Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Jul 11, 2008.
Wow... shorter than I realized. Keep on plugging (pun intended) away at it Duncan!
Yes very short, Duncan, you need to add more micro at the dull and sharp end.
Duncan, I don't know how you plan to guide a router around the windscreen channel but being a flat wrap windscreen the recess must align with the opposite edge so the glass fits against a flat surface. I would use a 1X2 sanding stick about 4 feet long.
At one en have a piece of 36 grit on a wide side and beyond that a step up to the flange depth to ride on the perimeter of the fuselage while the free end rides on the opposite side to establish the correct angle. Then finish up with a sanding block run around
Routered recess for windshield
I plan to do it backwards. Router out a nice clean recess of about 3mm all round the cockpit. (I have already filled in the recess which was there, but which was very messy.) I will end up with a very clean recess after the routering. I will make the mold directly from this.
The windshield will obviously not fit flush with the base of the recess, so I plan to add the backing afterwards. Once the actual fuselage has been vac infused and bonded together, I will tape a few layers of duct tape to the bottom edges of the windshield, fit the windshield, and pack resin/flox behind it to make a perfect (inclined) fit. Once removed, I should have a strong backing on which to mount the rubber (same depth as the tape) and the windshield.
I wouldnt cut a recess. You loose design flexibility. I would stick waxsheet into the mold as thick as the windscreen where the fuse ends and the windscreen starts. At least thats the route i will take
Razorback plug - sanding coat completed
The Razorback plug is looking great. I have now completed all the major irregularity rectifications. I did this by running a straight edge over the plug, noting where the hollows were, and then painting on some runny micro in those areas. Then a quick scraping with the straight edge, and hey presto, those gradual hollows gone.
When a straight edge revealed 100% contact with the plug surface (ie no slivers of light under the edge), I went out and bought a new LPHV spray gun, and sprayed two coats of Sayerlack. She is looking GOOD... However, I have a little Sayerlack left, so I'll give her another coat tomorrow.
When it is dry (Sunday), I'll go over it with 180 grit, then 240 and 400. At that point I'll be ready for the molds...
cabin mock-up in progress
HITC (Alan) and his wife joined Glenda and me for Christmas and we had a great day. Part of which included a mandatory trip to the RTFM workshop to give give Alan his first look at the Razorback. I was relieved to get the thumbs up for most of it. But Alan's experienced eye easily picked up a few irregular spots on the plug. Bugger. Hadn't seen them at all.
We discussed the wing at length, and as a result I am now making a quick and dirty mold of the central section of the plug. The plan is to use this to sit in and work on the placement of the wing, the seat, and the panel. Besides, I can't proceed with the actual molds till I get some money to buy more Sayerlack. And that might be another month or so.
We also looked at the engine, discussed the prop and the r we drive. Oh, and how to mount the engine on my bench so that it would be safe to fire it up.
All in all, a very profitable morning for me. Followed by a Christmas barbie on the deck and then a swim down at the river.
The cockpit mock-up
I took the rough cast of the cockpit area off the plug today. It is a bit floppy, since it is only glass (not a foam sandwich), but that's OK. I have made a simple crade for it to sit in, which will pull it into shape, and allow me to sit in it. I'll probably beef it up with a few more layers of chop strand, but it just needs to be rigid enough to accept me sitting in it.
THEN, I will cut out the seat-back bulkhead by a process of incremental approximations. Trying to figure out the shape of a sloped internal bulkhead is a bloody awkward thing to do. But I'll get there. Any bright ideas on how to do this?
Once the seat back is in place, I'll mock up the wing spar, and fit it to the cabin to check on leg clearance. It looks very likely that it will all fit very nicely.
Finally, I'll cut out the bottom of the cockpit mockup to accept the wing D-tube, and figure out the fairing required and when I'm happy with it, I'll simply fit the cockpit mold back over the plug again, copy the D-tube cutout directly onto the plug and mark the fairing outline onto the plug also.
Sigh... The lengths we non-CAD types have to go through to get things right. But at least I'll NOW it's right...
Some photos to follow (forgot to take any today).
Razorback Wing layout
Alan (HITC) had some extremely useful advice/insights to share regarding the Razorback when he visited on Christmas day, and I have taken most (except one) on board. He didn't think (given how far back the wing is positioned) that a high wing would be very practical considering that all upward vision would be lost. So I think a low wing is probably the only feasible option.
I like the idea of a mid-wing configuration, but the spar is not coming to the party, and so that's out. However, a low wing is do-able. I have been playing around with wing plan layout for months, and have oscillated from very high AR to swept back, to inverted gull wing, to tapered, hershey bar and elliptical. Every time I think I have hit on the ideal wing, I lose interest in it and draw something else.
At the moment, I'm inclined to give the plane a low wing, inverted gull forward sweep. And a reasonably low AR.
Something like this.
The various lines on the wing are (from R to L) 1/4 chord line, 37% line (spar), 50% chord. The wing has 8.25 deg forward sweep as measured along the 1/4 chord line. .35 taper ratio, 19.15ft span, AR=6.07, Area = 60.3 ft^2. With big extending split flaps I'm aiming for a stall speed of 38kts
The reason for the inverted gull configuration is to improve the otherwise rather poor downward visibility. And to make the plane very visually attractive. Of course.
PS I have researched just about everything I can find on the web regarding FSWs, and most posts seem to be rehashes of a very few basic observations:
(1) FSWs tend to tip divergence. Extra uni Carbon cloth laid in line with the tip twist (ie at about 45 deg) will make the tips rigid enough to withstand the tendency, without much weight penalty. Some details also on exactly how to do this are also available.
(2) FSWs are not affected by tip stall, since the airflow is inwards from the tips into the root of the wing. This gives greater CL at high AoA. However, I can't see this being much of an advantage for the sort of flying I want to do.
(3) Some yaw instability. But this can be overcome by ensuring a larger than "usual" rudder. The Razorback has a HUGE rudder (I like the look), so that would seem to be taken care of already.
And in essence, that's about it.
What remains, for me, is to determine how much forward sweep is too much. The X-29 was highly unstable. On the other extreme is the LET-13 glider, which has a very slight forward sweep, and is very well behaved. I notice that the Rutan Boomerang has about 13.5 deg FS on the main wing, and about a degree more on the smaller wing. I have not read that this plane is at all difficult to manage.
So how much is too much? I've gone for 8.25 deg, and lost about 5% of possible lift because of this. So I raised the wing area accordingly to ensure my stall target is met.
After spending a few hours reading the entire thread, I can only congratulate you for the incredible ambition to materialize your dream, Duncan!
I’ve also learned some useful lessons from the inherent wrong decisions taken along your +5 years of efforts (or maybe too many… indecisions - please don’t take it as a harsh critique though ) and I hope to shorten the “birth-pains” for my microlight project, mainly by starting to work on it only after the complete “freezing” of the theoretical design, including all the minor details.
Strictly ontopic now: I’ve also studied the forward-swept wing advantages and drawbacks (including J. Roncz’s older 12-episodes “lessons” from an 1990 EAA publication) and I guess it’s a good choice for your actual configuration.
The only 8.25 degrees of sweep don’t seem to have a really big influence on the general aerodynamic properties of the wing, including tip-twist at higher AoA (the advantage of developing the stall at the root being present although), but I think that your taper could be a bit reduced - I say that not being some expert, just reading previous comments of the late Orion, a true aeronautical knowledge source, on some similar designs.
P.S. An "inverted" rounding of the wing-tips (from the leading edge) would also be strongly advisable for a better tip-vortex management, IMHO.
Mmmm... Let me take a look and see what it looks like. SOUNDS sensible though...
When starting from absolute scratch (ie not even an interest in RC or plane watching prior to beginning this journey) I simply didn't know what to build. The only thing I DID know was that I wanted to build SOMETHING. Hence the frequent change of course. However, indecision and/or changing of one's mind is a common malady among home designers and builders. I know guys who have invested many thousands of dollars and a number of years in a particular design, and during the construction period have decided that they want a different plane altogether. Happens more often than people care to admit I think...
Happy New Year. And may 2014 be kind to you and yours...
Happy New Year and best wishes to you too, rtfm ! - I simply forgot that in Australia you’re already living in 2014… (6 hours more waiting here in Romania.)
I send naturally the same good wishes for a New Year full of accomplishments, but also of great ideas to all the HBA fellows - and very often enormously helpful contributors.
Duncan, One way to fit an intermediate bulkhead is to hack out one undersized by about an inch. Then extend lines from the center to the edge every inch of the perimeter. Prop it in place and measure to the fuselage writing the measurement from the test piece to the fuselage on the drawn lines. Then lay it on your bulkhead stock and mark off the distances you recorded. Connect the dots with a flexible
batten and cut on the line.
This is really looking like something from my own heart, looks just like what I used to imagine. Good luck!
Re: Razorback Wing layout
It's beautiful. I'm a big fan of forward-swept, relatively low aspect ratio wings.
I am a bit concerned that the spar is so far back. If you're worried about the outer wing twisting as the wing loading is increased (a classic issue with FSW), then putting the spar so far back is going to make it worse. The huge majority of the lift is going to be ahead of the spar, and it's going to try to twist the wing in just the way you don't want it to.
The X-29 was unstable, true, but not because of the wing sweep. The CG was quite far back, to make the plane as maneuverable as possible. My father-in-law didn't fly it, but spent hundreds of hours in the simulator at Grumman. He said that if the four computers went out, the plane would start tumbling in less than a second.
Count Pierre Louis de Monge de Franeau was a renowned Belgian aero engineer and he certainly didn't have anything against the forward sweep. So if you want to end up with something as elegant as his design, the Bugatti 100P, you just go right ahead, yours is looking very nice!
On a slightly different note, and to do with some of what we discussed about your engine at Christmas. I'll throw this out to the forum for their learned and helpful comments too -
Engine test running - I mentioned that I don't think it's the best idea to run it bolted hard to the bench, I think it might vibrate the bench a fair bit and you don't want an engine on the loose at full throttle ... I suggested making up a soft mounting for it but on second thoughts why not make up the real mounts that you will use to install it?
Also I think you need a test stand which gives you easy access to the engine, as you have it at present you can't get at the engine without sticking your arm through the spinning prop and that would be easy to do if something wasn't going as planned.
Here are some pics of your engine that I took on Christmas day, so others can help with ideas -
Here is a pic of the Universal mounts I suggested using -
And here is an ebay listing selling them cheaper than I have ever seen them - by a long way, in Oz they're about $30 to $40 each! You need the 1.25" size with 0.39 hole for 3/8 plate thickness.
Here is a link to his ebay store. These look like better quality, note that it's a set of two so you need two sets for a total of US$33.65 incl postage, about a quarter of what we pay here.
Here is a diagram which shows the installation rather poorly -
And here is a screenshot of the engine mounts on the OzMoz, I've used eight for the 912 but you'd only need four. My engine bolts solidly between the two upper angles, yours bolts downward so you would need two flat bars which you could orientate transverse or for-and-aft.
Moving on to your problem with 'loose bolts' and actually bolting the prop on. As you discovered from the Valley Eng site the prop has to be bolted on before fitting the upper pulley to the re-drive and frankly I think that's a really bad idea because it means that you can't re-torque the prop bolts without pulling the whole drive apart. Also that rear nut on the upper shaft doesn't appear to have any locking mechanism and that'd be disaster if it came loose. I put it out to the forum here but I think both of those things need addressing. While you have the drive apart again I think you need to devise a means to retain/contain the bolt heads so you can torque the prop bolts as loose bolts and wooden props are also a recipe for disaster.
Wooden props shouldn't be driven by lugs or spigots as metal ones are, neither should they be driven by the bolts, and that's what happens when the bolts are not tight enough. Wooden props should be driven by friction between the hub and the prop plate and the hub and the drive flange, so the bolts need to be just so.
Here is a link to a thread you should read about prop failure on Jabirus and which discusses the wooden prop bolting thing. I've never found a need for using Belleville washers but the point is valid at higher horsepower perhaps because the wood density/compression capability doesn't increase with horsepower for bigger engines so the right amount of clamping force to satisfy the friction needs for the drive gets more tricky as power goes up.
I bolt my engines to the back of my 1979 Toyota pickup truck bed. Just above the tailgate. I remove the tailgate and mount engine so the prop is behind the bumper by about one inch and prop tip is about 12" from the earth. The engine is bolted to a 4x6 wood beam across the bed. Takes about two hours to drill and bolt to truck.
Re: Razorback Wing layout
I read a very interesting article about aeroelastic tailoring, and a couple of very interesting "basics" emerged from it:
(1) I will need two spars instead of one - at 15% and 60% chord (pity, but the spars aren't that heavy...)
(2) The uni CF needs to be oriented at 15 degrees from the 25% chord line
I will use pultruded CF rods (rectangular) inset into the wing skin to accomplish the extra required rigidity at almost no weight penalty.
Fascinating... So FSW in and of itself does not make for instability. Whew... That's a relief...
And thanks HITC for the pictures of the Bugatti 100P.
I'll place an order immediately for the engine mounts. I love it when a plan comes together...
I did manage to see how to tighten the prop studs without removing the prop, BTW. It requires putting a couple of bends in a spanner, but easy enough to do. Haven't done so yet, but will report when/if I manage to do so.
Duncan, I would add a mount to the 4 you are ordering. With the short front to back spacing of the mounting holes compared to the height of the thrust line the engine will be trying to lean forward due to the thrust. A fifth mount from the top of the engine straight back to the firewall would stabilize this movement.
Re: Razorback Wing layout
Such modest sweep doesn't necessarily result in a divergent wing. I don't think that 8 degrees requires anything a non-swept wing doesn't as long as it's torsionally stiff (complete skin, no fabric)
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