9 lbs) it works out to 21.6 ounces per square foot. As a rule of thumb:

Gliders - 7 oz/sq ft or less
Trainers - 16-18 oz/sq ft.
Sport aerobats - 18-22 oz/sq ft.
War birds - over 22 oz/sq ft.

In general for planes that are loaded at 18 oz/sq. ft people tend to describe them as "floaters". They will take off easily, and if balanced correctly, will sink slowly in a neutral trim condition. In case its not obvious, the higher the wing loading, the faster the plane must fly (generally) to keep it in the air. Higher wing loaded planes do better in the wind, as they don't get thrown around as much, but will also stall easier, and have to have more speed on landing.

Brad