Former Member Posted February 22, 2018 Author Share Posted February 22, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Former Member Posted February 22, 2018 Author Share Posted February 22, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Former Member Posted February 22, 2018 Author Share Posted February 22, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Former Member Posted February 22, 2018 Author Share Posted February 22, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
The Wright Stuff Posted February 22, 2018 Share Posted February 22, 2018 Interesting David. Just to prove that I am still following with interest, do you know from the book what the actual definition of 'cruising speed' is in this context? Obviously, there are various definitions based on cruising altitude, maximum efficiency, etc, but I wondered whether there was a unique aerodynamic definition (presumably based on a particular point of the lift coefficient curve). I ask because basing it purely on fuel efficiency doesn't make sense in the diagram, because (presumably) this is dependent on the thrust (number and type of engines), which doesn't affect either the weight or the wingspan in any fixed way... Quote Link to comment Share on other sites More sharing options...
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Former Member Posted February 22, 2018 Author Share Posted February 22, 2018 [This posting has been removed] Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted February 22, 2018 Share Posted February 22, 2018 I was just becoming comfortable with that graph Heavier and lighter, larger and small wings all following a theme And now this is questioned again I was happy in my ignorance with the graph Quote Link to comment Share on other sites More sharing options...
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John Stainforth Posted February 22, 2018 Share Posted February 22, 2018 Many thanks to all for the feedback. I gave the wrong units for the cubic wing loading - I meant to say kg/m3 not g/cc. I could just as well have used imperial units, but I am a big fan of metric and air has a density of 1.225 kg/m3, so these metric units are rather easy to compare with that, e.g. a cwl of 5 kg/m3 means an aircraft that has an effective overall density about 4 times greater than air. I also made a small error in my approximation for the wing area of fighter jets in that I did not allow for their sweepback, so the cubic wing loadings are somewhat exaggerated - they should be reduced by about the sine of the angle of sweepback. I have used logarithmic scales for both the weight (actually mass) and wingspan axes of the plot. The equations of the straight lines are given by the power law M = cL^3, where M is the mass of the aircraft and L a measure of its length scale and c is the cubic wing loading, which has units of density. So the logarithmic relationship is log M = log c + 3 log L David, when I said that cubic wing loading does not have to be scaled, I only meant this was broadly true, and it certainly does not mean that we should give our scale models the same cubic wing loadings as their full-size counterparts. I agree with you about this: models of the heavy wing loaded fullsize planes would not be much fun to fly and would have to be flown unrealistically fast to stay in the air. My Hanger 9 Spitfire is one of those red dots on my graph and it has less than half the cubic wing loading of a fully laden full-size Mk 1 Spitfire. (The later marks of Spitfire had increasingly higher wing loadings). Likewise, I am building my model S6b to have less than half the cubic wing loading of the fullsize. Quote Link to comment Share on other sites More sharing options...
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Steve Dunne Posted February 22, 2018 Share Posted February 22, 2018 Just to clear my confusion, are the cubic units ranges metric units of 2, 4, 8 and 16 gm/cc, or imperial units of 2, 4, 8 and 16 oz/cu foot, as discussed on previous posts? Posted by John Stainforth on 22/02/2018 15:58:03: ...I gave the wrong units for the cubic wing loading - I meant to say kg/m3 not g/cc. 3,... Thanks, John - that makes sense as kg/m3 are the same as oz/cu foot to about four significant figures, for all practical purposes interchangeable. Again - excellent information and very useful. I might just do a little retrospective calculations on my models... Steve. Quote Link to comment Share on other sites More sharing options...
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Geoff S Posted February 23, 2018 Share Posted February 23, 2018 It's interesting that a 747 has a power to weight ratio of fewer than 100 watts/lb. My electric models have the same ratio but they're aerobatic at that. I don't think the payload would appreciate a few rolls and loops on the trip from Heathrow to New York! Geoff Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted February 23, 2018 Share Posted February 23, 2018 Posted by David Mellor on 23/02/2018 15:48:59: I have a question... we know that the smaller the plane the harder it has to work to get lift. All other things being equal, the choice is to make it lighter or accept a higher (than scale) cruising speed. So why, then, don't we need proportionately more power (compared to full sized aircraft) as our models get smaller? Dave In amongst the exponential and logarithmic considerations detailed above, does drag increasing with the square of speed have anything to do with it? Edited By Martin Harris on 23/02/2018 16:07:55 Quote Link to comment Share on other sites More sharing options...
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Jez Saunders Posted February 23, 2018 Share Posted February 23, 2018 Going straight to structural integrity we are lucky here that we can get away with a "weaker" structure, eg a mouse has speed and agility to get away from a predator , if an elephant had the same scale speed and agility it would break it's neck at the first evasive manover. .? That is unless someone know different ? Quote Link to comment Share on other sites More sharing options...
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Jez Saunders Posted February 23, 2018 Share Posted February 23, 2018 Thanks David, not really into the maths but going to your models of equal wing loading but different inertia characteristics problem it most be the difference in mass ? Perhaps the mouse and elephant example could be applied here ? Quote Link to comment Share on other sites More sharing options...
John Stainforth Posted February 23, 2018 Share Posted February 23, 2018 David et al, I also plotted empirical data of engine size (cc) versus aircraft wingspan and weight for some fullsize and model planes: The size of engine does go up roughly cubically with the wing span, and roughly linearly with aircraft weight. The slope of the line in the last plot is roughly 1.3 cc per lb of aircraft weight. The power of piston engines increases linearly with swept volume very roughly at about 1 hp per 10 cc (although 2-stroke engines are quite a lot better than that), so the line in the second plot also represents about 0.13 hp/lb, which lo and behold is about 100 W/lb -again about the same as a Boeing 747! The more powerful models and fullsize piston planes have about twice that power to weight ratio. Scaling of physical laws is indeed and intriguing subject. Quote Link to comment Share on other sites More sharing options...
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Denis Watkins Posted February 23, 2018 Share Posted February 23, 2018 Many many years ago John, my dad and I graphed our relatively small planes and the average in cubic inches was Almost exactly your 1.3cc/lb, at .08cu inches/lb Whereby a 6lb model was flown on a.46 2 stroke, in round figures, 6 x .08 = .48 This data came about from just 5 models So I find your work and Davids quite amazing. Quote Link to comment Share on other sites More sharing options...
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