Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Aug 3, 2017.
Don't be rude!
That's the burden we cash-poor folks have to bear. Time on the Laptop is free.
The "Mignet Formula" is a myth. It doesn't exist beyond the following:
Pivoting front wing, with at least 12 degrees of swivel
CG at no more than 25% of combined chord
Suitable (ie very low pitching moment) airfoil
...and (possibly) rear wing set at about 6 deg
Beyond this, nothing, and certainly no universal agreement. Size of wings, relative size of wings, angle of rear wing, airfoils, wing plan view, horisontal wing separation, vertical wing separation, ailerons, Cosandy flaps, build materials, fuselage shape - just about every other design aspect of Pou's is a matter of preference. Just check out the published list of Flying Flea details.
As for wings of different chords - I agree, most Fleas have wings of equal chord - but I strongly suspect this has nothing to do with aerodynamics. They are just easier to make that way. The front wing is significantly more loaded than the rear, so I don't see an issue.
Take both the Balerit HM1000 (1986) and the Courduan HM1100 (1996) designed by the Mignet family themselves. Both have different chord wings.
Hi Victor Bravo (and Radfordc)
Yes, I like the look of the convex tail section. But that's just a preference of mine. I suspect that a straight underside tail section is simply easier to make, which is why this is what we see nearly all the time. And I think our eyes just get accustomed to the look.
But there are some stand-out designs which have the convex underside tail shape - just look at Rutan's Quickie. Now that's a sexy curve. (And from what I can tell, it flies pretty well...)
I've been running some numbers, and based on published weights for foam, plywood and the wooden stringers, the entire fuselage of the KC-Flea will weigh in the region of 16.04kg (35.28lbs).
Rather than glass the foam core, I think I'm going to clad the sides and the edges with 1.5mm Gaboon plywood. Very simple to do, quick, and ends up with a great finish. I will have to router out a 12mm square corner all round the sides so I can bond in a 12mm square piece of Hoop Pine to give the plywood something to bond onto besides the foam itself. At 4g per linear meter, that's only another half pound or so.
The foam: 10.93kg
The stringers: 2.88kg
The plywood: 2.23kg
Total: 16.04 (35.28 lbs)
I have not included the weight of the T-88 or the edge reinforcements in this total yet.
Half the reason for the low weight is the fact that the plane, being a Pou, is extremely small. Only 2.4m from firewall to tailstock.
Based on the weights achieved by the DIY plane (fuselage 31kg) I'm doing very well. His fin & rudder came to about 5kg, and since I'm building mine in the same way (except with 1mm ply covering) I'm aiming for about 6kg. That makes the complete fuselage somewhere in the region of 22kg.
It will be interesting to see what happens when I actually start building. No funds at the moment, so all I can do is plan, draw, and dream.
OK - here's the ultimate easy build plane.
The KC-Flea mentioned above is great - but I thought it could be improved. This version is even simpler - and easier to build. I might just grab some foam and try out my CNC hotwire on it. Two sides of 100mm thick foam per side. Then lay each half on its side, and hotwire the plan view.
Can it really get simpler than this?
And yes, the scalloped tail is back!
The original Rutan Quickie was reportedly NOT a very good flying airplane. It met the stated goal of flight on an 18HP Onan generator engine. But a "nice flying sportplane", my understanding is definitely no. IMHO don't use that as an example of one of Rutan's "better" results.
With Duncan's latest narrow foam Flea sketch, we're getting back to FritzW's Piojo, a design which really should be built. Foam, metal, wood, recycled water bottles... whatever... it needs to be built. But it cannot be made too narrow because there is torsional twist and "fish tail" bending that will happen on the Flea configuration more than on many "conventional" configurations.
I appreciate your concerns about torsional twist of narrow tails - but remember, this entire tail is less than a metre in length. Unlike most planes which can easily be three times that. The AeroMax, for example is also 14 inches wide at its widest point, with a significantly longer tail section.
Duncan, I think a lot of folks would feel better about the design if that structural member(?) depicted under the seat ran in a straight line from the firewall to the fin post. It could then do extra duty as an internal hard point for things like the lower engine mount, nose wheel, struts, cables, main gear, tail bumper, and fin post attach. Getting one component to do more than one job is a very Rutan way of approaching airplane design. Really, you could design a "naked flea" around that belly beam then flesh it out with foam to whatever shape you like.
Interesting. My understanding is that the flying qualities of the original Quickie were just fine, albeit not for a large/heavy pilot or high density-altitude situations. Climb rate, especially with the original Onan motor, was a bit anemic due to the high span loading and low power. The landing gear made for a bit of an unusual takeoff and landing, and I've heard reports, especially on Q-2's, of a small stone jamming in the wheel pant and creating a spectacular ground-loop, but I'd always understood the actual flying qualities to be just fine.
I hate to keep banging this drum in Duncan's threads, but I think a lot of folks would feel better if there was some actual analysis and engineering going on in his design process. Copying other designs and making small changes is a time-honored process, but only very small changes can reasonably be accommodated before the use of the "parent" design is invalidated and the new design is actually a complete unknown. Just in this thread alone, I've seen gross material substitutions, based solely on availability, with no attention to resizing the part for the new material, as just one example. First Duncan started with an HM-293 plans-set, I believe. The Rangi-Pou is a significant departure from that, with no aerodynamic or structural analysis that I've seen, and now the "KC-Pou" and its new derivative is another major leap from that.
Duncan, you can't just eyeball this stuff and expect to be safe. I'm worried that someday you're going to finish one of these things and actually try and fly it - and that you're going to get hurt or killed in the process. I'm not saying you have to go full-bore numerical design like I choose to do - it can be done more simply in many cases, and I'm trying for a higher degree of optimization than you are - but you can't just start with one airplane, make changes until what you have is a totally different airplane, and still expect to be safe when you take it up into the air. You have a huge resource here in HBA to learn and get help, and to develop your designs. Why not use it?
I try not to say anything very often, because I know it bothers you and, let's face it, you're not any closer to actually flying one of your designs than I am, so the danger today is small. But someday you're going to finish one, and someday your body is going to be suspended high in the air with nothing more than the fruits of your own design work to support it and keep you safe. I sure wish you'd at least do the absolute minimum of loads and structural design, let alone the most basic of stability and control work, before that day.
Not trying to rag on you, Duncan. I just don't want to see you go the way of someone we both admired, about whom you posted recently, who also didn't do any real engineering on his designs. And eventually it bit him. You know who I mean.
Also absolutely not to browbeat or harangue Duncan or anyone else....
Between all the trained people who hang out here on HBA, and some sandbags, you can do real world proof tessting at very very little cost without you personally learning or studying engineering analysis. I will bet that more than one of the "real" engineers on HBA would contribute a small amount of their time and tell you that you need X weight of sandbags hanging from Y location on the aircraft as the aircraft is supported at Z position, for fuselage twist, aerodynamic tail loads, flight G loads, etc. Many or perhaps most of these tests can be simpified by an engineer down to the point of putting the aircraft on sawhorses and putting the sandbags in the right place. I have the book "The Taylorcraft Story" by Chet Peek, and this book shows several potos of this exact type of testing.... weights and ropes... during the original certification of the Taylorcraft in 1935. Those methods yielded a classic, safe, and well-proven aircraft. My last T-craft is now 77 years old and still fying daily in Alaska!
The point is that you will be able to do several important basic tests with sandbags, rocks, buckets of water, scrap wood, and ropes, and you will very likely have the guidance of people who know where those weights and ropes must be used.
Just another example of urban legend supplanting reality. Same with the BD-5, flying wings..... Those that have flown them actually liked them. The original Quickie did have a bit of a gestational period. It did not meet the original goals as first flown and there was a period of modifications to correct both the yaw and pitch. I don't know the level or Rutans involvement with the program during this modification phase. The canard went through several iterations and the sales pamphlet documents at least 2 of those major modifications - if you know what you are looking at.
That last bit, IMHO, is the tricky part. You have to know what those loads will be before doing the designing or testing. We can use the numbers from the various CAAs do define some of the critical loads, but not all, especially with a non-standard configuration.
Way back in statics class when free body diagrams were introduced it's impressed on the students - "get the FBD right and the rest is just math".
I'm more than happy to do these tests. Any engineering type care to give me some guidance re: the X, Y and Z above?
Guys - we seem to be on different pages. I think we need to distinguish between doodling with designs (which I enjoy doing when I have no funds to buy materials - ie most of the time) and actually building a physical artefact.
I did get pretty far with the Razorback till the plug was destroyed in a hail storm and I just didn't have the necessary heart to begin again. Since then, I have doodled on the laptop, playing with various designs, which largely because I have worked only 1 year during the last five, I have not been able to actually build. I did have a period of prosperity (aka a job) for one year, during which I bought the wing and empennage kits for an AeroMax, which I planned to build exactly according to the plans. When the money ran out, I invested my last $500 in wood for a slightly re-worked HM293 (the Rangi-Pou). That project has reached the completed fuselage stage. While I wait for another contract, I am forced either to ignore airplanes entirely and do something else - or return to playing with Sketchup on the computer. So back to the Laptop I went, for more doodling. This latest effort produced the Kookie Cutter Flea - both versions. But these are thought experiments only.
Back to what I'm actually building... My Rangi-Pou design is very close to the HM293 - I am using the 293 plans every step of the way - differing only as far as cosmetics are concerned. Same construction methods, same materials, same design points. What is different is my choice of airfoil (not a random choice, but an aerfoil designed specifically for Fleas, and already successfully used in a number of Flea designs.
As for testing - just because I don't go on about it doesn't mean I'm not planning on doing so. In fact, I am going to base my testing on the Glider Design Handbook (chapter 2). https://babel.hathitrust.org/cgi/pt?id=mdp.39015000453756;view=1up;seq=8
But if any engineer here on the forum would like to offer any advice on how I should test my fuselage and wings - I would be more than happy to hear from you. This is a genuine request for assistance in this regard.
Wings are easy to test, but weren't you planning GRP or CFRP wings? Usually, only one critical load condition is tested, the one most likely to break the wing.
Changing the plan view of a flea will affect its stability, so 'cosmetic' mods may also have an aero effect. Get that wrong, it'll bite you above a certain speed. I'm planning droppable tail ballast for unrecoverable test flight discoveries.
Wings usually have hard points to mount to the fuselage, and lots of handy area to pile sandbags on. I can give you the loading schedule for elliptical.
Fuselages are more tricky as their weight and the loading is more distributed. You need to somehow approximate flight loads. I'm doing a tailless aircraft, and haven't looked into tail loads, you need someone else for those!
But your build manual for the Range-Pou switches material for the fuselage and mentions composite wings....is that manual a thought experiment as well....?
Maybe there should be a sticky for basic load testing?
I'm not sure what you're referring to.
The 293 plans specify "soft wood", and "hard wood" reinforcing. I'm using Hoop Pine with Maple
Plywood: 2mm Birch ply. I'm using 3mm Hoop Pine ply
Glue: the 293 plans recommend epoxy, but go no further. I'm using T-88
Metal fittings: 2mm steel. I was going to be using 4mm 6061-T6, but think I've found someone who can cut 2mm 4130
Wings: the 293 has wooden wings, with swept forward tips. I will use composite wings, also with swept forward tips
Undercarriage: The 293 plans have both tail dragger and tricycle undercarriage plans. I'm using one of their two methods to build the tricycle gear.
So, as I say - what major departures are you referring to? And do I detect a note of sarcasm in your comment about the manual being a thought experiment also? If so, that's uncalled for Ragflyer...
Most people would say a switch from wooden wings to composite wings is a major departure (not just cosmetic) from the plans... no?
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