Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Jul 11, 2008.
Have you had the chance to run your engine and Prop yet?
Not yet. Been a bit preoccupied with two cataract operations over the past three weeks or so. Still can't see very well, so I've been fiddling, rather than working.
I've been following Aircars' thread about silent props. Very interesting stuff. Anything come of his initial suggestion of de-cambering the tips of the prop?
Good luck Duncan, both with the job-hunt and your eyes.
Maybe this is an appropriate chear-up?
dumpert.nl - Moeder & zoon doen dansje
Best of luck with your eye op Dunc -- I just had a few days of much pain after getting something in my eye (brushing leaves off the plastic covering that keeps the entrance to my workshop clear ) --only a slight irritation on going to bed that night but felt like a had a piece of sand in my eye on waking up and needed to go to hospital after a couple of hours and found a scarred cornea (that still FELT like a grain of sand was there...) --a few days of ointment and dark glasses etc and now back to normal . Until you get something in them you don't realize how many nerve endings are there. Be careful when sanding polyurethane foam if you are using that -- it is like glass in your eyes .
Well, after three weeks of being laid up here at home because my eyes were recovering from cateract surgery, and because I'd run out of money and materials anyway to work on the Razorback, I have finally received a long-overdue cheque, paid all my bills and even had enough left over to order some more resin and Q-Cell. I'm off now to pick up the swag, and get back to finishing the plug.
So you can breath a collective sigh of relief that I'll be otherwise occupied, and not posting fanciful concept designs...
However, one thing I DID manage to do during my time off, was to work with X-Plane, and to create a pretty neat model of a tandem wing Razorback.
I swapped back and forth between my spreadsheets, Raymer/Roncz, and X-Plane till I finally had a model which flies beautifully, looks great, and is a faithful representation of everything I can glean from Raymer/Roncz. I had to change a lot of things (mainly the wing placement/planviews/sizes. But I think I have it right now. X-Plane agrees, and it lifts off at just on 50kts all by itself, climbs out straight(ish), stalls straight ahead in a gentle mush, flies very smoothly hands-off if required, and (with flaps deployed) floats back down to the runway. I couldn't be happier.
Here are a few screenshots taken from X-Plane.
Or with a bit more colour and a T-tail?
A slight setback...
I moved the Razorback plug today out of the workshop to an open shed where i have more space to work. And half way there about 30 metres) the bloody stand broke, dropping the plug onto the tarmac. Bugger. Fortunately not too much damage - just a few clean holes punched through the surface, and the tip of the tail broke off. Nothing i can't fix pretty easily. But disheartening nevertheless.
Sorry to hear that Duncan, sometimes life dishes out some cruel blows ...
Glad to hear the damage isn't too bad though.
Taking a saw to the plug!
Well, I've finally committed to a wing configuration. Tandem wing. After almost 100 hours of running various design configurations through the simulator, going back to check the maths and incrementally evolving a wing configuration which both looks cool and flies well - I have committed myself to a configuration, and took a multi-tool to my pristine plug surface, and cut away a great big recess in the roof to accept the rear wing. No going back now...
It is just a rough cut, and tomorrow I'll build an accurate mock-up of the wing centre section to place in the recess and then fine-tune the fit. Then I'll do the front wing recess, and by that stage, I'll have run out of excuses, and will have to start on the molds.
Here's my final design.
The Razorback incorporates a slightly forward swept relatively high aspect ratio wing (8.75), slight gull wing in the rear (also quite high AR), and a tiny trim surface at the end of the tail extension. This final surface is not an h-stab. It is only for trimming the plane. Stall = 43kts. Both wings have flaps on the inner sections, and ailerons/elevators on the outer sections. The flaps, BTW, are simply extending split flaps. In other words, the rather large (about 50% chord) split flaps on the bottom rear half of the wing slide backwards on rails, but do not deflect. The added area alone accounts for the required increase in lift. What I wanted to do was to avoid the exaggerated pitching moments of regular flaps. I had to do quite a lot of figuring to keep the pitch of the plane as unchanged as possible when the flaps are extended - by tailoring the amount of fore/aft extension.
I have learned a number of things about tandem wings which I think far outweigh their disadvantages.
First, even though on this design the wings are close-coupled, this does not seem to translate into twitchy flight characteristics. This is because the plane balances between the two wings, rather than sitting balanced on a single wing?
I have discovered that tandem wing planes are very forgiving of changes in CG. For example, fitting a VW engine (170lbs) (instead of the "standard" Valley engineering Big Twin (116lbs) only requires a bit of extra trim. My initial fears of a bigger, heavier pilot upsetting the CG are now over. It will be able to accommodate much heavier pilots with ease.
My particular configuration (larger wing in the front) has an interesting stall - sort of halfway between a mono wing and a true canard. There is a gentle dropping of the front wing at about 9 or 10 degrees AoA, as the front wing lift curve starts to flatten. But if you hold back stick, it stays up until about 14 degrees AoA, when it stalls like a mono-wing, quite cleanly. I really like this stall behaviour.
The plane is almost impossible to slow down, so I think I will be installing drag brakes which will extend from the fuselage (horisontally) directly in front of the wing roots. This will both slow the plane down, and reduce lift. And of course, withdrawing the brakes has immediate effect.
As usual, comments welcome.
Those drag brakes are a bit odd... they'll blank the trim tab, at a minimum. I'd strongly recommend either traditional drag brakes or a Rutan-style belly board, both of which have better-understood characteristics.
Or the Glasflügel style dragflaps. By far the best solution if you have space for them (they don't occupy the place of slotted/fowler flaps)
What caracteristic would you point out for those flaps? They dont increase the chord length of the wing and the flap chord looks pretty short. There is no pitching moment data on those flaps. What i like is their simplicity.
You can brake with prop!
Yes, drag brakes are a bit of an issue at the moment. I remember you mentioning the Glasflügel drag flaps before, but couldn't find the reference. I'll go take a look.
Duncan, I had a problem slowing my X-plane model as well. I had set the minimum RPM above the 1200 RPM resonance of the Mazda PSRU.
Well, at that RPM the Mazda makes enough power for sustained flight with my light wing loading. Changing the minimum RPM down to 700
made it behave. Later changes that allow more AOA for slow flight actually made enough drag for a decent vertical speed.
I can understand that. However, I've set my 50hp motor to idle at 600rpm, and it still floats forever. on a total of 69ft^2. If I add flaps, of course, the approach is much steeper, and things are OK. I was talking about the non-flapped scenario.
Which flaps are you referring to? The ones I described, or the Glasflügel drag flaps?
Oh? How does that work?
Usually you will feel the plane start to slow about the same time grass, dirt and bits of prop begin hitting the windshield. Its really just a one time use deal....
The "glasflugel" type drag flaps actually first appeared on the DH Vampire (might be easier to google a cutaway drawing) and also the Caproni Ca21 glider and Rutan's solitaire --the mini Nimbus might have been the first to embody them --or the Club Libelle (other types that might have pix)
Propeller drag is not really an adequate glide path control --except if you had reverse pitch (beta) and then it is the ultimate but can really mess up the flow behind it (somebody might be able to find the you tube video of a Pilatus Porter disgorging parachutists and then rolling onto it's back and using reverse pitch to maintain a vertical nose down descent --at which time one of the parachutists (still in free fall) RETURNS and REENTERS the Porter -- the pilot of the Porter was Henning Heinemann (we were watching a series of oddball flying videos and he casually mentioned this..... you can google his website and contact him if you like , he left for the states a few years ago after a year or so in Australia after delivering a yacht here I think )
Propellers in "windmill' mode can create a LOT of drag --one tragic example concerns the company that had a contract to tow gunnery targets for the Royal Australian Navy in the early to mid 80s . after A friend had an ultra light flying accident I briefly took over his role of winch operator until he recovered . we used a WW2 RAF winch and a two blade windmill (we had to design and build the windmill also ) with 4000ft of steel cable about 1/8 inch dia and a drogue that was like a windsock going backwards . The first aircraft was a Piper Aztec stripped of all unneccesary weight (later adding a Navajo) -- the job was mostly boring being repetitive runs on three courses and watching to see if the radar laid guns actually hit the drogue but after a few hours of that we retracted the drogue and feathered the windmill and had "freedom of range" -basically to act like an attacking aircraft and to use any cover (the sea cliffs and radar scatter zone near the sea) to try to "bounce" the gun operators (not shooting real ammo thus time)That was quite exciting ...
Anyway after I had handed the job back and a new company got the contract and bought our equipment they had a very bad ending --it seems that they had an engine failure while winding the drogue in and the added drag of both the drogue AND the active windmill was too much for the remaining engine to overcome and the they went into the sea --the wreckage was never recovered . the critical engine was the one in front of the windmill which itself benefitted from the slipstream of it's engine (more airspeed to help haul the drogue in ) --most likely it was that one (or maybe both -they overheated at full power but low speed when towing) that failed . We practiced for cutting a drogue free and for feathering the windmill but not for having both at once -- and not for an engine failure at the same time. Looking back on it it was not so well conducted or trained for --the crew before mine got killed after hitting the mast of the ship during 'freedom of range' and the crew after as described. Propellers as airbrakes in 'windmill mode' (reverse pitch) can be too much of a good thing and not easy to rapidly modulate like a dive flap .
Whenever you lower the engine RPM below flight idle speed, your propeller starts to create a lot of drag (of course, depending on the propeler size, shape, airfoil, surface, pitch) and that can lower the glide angle significaly. Maybe X-plane program cant handle this situation realisticly.
Many aeroplanes crashed at such situations as they were unable to keep proper speed with windmilling props while trying to glide to distant airstrip.
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