Bob Llewellyn

The bulge below the centre section was developed from theory in the 1950s, as discussed in Stinton's Anatomy of the Aeroplane (the other book); if you look, you will see that the airfoil is also inverted at the root. Compressibility, it's all about compressibility, and pre-dates CAD FEM by a while.

I wonder how much of the Jabiru cooling story is told by the pressure field around the cooling outlet(s)? Given the std jab cowl outlet location, even between two "identical" Jabs, a slight difference in wing static incidence or control surface rigging could produce a substantial change in outlet zone static pressure; as could a small change in climb speed or climb weight. Moving the outlet rearwards on a Glassair definitely improves things... "Faster flying aircraft" are also low drag, so one would not expect large static pressure fields near the front. Perhaps the simple solution is to cut the noseleg off all Jabirus? Is Thruster.... is good! There is no accident in the position of the Hawker Sea Fury cowl outlets, ditto the Douglas Skyraider, Thorp T-18, P-51 Mustang, DH Mosquito... whereas the early US GA cowl with a hole at the back on the bottom for the hot air to fall out, has very little to recommend it.

True - the Transavia Airtruk and Pawnee Brave 400 and Fletcher FU-24-400 all have a payload greater than 50% of the takeoff weight. So now we know what efficient aeroplanes look like!

The system was an "open throat" wind tunnel with no flow straighteners*, which is a form that has done quite a lot of useful work in comparative analysis. *Though the massive tip vortices from a typical desk fan give me the shudders... This became a major issue for Lanchester - the drag need be no more than the friction drag; but everybody was jumping on "form drag" and ignoring the subdivision of drag sources - and is demonstrated very well in the Cd for a teardrop body of length:diameter 3:1; the Cd is ~3% of the maximum sectional area. Compare this to the ~25% of the Cherokee fuselage. Taylor's Mini-Imp fuselage should have approached 0.05 (5%), or 1/5th as draggy as a Cherokee fuse. Truly we need to think outside the box (and inside the fish? DH Mosquito supposedly had fish input to the fuse...)

The problem is the issue of forced convergence - FEM works well with essentially two-dimensional plates of limited number; and even in such cases, the algorithms have to have a predetermined "fudge factor" to garauntee that they converge (I've had some glorious non-convergences reported from Inventor...). As soom as the program takes over the meshing, the model becomes mathematically unstable; if forced in a pro-stability direction, it becomes inaccurate. Chaos theory shows that a large accumulation of small inaccuracies gives wildly variable results from the one dataset. FEM is a tool, that translates reality into 2-D triangular plates with imaginary thickness, and averages the properties as if the plates were rectangular. It gives very precise answers, but the accuracy is suspect in all but fairly simple structural models with intelligent - not automated - control of the meshing. Computer programmers neither understand this, nor care. It is my understanding that the Joint Strike Fighter FEM teams entered an intractible dispute, and they had to build two extra prototypes to resolve the FEM disgreements (back about 2006/7). Hoerner himself states that there is no credible method for predicting laminar to turbulent transition around a 3-D shape, and the physics haven't changed...

the 7.1lb (last) drag term in the example is for wing parasitic plus tail parasitic, not just tail! Good night.

Induced Drag is estimated as wing area (S) x 1.2 x Cl (lift coefficient) squared / (PI x A(spect Ratio)) for a constant chord wing (square tips) of AR 5 ~ 8ish; 1.1 x Cl / (PI x A) for a straight tapered wing in the same aspect ratio range; and Cl / (PI x A) for an elliptical wing - but forget that one, unless you like snap rolls as stall warnings... You calculate your Cl at any speed from: Lift = 0.5 x rho (air density) x V squared x (S(wing area) x Cl), rearranging for Cl. Assume your maximum Cl (unflapped) will be ~1.45, and you won't be far wrong at all. Forget the "this airfoil section gets 1.8!" claims, those are not figures for 3-dimensional wings, OR at real Reynolds numbers, OR in typical air. Your fuselage drag coefficient will be in the range ~0.25 (Cherokee style) to ~0.15 (Sonerai), ignoring cooling drag, based upon the maximum cross-sectional area from in front; calculated from Drag = 0.5 rho x V squared x max cross sectional area x Cd (say 0.2 to start with). Cooling drag depends a lot on what's inside the cowl, but a good starting point is (imperial units): (5 + 5 x number of cyls) x 0.025 x 0.5 rho x V squared in feet/second. Most GA aeroplanes do worse than this. U/C, faired and spatted, will typically be 0.4 x frontal area (i.e. max section from in front) x 0.5 rho x V squared. The wing and tail surface parasitic drag will be ~0.008 x projected area (plan, in the case of wing & HS) x 0.5 rho x V squared. Calculate thussly the total drag at, say, 40kt; 50kt; up to 120kt by 10 kt increments. The lift remains the weight of the aeroplane, so you have the L/D at each speed. This gives you the (unpowered) sink rate at each speed, which - multiplied by the weight - gives you the (thrust) power required for level flight. Whack that lot into a spreadsheet, and it'll be within a bull's roar; which will give you a fair idea of the performance effects of different configurations. In the next day or so I'll try to post some general guidance on propellor performance. Have fun! ps an eg: With an 800lb aeroplane, AR = 6, constant chord wings, 80 sq. ft, at 50kt: Cl = 1.18; induced drag = 70 lb; assume 6 sq.ft frontal area, fuse drag = 10.2 lb; a 4 cyl donk drag = 5.3 lb; two 5.00 x 5s on springlegs in spats etc drag = 4lb; with 24 sq ft of tail, drag = 7.1lb total = 96.6 lb drag; L/D = 8.28; DHP = 14.8 Assuming a prop efficiency of 55% (4-stroke, direct drive), this requires 27shp for level flight at this speed; with a 45 hp donk, the ROC would be 410 fpm. Check against real aeroplanes...

A useful starting point for design estimations is that, in incompressible flow (ie well subsonic) for a very fair fuselage with elliptical wings: 1) The total drag is least when the parasitic and induced drag are equal (as Dafydd mentioned); 2) The minimum sink speed occurs when the induced drag is 3 times the parasitic drag; 3) The best L/D speed is 1.315 times the minimum sink speed. It's virtually impossible to do better than these, as they are a result of the basic physics. Note that (2) predicts that soaring aeroplanes should have high aspect ratio wings to minimise induced drag, and (3) demands that transport aeroplanes fly at less than twice their stall speed for good efficiency. (3) also means that high speed ratios require huge power. In a small powered aeroplane it'll be virtually impossible to achieve a speed where the induced drag becomes 3 times the parasite drag, as the Reynolds number range makes it a great challenge to avoid premature separated flow from the fuselage.

Reports 485, 518, and 522 tell almost everything about light aeroplane undercarriage. 518 covers spats and faired struts vs "pants". The theory behind any form of streamlining comes back to Lanchester (1892); any body of "icthyoid" shape (i.e. fishlike) owes all of its drag to skin friction. Which is to say, if separation (and compressibility, but Lanchester didn't know about that) is avoided, only friction drag remains. All of this stuff about "form drag" is simply a way of combining typical friction plus separation in a way that avoids the computational complexities that CAD so neatly deals with. Incidentally, fish don't use much fairing to avoid interference drag (sharks and whales do...); but air has 14 times the kinematic viscosity of water, so it's much less critical once you're out of the sticky stuff and underwater... Such highly successful small "racing" aeroplanes as the Miles series and early Lockheeds used "pants" as a structurally simple way of fairing the U/C legs, brakes, wheels and tyres all at once. The drag reduction was substantial. The reduction in directional stability was also substantial, but as everyone was used to directionally unstabke fighters, who cared? It's worth looking at the 1930's european development of small racing aeroplanes, e.g. the French Deutch de la Muerthe Trophy racers ~ 20ft wingspan, 240~350hp, averaging ~ 380kph (207 kts) over two races of 1,000 km each in the one day (in 1934). The main drag from typical spats comes from the airflow within, and how it leaves the spat. Putting closing doors on the bottom of a spat reaps dividends, and provided they're not made of Unobtanium, it's safe to land with the gear "up" ...

There's a NACA report - a series on undercarriage - which looks at spats, leg fairings, pants, and finer points of fillets and geometry - when I reboot my desktop I'll post the number...

As soon as the air next to a surface begins slowing down, it creates an "adverse pressure gradient". Seperation is imminent. If a low wing is rigged with a small positive incidence, such that at cruise the fuselage centreline is aligned with the airflow, an anti-interference fillet has to grow in radius as it moves aft; and normally, the rear edge of the fillet has to turn up - see also Grumman 4-seat lighties, which twist the root of the flap/TE up. You can also live without fillets if you place a VG at or just ahead of the LE to energise the junction airflow. The cat can be skun many ways; but air dinna like flowing around solid bodies at high speed...

Eagles... NO sense of humour...

Didn't Hawker use bolted tubes in the Fury, Hart, Hind, Hurricane ?

Small horses. Once upon a time I had an overpropped Rotax 503; would not get properly on the pipe at static (or, indeed, below about 25kts...). I fitted a 50mm PVC elbow and about 40cm of straight pipe to one carby; it jumped the chocks... we strangled it, and fitted another inlet runner to the second carby, and fired it up (tied to the scenery, elephants etc). Gutless. More gutless than with no runners. I angled the runner to the front carby, to separate the inlets. Very little change until I had it nearly in the prop, then it roared. So I threw away the plastic and got another prop...

It's a big enough deal that Heinkel made the wings on the He111 perpendicular to the fuselage surface, even at the expense of "gulling" the wings outboard, and faired the roots (See also Corsair...). Airliners put external mitres on their fuselages where the wingroots are (there is a compressibility reason as well, but the interference drag is still valid). Cessna stuck the wings on top of his smaller products, which neatly avoids any problems of interference from the wing supervelocity (higher speed air over the top). Thorp kept the PA-28 fuselage constant width to the wing TE, and added fillets reducing the thickness/chord of the root, thus reducing local supervelocity. Henry Millicer positioned the maximum width of the AirTourer at the wing TE. Vans start tapering their fuselages foward of the wing TE, and do not fit fillets, and did not offer any sort of radiused fairing on the earlier products (that I'm aware of); which gives the interesting characteristic that, if the propellor actually stops rotating in flight, at best L/D speed the separation from the wingroot/fuselage blankets the tailplane sufficiently that, if the CG is fowards - say, with an IO-320 driving a CS metal airscrew - there is insufficient elevator power to flare. (Any RV driver who thinks it's normal to not be able to three-point with the flaps out, or to need a blip of power to flare, has been demonstrating the point). Rutan stuck the fan on the butt end of his earlier designs, effectively vacuuming the interference wingroot area, and in any case the Vari-anythings have ample fillets and generously radiused fairing. The P-51 Mustang used fillets; so did the Bonanza. Aircraft that have gone to the trouble and expense of fillets or fairings include most WW2 fighters, most production low-wing GA aircraft post WW2, and most airliners post WW2 (look at the Super Connie!). Provided you stay at a lift coefficient of less than 0.3~0.4, interference drag from the wing-fuselage junction will be insignificant if the fuselage incidence is appropriate for cruise. For a typical small aeroplane, the increased parasite and induced* drag during the takeoff-climb approaches the weight penalty of an extra passenger. *Not only does the stagnant wake increase in area by 1~2% of the wing area (or 15~30% of total drag) at best L/D, but the irregularity caused in the spanwise lift distribution increases the induced drag measurably) Consider the venerable Piper Cherokee; unspatted and in average paint, it'll get a best L/D of about 1:10 at ~65 knots. At 2,000lb, that equates to 40 (thrust) HP to stay up. Now, add 4.5* square feet of extra stagnant wake from putitative wingroot interference drag - a force of ~63lb if the wake be truly stagnant, but ~40lb is likely nearer the truth. So, an extra 8 DHP, or a 20% increase in the power required for level flight. In climb terms, the cost is 132 fpm. If we assume that the Cherokee at 2,000lb will manage 832 fpm ROC sans interference drag, then the 132 fpm that would be lost, could be recovered by reducing the weight by177lb. *Turbulent transition and interference separation both develop - spread - at ~15 degrees to the airflow; so on a 5' chord, if the inteference turbulence starts at the LE, it'll be about 1.34 ft out along the TE and 1.34' up the fuselage side, and oscillating to give an effective area around the square of this distance. Check wooltufting and smoke-trail wind tunnel pictures, there'll be enough on the web; or I can find you a couple of NACA report numbers. To check the sanity of this, a fixed pitch propellor optimised for ~100kts cruise will do very well to exceed ~70% efficiency at 65kts; and, with an unspatted 6.00 x 6 hanging off the noseleg, propulsive efficiency would be less than 65%. As such a prop would also hold the engine revs back in slow climb, the available THP will be around 150 x 0.9 x 0.65 = 88 THP; less 40 to eliminate sink, leaves 48 THP to climb with, giving 790 fpm ROC at 2,000lb. So, the numbers are in the ballpark, and the fillets on the Cherokee are worth about 12 SHP in climb, or at least one passenger, or a lot of your fuel - or not operting out of a strip where you need the performance!

No worries!

not unless you used a suck-through carby or injected into the eye of the compressor...

Or you could just stick two Rotax 462's on the wings, and have twin engined reliability!!!!

Good question. There are two separate (but overlapping) issues here; strength, and stiffness. Lurking in the background is longevity, or ongoing airworthiness, or fatigue - take your pick! Because air is thin stuff, most of an aeroplane's structure is also thin, gathering dispersed loads (air pressures!) and feeding them into more major structural members. Apart from the most major of loaded elements (e.g. wingspar root attachment), the maximum strength of aeroplane structures is generally set by buckling; and buckling is a function of stiffness. For example, alloy steel has about three times the working strength of PH aluminium, and is about three times as dense - so, why not use very thin steel instead of thin aluminium? Buckling - because the aluminium bit will be three times thicker than the steel equivalent, it'll have ~27 times the buckling resistance (up to yield - but the working stresses are such that, in sheet and thin extrusions, an aluminium structure works out lighter than Chrome Moly). The other stiffness issue is aeroelasticity, or "flutter" - if the airframe (or any bit of it in the breeze) is springy enough, it can interact with the airstream and resonate - the first sign of, say, aileron flutter is generally the instrument panel blurring because the shaking is so violent that you can't see; rapidly followed by either the aileron (if you're lucky), or the wing falling off. Getting back to your specific question, rag does nothing in compression; which is to say, it contributes naff all to the overall strength/stiffness of a structure, though it's a lightweight way to catch small airloads and feed them into a skeletal structure. A steel tube skeleton - i.e. a fully triangulated spaceframe - is itself a very effective structure, and can be designed to be pretty efficient. However, combining the two gives less than optimal material usage. Welded 4130 (chrome-moly) spaceframes are ideal for engine mount frames and cabin structures (that have to feed lift truss loads, landing loads, and powerplant loads around the crew); they are less ideal for such things as tailplanes. Glass/Epoxy is less efficient in sheet form than aluminium; and it's less efficient in beam form than steel. However, it's much much easier to arrange to have just the right amount where needed - whereas variable thickness aluminium skins are impracticable on light aeroplanes! Also, judicious usage of core materials (coremat, various foams) can make "sheet" structural elements in glass/epoxy look very competitive in stiffness/weight. Compare the airframe weights, if you can get them, of the Lightwing and the early Jabiru (pre- Light Sports Aircraf category Jab LSA); the Jab structure is a tad smaller % of the MTOW, for an aircraft with much the same payload (i.e. 2 seats), a faster cruise, a similar stall, and more endurance. Both aeroplanes have enviable accident survivability records. Carbon/epoxy is a clear winner in the stiffness/weight stakes for things like wingspars and D-boxes, but has low toughness and low lug effectiveness - it's very hard to get a light but reliable bolted or pinned connection, such as to an engine mount. Compare the airframe weights of unlimited sailplanes of today, to Blaniks and the wood, steel and rag generation (with the same payload). If you're aiming at LSA, then a welded steel cabin structure, ditto engine mount, aluminium rear fuselage, glass/epoxy/foam tail surfaces, and a glass/epoxy/coremat/foam wing with aluminium struts is about the optimum for structural efficiency - but the joins between the sections better be well designed!

Obsolete before I wrote it!

Unless the air velocity is extreme, or some sort of resonance or interference exists, the air differential will be very small; fuel is much more sensitive to centrifugal force, partial vaporisation etc. If there's enough fuel at 3 pots but the 4th is wrong, one needs to twiddle the 4th injector only... Rotax, under the ASTM for LSA certification, satisfy themselves. Thierlet were the first, and possibly still only, people to get any National Airworthiness Authority to accept non-aviation electronics on a light aeroplane under any Design Standard requiring an independent umpire to be satisfied..., to the best of my knowledge...

Nope, not missing that. As I said about 3,000 posts ago, if one regards the EFI as an electronically calibrated high power jet - replacing the enrichment function of the needle - then the mythical complete EFI failure mode is to be fine at ~80% power or less (post #126). FMEA buddy; you'll learn to love it

Aha! THAT's why you bought #0001 - you're not good with big numbers! I must say, I've long suspected it... Most aircooled aero donks run stoichometric - i.e. the "correct" mixture (nominally 14.2~14.7:1) - up to ~85% power, then progressively richen up to ~12:1 at 100%. The main slide needle in Bings (both by shape & position) controls this enrichment. If you re-needled your Bing to give stoichometric all the way up, then EFI failure should only affect you on takeoff/climb; and you'd kinda notice the rev drop!

then she must be happy!!!!!

I put it to you that we feel we do not get full value for our tax dollar; how reasonable this is, I am not sure - though there is a lot of froth and bubble about corruption and inefficiencies, in terms of GDP it seems a trifling amount. I think the lack of meaningful public consultation, even more than the opacity, turns most of our tax into a grudge cost.