Tom Mann Posted May 22, 2007 Share Posted May 22, 2007 Help me out educated masses. Take a full size Cessna 150 (Stall speed (dirty) ~50kts, wingspan ~10m).Why won't a 10cm wingspan model with the SAME wing loading as the full size fly at 0.5kts (about 25cm/second)?I can only think that the relationship of lift to wing span or area is not linear.Can anyone explain?tom Quote Link to comment Share on other sites More sharing options...
Phil Meredith Posted May 22, 2007 Share Posted May 22, 2007 errr ummm ????i think this was explaine i a recent mag i have wilphill see if i can find it Quote Link to comment Share on other sites More sharing options...
Mike Rolls Posted May 22, 2007 Share Posted May 22, 2007 Mr. Reynolds is the culprit. The 1/100 scale (or whatever) model is operating far less efficiently than the full size.Mike Quote Link to comment Share on other sites More sharing options...
Myron Beaumont Posted May 22, 2007 Share Posted May 22, 2007 Ever heard of Reynolds numbers ? Quote Link to comment Share on other sites More sharing options...
Mike Rolls Posted May 22, 2007 Share Posted May 22, 2007 BTWIf the wing loading is the same the envelope for flying spped will be roughly (and I mean roughly) the same - so your 1/100 scale model would still stall (theoretically and ignoring efficiency) at the same 50 mph.Mike Quote Link to comment Share on other sites More sharing options...
Nick Chudley Posted May 23, 2007 Share Posted May 23, 2007 That's right Mike. You've got to fly fast enough so that lift = the weight of the plane. If you could build one where the wings were full size and the weight was about 1/100, then you could have a stall speed of 0.5 mph, in theory. Have a look at a model called a mini I.F.O. at www.wildrc.com. It weighs next to nothing and has a sizeable wing area. Brilliant fun to fly. Even on windy days as long as you don't mind climbing trees to get it back. Quote Link to comment Share on other sites More sharing options...
Tom Mann Posted May 23, 2007 Author Share Posted May 23, 2007 OK thanks all for responses!Mike - Wing loading is relationship between weight and wing area right? So if 10cm wing, the same wing loading as full size would require very light weight to make the same ratio. I can't believe it would need to fly at min 50mph to stay in the air?I looked at Reynolds number - it seems that you're saying that it is the turbulence of the flow at this small size that disproportionately knackers the lift on a baby model.Apparently there's something else about moment arms that is non linear i.e. it is disproportionately easy to roll a small wingspan, hence the 'twitchyness' of small models (and difficulty in flying in a smooth scale manner). This could be cured a bit with clever avionics, it's just the airspeed issue that seems insurmountable.How are we gonna crack it? More wing area for same weight? Triplanes? Ultra light weight must be the first target shirley.I've seen those fairy-liquid dipped efforts flying indoors at v slow speeds -and then there's the plane that man pedalled across the channel - lots of area, low weight. Quote Link to comment Share on other sites More sharing options...
Mike Rolls Posted May 24, 2007 Share Posted May 24, 2007 TomSorry - we may be at cross purposes. You said the same wing loading. Wing loadign is a simple number, as you surmise, weight divided by area, so a model with, say, 2 square feet of wing weighing 2 lbs will have the same wing loading as a model with 3 square feet weighing 3 pounds, and so on. A Piper Cub probably has a wing loading of 7-8 pounds per square foot. (L-4 has 169 square feet of wing area and a max all up weight of 1260 pounds). A 1/100 scale model would have only 1/10,000 of that area or 2.4 square inches, but to have the same wing loading would weigh 2 ounces. Ignoring the fact that such a tiny a/c would be far less efficient than the full size it would still need to fly at the same speed.Reynolds - the number is effectively an expression of three things - the airspeed, the chord of the surface, and the density of the air. For practical purposes we can ignore number three - it is the first two that work in favour of big a/c as opposed to small ones. If you look at the data published about aerofoils you will find in some instances a 'critical Reynolds number' below which the characteristics of the section can be markedly different - always (as far as I am aware) sigifying reduced effeciency over and above tha lost from simple scaling down.Small models tend to get thrown about more than big ones simply because they are seeing, in effect, proportionately greater forces acting upon them. It's simple physics, and why a BA 737 can cope with much more turbulent conditions than a small model.HTHMike Quote Link to comment Share on other sites More sharing options...
Alistair Taylor Posted May 24, 2007 Share Posted May 24, 2007 If you could scale down the air molecules to 1/100 you'd be getting somewhere.....wikipedia has lengthy explanation, but in modellers terms (not that I'm saying we're a bunch of simpletons), if you scale the aeroplane down to 1/100, you have to do something to the medium it is moving in to maintain the same reynolds number in order to have it perform the same as the full size.The example wikipedia uses is a 1/13th scale model of an aeroplane, which would perform almost exactly at the scale speed etc of the original aeroplane, if it were "flown" in water (!). The reason being that water has almost exactly 1/13 of the reynolds number of air.i.e. in a "water tunnel" (moist equivalent of wind tunnel), the 1/13th scale model would take off/stall/roll etc at 1/13th the speed of the full size.If flown in air, the 1/13th model would need to fly at four times the speed to achieve the same Reynolds number - hence the reason fun fighters are so ballistic - they have to be.AlistairT Quote Link to comment Share on other sites More sharing options...
Tom Mann Posted May 29, 2007 Author Share Posted May 29, 2007 Thanks ever so much Mike and Alistair. Right then. We're stuck with air so ditch that factor. From your excellent explanations we need ultra light aircraft with large wing area and large chord.This fits the description of a nippy little foam number I got for my son recently but still isn't front room material.Biplanes have twice the area but presumably drag and interference between the wings means you don't get double the lift - however you must get more. So a biplane or even a triplane is probably the way to go? Has anyone seeen or attempted a really tiny bi/triplane fitting this admittedly ugly description (a delta biplane???). Really we need about 0.5m/s tops for front-room flying.tom Quote Link to comment Share on other sites More sharing options...
Richard Treherne Posted May 29, 2007 Share Posted May 29, 2007 Hi TomI really do get lost with all the physics involved with tiny models, but have you seen the offering from silverlit (picoo) fame? It's called Palm Z and due out in June. Looks like it could fit the requirements for indoor flying. Quote Link to comment Share on other sites More sharing options...
Tom Mann Posted May 29, 2007 Author Share Posted May 29, 2007 Spot-on - an ultralight biplane!I'll find out the flying speed and post heret Quote Link to comment Share on other sites More sharing options...
Tom Mann Posted June 2, 2007 Author Share Posted June 2, 2007 OK here it is on video. Sitting right on the stall the whole time and almost slow enough!!http://www.youtube.com/watch?v=ntSLZ8w1YHY Quote Link to comment Share on other sites More sharing options...
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