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Larger planes more stale?

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Old 10-30-2019, 04:02 PM
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obrien135
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Default Larger planes more stable?

Not taking into account wind conditions why are larger airplanes more stable in response to control inputs than smaller ones? Maybe this only applies to trainers that's all I'm familiar with. I know some plans are designed to be super responsive but I'm talking aboutIwhy is a sport Cub s so hard to control in the advanced mode compared to an apprentice as I'm told? Is it because they are overpowered or too light for their size or shape or form the amount of travel I their control surfaces?

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Old 10-30-2019, 07:55 PM
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speedracerntrixie
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To greatly simplify the explanation, the biggest factor is called Reynolds Effect. The more air molecules that are on a wing at any given time the more efficient that wing is going to be. A wing can be shrunk down but an air molecule can't. Mass has a big factor as well. I also preach ( limited success ) that the way an airplane is set up has a huge hand in the way it flies. Think of your airplane as a symphony. All parts have to be in harmony with one another. This is where thrust angles, wing and stabilizer incidences, center of gravity and such come into play. IMO everyone should learn to manipulate these things to get the desired flight charictaristics. IMO the old " she took just a couple clicks of trim to fly perfectly " just doesn't cut it, it just shows limited knowledge.
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Old 10-31-2019, 01:59 AM
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obrien135
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Default Wing loading?

Is wing loading the square or the square root of the size of the plane or the airfoil? Does this affect the mass for size of the plane in a way that affects the stability to control input? I have a book on model airplane design but I can't find it
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Old 10-31-2019, 05:50 AM
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obrien135
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Default Apprentice compared to sport Cub s

I believe the sport Cub s is about 2 oz and 24 in wide the apprentices according to the internet 49 Oz and probably two or three times as wide so it's got more mass per size is that how stability increases with an airplane or controllability? because I hear an apprentice is easier to control than a sport Cub s because it's bigger. Is that even true?
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Old 10-31-2019, 06:15 AM
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Wing loading is the weight the wing has to carry. Usually described for our models is ounces per square foot. Mass is different. The Cub and the Apprentice may in fact have similar wing loading but the Apprentice flies better because of a higher Reynolds Effect and higher mass. The Apprentice also has a more rigid structure that maintains those all important sdjustments better.
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Old 10-31-2019, 06:19 AM
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obrien135
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Default Apprentice compared to sport Cub s

I believe the sport Cub s is about 2 oz and 24 in wide the apprentices according to the internet 49 Oz and probably two or three times as wide so it's got more mass per size is that how stability increases with an airplane or controllability? because I hear an apprentice is easier to control than a sport Cub s because it's bigger. Is that even true?
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Old 10-31-2019, 06:48 AM
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Yes, an Apprentice would be much easier to fly then the Sport Cub. I once had one of the UMX P-51 airplanes and found it quite a challenge to fly. It wasn't that it was difficult, it just wouldn't hold a line and that increased the pilot workload. A biginner has a tough enough time with planned control inputs let alone a multitude of correction inputs. I have followed you threads for a while now, a good plan of action would be to invest in the Apprentice and a club membership. It appears you are trying to learn to fly without any basic aeronautics knowledge. Having that knowledge would make the process easier. A good club instructor will not only teach you the mechanics of flying but should also teach you the how and why.
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Old 10-31-2019, 07:19 PM
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Another part of the "bigger flies better" issue is dealing with turbulence. Air is very rarely still or consistent. There are always currents and/or thermals, little swirls, and pockets of turbulence. So let's say you fly through a small horizontal swirl of air and the edge of it pushes on your wing. In a small plane, maybe 1/3 of your wing feels that push, and there is little resistance to it from inertia or.wing area. You'll see the result as a quick roll that you have to correct. On a plane twice as big and 3 times the weight, maybe only 1/5 of the wing area is affected and it's a much smaller force compared to the wing's ability to resist moving, so it's seen as only a little bump.
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Old 11-05-2019, 05:14 AM
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As a full scale pilot, I can assure anyone wondering, that bigger flies "different" beyond any doubt! "Better" has too many variables involved. What YOU consider better may be something others might disagree with.

I really just wanted to add though, that smaller stuff can feel MUCH bigger when equipped with a decent stabilizer that's set up correctly. IMHO, that's one of the better reasons to do that (add the extra weight and complexity). Planes that are all over the place in the lightest cross wind can be MUCH more docile/solid feeling (like a bigger plane) on landings for instance , making their pilots look much better than they really are. Too, they can sometimes allow flying in conditions that leave others on the bench. If nothing else, they can give a pilot the extra degree of confidence that will allow him to at least give it a try. -Al

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Old 11-08-2019, 08:16 AM
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"BIG" planes are NOT more stable by virtue of size.
Though the comments about scale effect, or "Reynold's Number" are true, any RC plane flying more than say....15 miles per hour with a 12 inch cord wing is not flying in the dreaded "Low Reynolds's" number region. By this I mean, all the really nasty persistent laminar flow and separation bubble effects mostly happen with slower planes that don't have intentional or inadvertent turbulation.... (The effects can happen up to 500,000'ish Rn... but are much less likely especially with the manner our wings are fabricated.)
What you are more than likely experiencing with a large model is improved DAMPING!
If a plane has high enough damping, you can easily fly an unstable airplane.
Damping: Tendency for a disturbance, in pitch, yaw and roll, to be quickly stopped. Aerodynamic forces easily and substantially overcome inertia.... with low tendency to oscillate.
Add to the improved damping the fact you can see a large plane better.....

IMHO
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Old 11-08-2019, 08:38 AM
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Mike, I agree but unfortunately I have brought that up many times. I have tried to explain that the damping is directly related to having all your setting in tune with one another. Thrust angles, wing incidences, CG, aileron differential etc are the tings that should be working together. Most of the time I simply get crickets on the other end or arguments about how a certain designer didn't have right thrust etc on the plans so it's obvious the airplane doesn't need it. I'm almost convinced that guys don't want to learn.
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Old 11-09-2019, 08:34 AM
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Speedracer has a good point about looking at the aircraft as a system where all the parts work together and have an impact on one another. One aspect of the size issue is how tolerances impact the performance of the aircraft. Through the years I have had several 1/12 scale warbirds. These were generally small airplanes with a wingspan of three to three and a half feet. When everything was set up just right they were as stable as the big warbirds. Where size come into this is that 1/32 of an inch misalignment doesn't impact a large airplane as much as it does a small airplane. Control surface gaps, flying surface alignment, slop in the linkage all add up to have a bigger impact on small airplanes just because the measurements involved add up to a larger percentage of the dimensions of the aircraft and its control throws. You can build a stable, easy flying small airplane but you have to pay closer attention to the tolerances. An airplane with tight tolerances is easier to trim. Another aspect of size and stability is the pilots perception. A larger airplane provides more visual feedback to the pilot. The pilot can perceive and respond to small changes in the aircraft's direction of flight more easily with a larger model. As a result the pilot responds more quickly and with smaller control movements. There is less divergence from the original flight path and thus a perception greater stability even when both the larger and smaller aircraft are capable of flying "hands off" for the same length of time.

In re-reading the original post I think the answer is a lot simpler. When the switch is in beginner mode the little box in the airplane does a lot of the work for you. It can respond more quickly and with more sensitivity than most pilots giving the impression that the airplane is more stable than it would be without the box.
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Old 11-09-2019, 01:48 PM
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I agree with you, Speed, 100%. Setup is the biggest factor. I bought a Sweetater to fly SPA with a couple of years ago. The guy I bought it from said it didn't fly well and gave me a good price on it. I figured I was pretty good at flight trimming, so I could probably get it at least tolerable. He had the CG at the neutral point where the plane would hold an inverted 45 degree upline. I moved it 1/4 inch forward of that for my first day of flying it to be safe, and found the plane got tossed around like a sheet of paper in the turbulent Texas wind. It was also snappy in the stall tests I did, and needed me on the elevator constantly on landings to maintain a good glide. I would up moving the CG forward by 3/4 inch, a pretty big change on a 12 inch chord wing. That settled the plane right down and made it a joy to fly. Yes, it needs a mix to do knife edge flight properly and requires down elevator to hold an inverted line now, but I am totally cool with that if I means I'm not constantly chasing the plane throughout the flight.
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Old 11-09-2019, 04:50 PM
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obrien135
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Default I see what you mean

it seems like whenever I try to fly a plane without stabilization and he starts thick movement causes it to go into a spin or a diving out of control . It doesn't seem to have anything to do with wind fluctuations. They've all been small planes up until now
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Old 11-10-2019, 08:54 AM
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Originally Posted by obrien135 View Post
it seems like whenever I try to fly a plane without stabilization and he starts thick movement causes it to go into a spin or a diving out of control . It doesn't seem to have anything to do with wind fluctuations. They've all been small planes up until now
Obrien, from reading your post and considering what others have said it sounds like you airplanes may be set up with the balance point (commonly called center of gravity or CG) a little far back for your level of flying experience. When the stabilization is on, it may be covering up an aft CG condition. Hold the airplane so that the weight of the fuselage is below the wing. Find the point where it balances with the battery in the airplane. There should be a balance point shown in the manual. For stability you will want the airplane to balance at, or just in front of that balance point. This is one of those things that is easy to over do. The saying goes that a nose heavy airplane flies poorly and a tail heavy plane doesn't fly long at all. If you do need to adjust the balance, do it in small increments.
As I type this it also occurs to me that the stability assist may be covering up an issue with over control. Many of us, myself included, as we are learning to fly end up chasing the airplane, responding to its movements with larger than needed movements of the controls. It can be difficult to remember as the airplane is darting around the sky to make smooth small corrections with the sticks. At least some versions of stability assist will limit the movement of the control surfaces no matter how much you move the sticks. As a result you may be overcontrolling and it doesn't show up until you turn off the stability assist.
Speed racer is giving good advice. Even if you have to drive an hour to get there, interaction with a local club and feedback from other fliers can be immensely useful. Just having a set of unoccupied eyeballs on the airplane while you focus on flying can provide a lot of useful feedback.
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Old 11-10-2019, 09:18 AM
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Thank you very much for those this is all been very helpful
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Old 11-12-2019, 01:39 PM
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Much of why bigger flies like it is more stable has to do with the moments of intertia. Small planes need lower wing loadings to fly well so they tend to be very light. And what weight they do have is carried in close to the true Center of Mass. Take a long broom handle that might weigh 1 lb. Then take a short 2x4 that also weighs 1 lb. Hold each at one end and swing them back and forth. You'll find that the short 1lb 2x4 is easier to speed up and easier to stop than the broom handle.

It's the same thing with big models. Even when built lightly such as is the case with 6ft and bigger old time free flight models adapted to RC they respond to control inputs with a slower and more stately motion than small flitty models around 3ft span.

All that other stuff mentioned also enters into it as well. But even with two wildly aerobatic models the smaller one will be more "nervous" than the bigger one due to this mass-moment issue.
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