Scale Flight Qualities?

[GeeBeeFan]

As per usual I'm designing a new aircraft and I was wondering a few things about scale aircraft flight. How do the flying qualities of a scale model translate into a full size aircraft? I heard that if it flys well as a model the full size plane will fly well, is this true? If you know me well, you know I'm working on something that's fast, or at least Looks fast. I'm also wondering what the drawbacks of the staggerwing layout is. I seem to remember something about control effectiveness? Again for a bi-plane, would there be any negative qualities of making the upper wing thinner, smaller chord, and longer span than the lower one?? See, the design I'm working on uses the lower wing as the mounting point for the landing gear, so it has to be stronger than the upper wing. The bulk necessary for the lower wing is not necessary for the upper wing so therefore it can be smaller correct? Okay, one more Is there any rule or some such that will tell me the minimum distance between the upper and lower wings? I'm working on a small plane with both wings attached to the fuse. I guess that's all of my questions for now.

Thanks,
~GB Fan

[vintage1]

Originally posted by GeeBeeFan
As per usual I'm designing a new aircraft and I was wondering a few things about scale aircraft flight. How do the flying qualities of a scale model translate into a full size aircraft?



Some things, like the CG and the moment arm - esseintially how long the rear fuselage is - translate well.

Other thigs like wing section, don't, because the airspeed is so different, but you can't scale down gravity or air viscosity...


I heard that if it flys well as a model the full size plane will fly well, is this true?


Not necessarily, but obviously there are similaritioes, or wind tunnel models would be no good.


If you know me well, you know I'm working on something that's fast, or at least Looks fast. I'm also wondering what the drawbacks of the staggerwing layout is. I seem to remember something about control effectiveness? Again for a bi-plane, would there be any negative qualities of making the upper wing thinner, smaller chord, and longer span than the lower one?? See, the design I'm working on uses the lower wing as the mounting point for the landing gear, so it has to be stronger than the upper wing. The bulk necessary for the lower wing is not necessary for the upper wing so therefore it can be smaller correct? Okay, one more Is there any rule or some such that will tell me the minimum distance between the upper and lower wings? I'm working on a small plane with both wings attached to the fuse. I guess that's all of my questions for now.



I don't think anything is really critical in a sport model. There is some relationshipo about the distance between wings being grater than a fraction of the chord - _ seem to remember about 2/3rd chord at least distance, or the wings interfere and don't work so well.

Speed is all about the power to drag ratio, so a slim fuselage and not to blocky stubby wings will fly faster for the same power. Its a mistake to think that heavier models will fly faster. They HAVE to fly faster to stay up, thats all. That means they won';t land easy, and may snap at you.

Keep it light as you think won't break at teh speeds you want to fly, and don't use too much control movenmment on a plane that flies fast - it can get it into uncontrollable states!

A fast plane needs a coarser pitched prop than a slower one. Maybe an inch more pitch and an inch less diameter - for example a direct drive speed 400 loves a 8x4 prop for climb, but prefers a 7x5 for speed.

Use a slightly thinner wing section for speed as well. Maybe only 10% depth on teh chord section.






Thanks,
~GB Fan

[Ollie]

If you put a working strut between the wings at the landing gear attachment and wire brace between the wings with flying wires and landing wires, then the wing airfiols won't require much spar depth. This is because the landing shock forces will become tension in the bottom wing spar and compression in the top wing spar with the flying wires carrying the shear loads. In cantilevered wings (no struts or wires) the landing shock loads are transmitted through the lower wing spar alone. The big advantage of a strutted and wired structure is that the tension and compression loads are reduced by the ratio of the wing spacing to the cantilevered spar depth. This could give a ten or twenty to one reduction depending on the particular case. With strutting and bracing, you can use very thin airfoils.

The down side is that the struts and wires add lots of parasitic drag which ups the thrust requirement for a given level of performance.

Structural designs do not scale well between models and full scale. This is because, for the same materials, the weight goes up as the cube of the linear dimension while the strength only goes up as the square of the linear dimension. This is why elephants don't jump. Elephants always have at least two feet on the ground. If they jumped, they would break bones when they landed. Mice, on the other hand, can jump off a roof without hurting themselves even though their bones are made of basically the same material as the elephants'.

[GeeBeeFan]

So basically build the model, if it flies well Great! Then build the full size, if it flies well Even Better! Is that about right? I was thinking that I should put a wing strut in there but it always adds drag. If I just put a strut in and no flying wires will the wings still require the bulk to deal with the shear loads? My plan is to keep the drag down, of course, but my method was to make the lower wing the standard cantilevered wing and have the top wing be a skinny but longer wing to carry most of the flight loads. So basically, if the top wing doesn't carry any of the structural loads, why does it have to be as big as the lower wing? I guess what I'm trying to say is, do both wing sections have to be the same or similar for proper aerodymanics? About the structural issue, I never intended to build the full scale plane out of balsa, so I think I'm safe there. I think it's more an issue of having the same power to weight ratio on both examples and the should fly similarly.

So, Use a wing section that performs well on a model for the model and when it gets scaled up to full size, switch to a wing section that performs well for a 1:1 plane, correct? I think the comment about if a scale model flies well then a full size version will also fly well, but not the other way around, was talking about computer controlled aircraft, because R/C planes dont usually have the same level of control. It was more of a stability thing than a performance issue.

Thanks again,
~GB Fan

[vintage1]

well we covered stability, performance and structural strength.
On the latter, Boeings don't use much balsa wood. Nuff said.

On performance - thats more about power to weight, and that is different for full size really.

Stability maps quite closely, except around the stall, which is markedly different because teh air don't change but teh model size and speeds do.

There is also teh issue of whether you want the same stability in model and full size.

[FlyByMike]

Vintage1,
Ya know, I'm actually not noticing your teh's anymore :D
-Mike

[Ollie]

GB Fan,

If you expect to get in this thing and fly it, the risks to life and limb will be much greater than the risk of crashing a newly designed R/C model. You need to give yourself a course in aircraft structural engineering to reduce the risk of suicide or negligent homicide depending on who is to fly it.

[GeeBeeFan]

Well I know, I'm not speaking exactly about this aircraft, well I guess I am acctually. Eventually I want to build my own high speed aircraft and I'm just wondering about the scaling properties of aircraft so I dont have to spend thousands of dollars on a full size plane that doesn't perform! I'd rather build a scale test aircraft to verify the design. I guess I'll just have to build it and see!

Thanks again,
~GB Fan