Building Light

All those construction tips about building light caused me to re-assess my
glue habit. I love to lay on the epoxy which makes for a more prang-proof
model. I decided to put my most recent model (920 sq. inch wing area) on a
scale and found it to be 6 ounces over the HIGHEST recommended weight of
8lbs. Two full containers of epoxy (epoxy plus hardener) weigh 9 ounces so
I used the equivalent of two-thirds in constructing the model. I suspect
that contributed to the "excess" weight. The effect however, was to only
raise the wing loading from 20 ounces/square foot to 21. Five-percent isn't
much so why worry about building light at the expense of strength when
the effect on wing loading is negligible. I would suggest that engines,
mufflers, covering, paint, and undercarriage components are far more
significant contributors to higher wing loadings than "light building
techniques". Now, if only V Mar would take heed!!!
Reply to
jflongworth
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Excluding the finishing process, these are "fixed" weights and only varied by choice of engine and radio equipment. The rest is up to the builder. If you're happy with building a flying brick, more power to ya'. Besides, you're gonna' need it...more power that is. It sure saves all that needless exercise picking up pieces when they just stick upon impact, huh? :o)
Reply to
Greg
Greg, the point I was trying to make was that any excess weight resulting from the building process is insignificant in terms of wing loading. Even if one pays no attention to weight saving building techniques, the additional ounces added are inconsequential in terms of additional wing loading. For example, is an extra wing load of 1 ounce per square foot really noticeable in terms of flight characteristics? Not likely, since the recommended flying weight for my model (as recommended by the manufacturer) can vary by as much as 16 ounces which I suspect is intended to accommodate variations in the components that could add weight either as part of the building process or in the form of accessories such as the engine. I'm not advocating flying bricks but rather that any weight added through the building process is not going to have any noticeable effect on flying characteristics. A more significant weight problem is poor aircraft design resulting in a nose or tail-heavy condition. It would be interesting to know just how heavy one could build in terms of wing loading without adversely affecting the model's flying characteritics. This would be a complicated calculation bearing in mind the variations in airfoils. I raise this not as a justification for building heavy but rather for the purpose of adding heavy items such as cameras. We often hear of full scale accidents where planes have exceeded the recommended weight, especially on take-off where the aircraft is only marginally over stall speed.
undercarriage
Reply to
jflongworth
Greg, the point he was making was that your use of glues has FAR less effect on the finished weight of a model than your engine selection or covering choice. I have been designing and building for 40 years and I agree.
Reply to
Paul McIntosh
overall weight is important, but where the weight is located is more important.
when I was teaching my son to fly I added 3 ounces of weight to the ends of the wing tips on a Kadet Lt40 and it really helped minimize the gust effect on those windy days, although it rolled slower when flying. (The plane was built light overall).
opinions will vary, jk
Reply to
jnkessler
I normally build and fly CL stunt so it's a matter of course to build as light as possible commensurate with adequate strength. Some time ago I built an RC model for my son based on my CL stunters. With 865 sq inches of wing and an Enya 80X it weighed 5.5 pounds and I wasn't trying too hard to keep the weight down. It flew magnificently, at low throttle and low rates it was like a trainer but at half throttle or above and high rates it was a fun fly. But the point is that I built it to FLY as well as possible, not to survive crashes. There's an article about it that I wrote for my club web page at
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Brian Hampton Adelaide, South Oz
Reply to
Brian
The point I didn't make was that broad, generalized statements, made to discredit a valid and proven building technique (verified by others in this thread), should be questioned.
While I will agree that finishing is one of the major areas of overall weight consideration, lumping the engine, muffler(s) and undercarriage into the group is to me, meaningless. When I start building, I know from the start what engine, muffler, radio equipment, landing gear and any associated mounts to safely attach these to the model I'm going to use. I also know what areas I have to reinforce for structural integrity. I consider these "fixed" weights. As far as I'm concerned, the rest is variable according to how light I want the model to build.
Giant scale builders found they could save literally pounds by choice of hardware alone. I'm sure you've taken a look at how a Q40 and Q500 is built. I doubt you'll see any competitive model in either of these groups with any extra adhesives added in the name of strength. They take more G's than the typical sport model will ever see and fly great. How about an area we have both participated...SWRA racing. Two cloned models go into a turn. One weighs 8 pounds and the other 10. Which one will exit the turn first? Which one will have the higher loading? It becomes a two-edged sword when unnecessary weight is added for the sake of "strength".
While it's true that experience levels will dictate our opinions on this and other aspects of this hobby, 35 years in the hobby have led me to a different place than you...not any better or worse, just different. Personally, I build for the most performance I can achieve from a model, not crash survivability. That's why I look at every nut, bolt, wood selection, finishing process and also, adhesive application (to an extent not to compromise safety) as a variable.
My point here is to get the most performance from your model, total weight savings is a valid point worth considering. Adding adhesives to non-critical areas where a properly made joint with CA will hold as good if not better, is, in my opinion, nothing more than a morale booster.
It all adds up in the end.
Greg
Reply to
Greg
If they enter at the same speed, they will exit at the same time.A heavier aircraft may require a higher speed in the turn.
With all due respect, your observations are akin to cutting all the legs off of a frog and telling it to jump. When it fails to do so, you concludes the frog is deaf...
Reply to
w4jle
Not necessarily. If they enter at the same time, and the ligher aircraft turns as hard as possible, right on the edge of the stall, it will definitely exit the turn first. The heavier one will exit later, and further out, having a larger turn radius, because both reach their stall angle of attack at the same actual, instantaneous wing loading.
-tih
Reply to
Tom Ivar Helbekkmo
Some editing to keep responses in order. Top posting sucks.
No, the lighter plane will always have an advantage. The heavier aircraft must generate a higher angle of attack to produce the same turn. That means more drag. Or get a larger radius turn from the same AOA. The heavier aircraft is making the same lift, but the g loading is less.
In the above example, if the 10 pound plane is making an 8 g turn, the 8 pound plane will make a 10 g turn _with the same AOA_ (Angle of Attack). Both wings would be generating 80 pounds of lift. The lighter plane will turn a lot tighter at the same control deflection, or need less deflection to generate the same turn.
Either way, the lighter plane has a big advantage. Note that even flying straight and level, the lighter plane will be faster. In the above example, it needs to generate 2 pounds less lift just flying along, and more lift equals more drag.
Reply to
John Alt
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I'm having a hard time finding the slightest bit of respect in your comment, much less any proof to back your opinion. Therefore, in like manner...
1) I like frogs and wouldn't cut off their legs. 2) Frogs aren't deaf...just some people.
Greg
Reply to
Greg
My FS Jodel climbs at 500 feet per minute on a warm day. If I add a 150 pound passenger, the increase both weight and wing loading is 12.5 percent. The climb rate drops by 40% to 300 feet per minute and the stall speed goes up by about 6 percent. Takeoff roll increases by at least 30%. Cruise speed is not noticeably reduced. Adding weight really does affect performance, and the difference is visible when you have instruments to prove it so.
Dan
Reply to
Dan Thomas
Turn radius is determined by airspeed and bank angle, nothing more. That applies to everything from models to a 747. Heavier airplanes are unable to bank steeply if they want to avoid a stall.
Dan
Reply to
Dan Thomas
Amen Dan, The angle of bank for a standard rate turn in full size aircraft follows the rule of thumb: Divide the air speed by 10 and add 50% to it. i.e.: 100 knots =10 + (1/2 of 10)= 15 degrees. 500 knots = 50 +(1/2 of 50)=75 degrees.
Weight only applies to stalling speed that would be a limiting factor in the tightest turn possible.
Reply to
w4jle
Dan Thomas said:
"w4jle" said:
OK... That's three ways of saying the same thing. Anyone want to add to the list? :-)
Getting back to the original problem (two identical planes differing only in weight entering a pylon turn at the same time), the lighter one will, as has been explained, exit first. If it turns as tightly as it can, the heavier one can't follow it. If both planes hold the same turning radius, the heavier one will be using more of its power to turn and lift, by suffering more induced drag, and thus lose more speed in the turn.
The weight difference will be observable in every conceivable maneuver. You can even deduce it by studying the two planes in straight and level flight.
-tih
Reply to
Tom Ivar Helbekkmo
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Increase the speed and turn radius will increase. I used 2 cloned models in the example and now you're changing the parameters to the heavier one being faster than the lighter one.
Greg (the frog abuser)
P.S. BTW, I did get my leg-less frog to jump tonight. Nothing a few beans wouldn't assist. Even though some would say it didn't legally qualify as a jump, he was really scootin' :o)
"ribbit"
Reply to
Greg
Nope. Pretty much covers it.
And slower to regain the speed once the turn's completed. Good points Tom.
Greg
Reply to
Greg
But the same AOA will not do for the heavier airplane as it will for the light one. The nose will need to be raised further while in the turn (with the same bank angle) to increase AOA to generate sufficient lift. Leaving the AOA unchanged will result in a larger radius caused by a skidding turn, which is uncoordinated flight, and therefore does not fit the bank/airspeed formula for turn radius. A skidding turn is also asking for a nasty stall and spin. A given airspeed/bank angle/turn radius will generate the same G forces regardless of the airplane's weight or AOA.
Cruise speed is the one performance parameter least affected by weight. Weight increases induced drag (drag caused by generating lift), and induced drag is already at a minimum in cruise. See my earlier post (#26) in this thread. Having instruments make so many things clear. Being an instructor of this stuff helps, too.
As far as the general argument goes, lighter is better if you want good performance. But light must not come at the expense of reasonable strength or minimal drag.
Dan
Reply to
Dan Thomas
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Dan, I realize you stated that speed is the least affected performance parameter of the examples you gave (and I agree) but my charts (Brake Horsepower vs. True Airspeed and Power Required vs. Speed) still say TAS is higher at a lower weight with a given HP value. In the racing format, it's one of the more important parameters we seek where even tenths of a second per lap can make a difference over 10 laps. Hence, the hair splitting and grasping for every last bit of speed no matter how insignificant it appears. Currently, we've pretty much maxed out the engine technology so that's why we look at weight reduction so closely.
Thanks for reiterating my position and especially, the safety factor I tried hard to impress within my second post.
Greg
Reply to
Greg
And there, folks, I made a mistake. Insufficient AOA in the turn will not only increase its radius, the airplane will descend as well. The descent will be much more pronounced than any skid. Too early in the morning, while thinking one thing and typing another.
Reply to
Dan Thomas

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