How do we define this term? - RC Groups
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Oct 13, 2017, 08:55 AM
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How do we define this term?


While the pot of controversy is simmering gently, I can't resist the opportunity of throwing in a few herbs and stirring it gently.

A question to ponder. "What do we mean when we declare that an aeroplane is 'over-weight'?"




I can think of various criteria, any of which might me lead me to say, "That aeroplane is over-weight for that particular task."

But, I can't think of an inclusive statement that would embrace all these different elements.
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Oct 13, 2017, 09:55 AM
An itch?. Scratch build.
eflightray's Avatar
'Over weight' for a model probably means it came out heavier than the designers expected weight.

For full size aircraft, it could depend on what and were the aircraft is --
https://en.wikipedia.org/wiki/Aircraft_gross_weight --- taxi weight, take off weight, landing weight, flight weight, etc

To me it usually means it's IC powered.


Ray.
Oct 13, 2017, 10:42 AM
San Antonio TX.
sensei's Avatar
The term that model airplane is overweight is simple, if you have a model that should have lets say a wing loading range of 25-35 ounces and it comes in at the 60 -90 ounce range, it is overweight. I have witnessed many overweight RC airplanes at places like Bomber Field here in Texas, I watched them tip stall and crash during things like slow turns, when applying elevator input during throttle off, and during engine out conditions. They land much to hot of a landing speed for the average sport pilot and are usually destine for a short lifespan. Most all I have seen are 80" spans or smaller, and again... The smaller they are, the less efficiently they carry weight. (Reynolds numbers you know).

Here is an example of a lightweight jet, pay attention to the flight but mostly the landing, it tells the story.

https://redirect.viglink.com/?format...h%C3%A9%202017

Bob
Last edited by sensei; Oct 13, 2017 at 12:52 PM.
Oct 13, 2017, 12:50 PM
Registered User
ShoeDLG's Avatar
Quote:
Originally Posted by bogbeagle
"What do we mean when we declare that an aeroplane is 'over-weight'?"
Suppose you have a given design. What effects would you expect from adding weight to it?

- Reduced acceleration/deceleration
- Increased takeoff, landing and stall speeds
- Longer takeoff and landing distances
- Reduced climb rate
- Reduced available load factor for the same structural stresses (depending on how the additional weight is distributed)
- Higher landing gear stresses
- Higher cruise airspeed (higher airspeed to operate at L/D max)
- Reduced range and endurance
- Larger moments of inertia (depending on how the additional weight is distributed)
- Lower service ceiling

All of these represent reduced performance or contribute to degraded handling qualities. Any one of them, could be enough to label an aircraft ‘overweight’.

Are there any potential benefits from adding weight? If you wanted to reduce the acceleration in response to gusts, you could add weight, but you could also achieve this by slowing down (if that was consistent with your mission). Gliders sometimes add ballast to achieve better penetration (higher airspeed at the same glide ratio - at the expense of lower climb rate in thermals). This is typically done when going cross country or traversing between thermals in strong winds.
Oct 13, 2017, 02:33 PM
Registered User
Add "Increased turn radius" to the list
Oct 13, 2017, 09:27 PM
Registered User

A Tale of Two Models.


This is from a sport flier's perspective, not a pattern competitor.

When I returned to RC flying this year after a 26 year layoff, my first plane was a 13 oz Ares Decathlon 350 trainer. It has a stabilization system, which I credit with keeping it right side up while I rebuilt my flying skills without benefit of an instructor. Its wing loading is just under 12 oz/sq ft, which is higher than many ultra-micros but low enough to stall and land quite slowly. It's a very nice model to practice approaches with, although on my grass field touch and goes are out of the question with the small size and rearward gear placement.

However, once the wind gets over about 5kts and thermals start popping, the Decathlon turns into a handful. It reacts to every gust, and the stabilization system starts becoming necessary for a respectable landing approach. I don't think I'd want to fly it in winds over about 10kt.

While building time with the Decathlon, I converted my ca. 1987 Great Planes Super Sportster 20 to electric power. The result tips the scales at 62 oz, with a wing loading just over 22 oz/sq ft. This is a pretty clean airplane, and has to be flown precisely to land at slow speed after clearing the tall weeds at the approach end of the runway. It would benefit from flaps, though I'm getting more comfortable with it as we get reacquainted.

However, once the wind begins to blow, the old beast comes into its own. It's very solid feeling, laughing at air pockets that would make the Decathlon crazy. With the wind down the runway the approach is much steeper and easily controllable. I added a stabilization system, but it's never necessary, and hardly noticeable unless conditions are very gusty.
Oct 14, 2017, 01:03 PM
San Antonio TX.
sensei's Avatar
Quote:
Originally Posted by jruley
This is from a sport flier's perspective, not a pattern competitor.

When I returned to RC flying this year after a 26 year layoff, my first plane was a 13 oz Ares Decathlon 350 trainer. It has a stabilization system, which I credit with keeping it right side up while I rebuilt my flying skills without benefit of an instructor. Its wing loading is just under 12 oz/sq ft, which is higher than many ultra-micros but low enough to stall and land quite slowly. It's a very nice model to practice approaches with, although on my grass field touch and goes are out of the question with the small size and rearward gear placement.

However, once the wind gets over about 5kts and thermals start popping, the Decathlon turns into a handful. It reacts to every gust, and the stabilization system starts becoming necessary for a respectable landing approach. I don't think I'd want to fly it in winds over about 10kt.

While building time with the Decathlon, I converted my ca. 1987 Great Planes Super Sportster 20 to electric power. The result tips the scales at 62 oz, with a wing loading just over 22 oz/sq ft. This is a pretty clean airplane, and has to be flown precisely to land at slow speed after clearing the tall weeds at the approach end of the runway. It would benefit from flaps, though I'm getting more comfortable with it as we get reacquainted.

However, once the wind begins to blow, the old beast comes into its own. It's very solid feeling, laughing at air pockets that would make the Decathlon crazy. With the wind down the runway the approach is much steeper and easily controllable. I added a stabilization system, but it's never necessary, and hardly noticeable unless conditions are very gusty.
Lets see, the Super Sportster 20 has a wing area of 383 sq. in. and you weigh 62 ozs. that would be a wing loading of like 23.3, but more importunely it has a cubed loading of 14.3. That kind of loading places you out of the sport category and more in the racing category doesn't it. That airplane with a 48" and span and airfoil type and your Decathlon with it's 29" span and completely different airfoil type is like comparing apples to oranges. Come on, you got to have something better than that to throw at us...

Bob
Oct 14, 2017, 01:44 PM
Registered User
Quote:
Originally Posted by sensei
Lets see, the Super Sportster 20 has a wing area of 383 sq. in. and you weigh 62 ozs. that would be a wing loading of like 23.3, but more importunely it has a cubed loading of 14.3.
The original plans give the wing area as 400 sq in. Using that number I get wing loading of 22.32 and cube loading of 13.39.


Quote:
Originally Posted by sensei
That kind of loading places you out of the sport category and more in the racing category doesn't it.
Haven't the faintest idea; I've never seen these categories defined. Why are they significant for sport fliers?

Quote:
Originally Posted by sensei
That airplane with a 48" and span and airfoil type and your Decathlon with it's 29" span and completely different airfoil type is like comparing apples to oranges.
The Decathlon has a 29.5" span and wing area of 152 sq in. Wing loading 11.94, cube loading 11.62 The cube loading isn't that much lower than the Sportster.

The Decathlon is slower due to its higher drag and undercambered airfoil, but both airplanes will climb almost vertically at full throttle. The Decathlon is fully aerobatic, although it handles differently due to the high wing, different airfoil and greater dihedral effect.

The wing loading is evidently a better predictor than the cube loading of the response due to turbulence.
Oct 14, 2017, 02:26 PM
An itch?. Scratch build.
eflightray's Avatar

Over-weight ?


It's a visual thing.

Most clubs have a few members who could be classified as 'over-weight' just by looking at them.

When it comes to model planes, you watch someone pick up someone else's plane, if they shake their head, ........ it's over-weight.


Ray.
Oct 14, 2017, 03:57 PM
San Antonio TX.
sensei's Avatar
Actually, it's a flying thing, when you take two exact airplanes as I have done several times, but in this particular case one is 30% lighter equaling 12 lbs. you can easily feel the difference in flight.

Bob
Oct 14, 2017, 04:19 PM
B for Bruce
BMatthews's Avatar
The question is likely also compounded by the idea that the "ideal" weight for any given model's mission will peak out on a bell curve. Values close to the optimum will be close enough. But as wel move either way the slope of the curve of wing loading vs desirability will steepen. And likely not in a symmetrical manner. And as the other threads have shown each person's "optimum" varies by some amount.

Now I'd be the first to admit that I'm highly biased on the topic. And while I can see that a moderately heavier wing loading is a good idea in some cases it's highly likely by inference that my idea of "optimally heavy" would not jive with what others in that style of flying consider "optimum".

I recall one particular fellow that built a Telemaster shaped model from the plans but that "hardened" the model up by a HUGE amount. Like can you imagine a 40 size Telemaster that weighed something like 12 lbs or more? I don't recall the exact figure. But it was IMPRESSIVE! ! ! ! Even more shocking was the lump of lead in the nose!

But in compensation for this he also extended the wing by one rib bay each side........ He said he'd done this deliberately to use it as a trainer for larger size heavy scale models...... Needless to say it didn't last long. I was there for the first and only three flights and it was pretty obvious that it was a crash looking for a nesting site. And it didn't dissappoint. Stalled in a steep'ish turn and snapped towards the ground. In trying to lift the nose before impact the G load overwhelmed the wing again and it flipped over and planted itself in a most grand style.

But that was WAY down the "too heavy" side of the bell curve.
Oct 14, 2017, 04:27 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by eflightray
It's a visual thing.

Most clubs have a few members who could be classified as 'over-weight' just by looking at them.

When it comes to model planes, you watch someone pick up someone else's plane, if they shake their head, ........ it's over-weight.


Ray.
Quote:
Originally Posted by sensei
Actually, it's a flying thing, when you take two exact airplanes as I have done several times, but in this particular case one is 30% lighter equaling 12 lbs. you can easily feel the difference in flight.

Bob
Ray, you need to end your posts like this one with a Vaudeville drum "Baddaboomp-CHING!" smiley.... For folks like Sensei....



Oddly enough for all that I preach about lighter is righter there's one model I've got which turned out better for a weight increase. Years ago I built a highly modified RO-8 glider with an Eppler 201 airfoil since the stock ribs in the kit looked like they were chewed out by a pet gerbil. And in truth were best used as fodder for the gerbil's cage. The glider came in at 34oz. It did OK and was better than what I'd had so I figured "job done". Some years later in the age of brushed electrics I built a new fuselage for the wings and tail. Given the motor and battery tech of the day it came out to 54 or 56oz. But this increase from around 6oz/sq-ft to more like 8.something turned out to be just what that airfoil needed. The heavier electric version is FAR more fun AND effective to fly. The slow speed side of the envelope was raised only slightly. But the higher speed side opened up tremendously IN THIS CASE.

So I'll eat enough crow to admit that there is a "too light" side of the bell curve for optimum weight for any given model and task for that model. And with hard work and determination we can build a model that is TOO light.
Oct 14, 2017, 06:04 PM
Registered User
Not sure about this cubic-loading malarkey.

Where'd it come from? Is it a proper engineering thing?

I did the sums ... a while back ... for a laden Mustang (full-sized), and the answer was something outrageous.

Coulda got the sums wrong, tho.
Oct 14, 2017, 08:24 PM
Registered User
Quote:
Originally Posted by bogbeagle
Not sure about this cubic-loading malarkey.

Where'd it come from? Is it a proper engineering thing?
I think it's a model airplane thing, trying to account for differences in size as well as wing loading.

Never encountered it in a full-scale aeronautical engineering context. That's not to say someone else didn't, though.

Wing loading (and maximum lift coefficient) tells you stall speed. I'm not sure what cube loading is supposed to tell you.
Oct 14, 2017, 09:19 PM
B for Bruce
BMatthews's Avatar
I know that the cubic thing works up to stuff like Piper Cubs and Cessna 150 to describe how such planes fly in normal use and compared to other similar size planes.

A FULLY loaded Mustang? As in full internal tanks plus full ammo load AND drop tanks? From what I've read of them the full load out takeoffs were nervous things indeed with shallow climb angles and an ever watchful eye on the airspeed. It wasn't until well into the escort flight and mucho fuel had burned off that they got their spirited performance back. So the value you got might have made sense.

For "point defense" intercept missions they would likely be filled only part way to enhance climb performance to intercept knowing that the overall flight duration would be short.

.... or you did the math wrong....


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