Bob Llewellyn

Ok Stevron, perhaps I can make it a bit more understandable. In the case of a sphere, moving through the air at aircraft-like velocity, the air will only flow along the sides from the direct front of the sphere to a point ~92 degrees around - that is, only just past the fattest point. This is a problem, because once the air has separated, it creates a wake which is the drag. Tapering the rear part of the sphere in a "teardrop" shape, with a total length of ~3 times the max. diameter, will persuade the air to stick until the wake represents 3~4% of the maximum cross section. Most lumps of aeroplane - spats, fuselage, cabin, wing, etc - fall between these two extremes; the drag of most small aeroplanes can be reduced by reducing the wake size, which can be done by reducing the separation - where the flow breaks away from the body and eddies around - which can be done with VGs. Back to our original sphere, if we put a ring of VGs a bit fowards of the fattest point, it is likely that the wake size can be reduced from ~95% of the maximum cross section (of the sphere), to 50~30% of that value (or perhaps less). This is one form of "boundary layer" control. As several posters have alluded to, progressive separation - especially approaching stall - is often used by aeroplane designers to give desirable handling, even at a performance cost. Regaining the performance can lose the good handling, possibly in an extreme way. On a poorly designed aeroplane - and there are a few out there - it is potentially possible, with VGs, to improve the performance and not impair the handling - if you get it exactly right. There is a fair bit of specialised knowledge involved. If there exists an "approved" VG kit for an aeroplane you own, then it probably does what it claims, and probably has the bugs ironed out. Otherwise, do you really wish to become a test pilot?

well, yes... I was hypothesising an aeroplane with inadequate horizontal stab, which inherently resisted entering a developed spin due to lack of elevator power, exacerbated by initial separation in the wing root area (so reducing downwash onto the HS); if VG'd full span, it could well be able to fully stall one wing, and - while the pilot sat in shock thinking "WTF?", it could quickly pass through the incipient phase into a developed spin; by which time the inadequate tailfeathers would not allow recovery nohow, and there's a reasonable probability that the prolonged rotation would begin to flatten before it hit the ground. Which would be bad.

"that's NOT on the agenda, Ratty... now sit down, and shut up. Are we likely to get a visit from Mole...?" At which point Rat responded "look, it's the AGM, and you work for the members, i.e. ME! so don't tell me to..." "SHUT UP!" yelled Endo. "Your membership is NOT paid up!"; at which point he gloated smugly. Rattus turned puce, which looked strange under the fur, and spat out...

How about when Lord Kitchener's coffin arrived at the docks before he sailed on the Edinburgh, which ran into an unexpected German naval force and was sunk with all hands? WW1...

Could be exciting then... an unrecoverable flat spin IS exciting, ce ne pas?

'Let's talk about that, Honey" said Don Quixote, pushing him aside. "Now, I represent the intimate entertainment and filthy acts union, and this is UNACCEPTABLE!...". At this moment, the entire tent vanished upwards with a noise like a Star Destroyer class vacuum cleaner (it was the GulfStleam again... in the cockpit, Ben and Maggot were wrestling... greek style...). The sudden disappearance of the burning tent, and the accompanying gust of wind as Rattus farted nervously, extinguished the flames on Turbo. In fact, like smelling salts but fouler, the wind of Rat's passage brought Turbo back to consciousness - and knocked out Hy. The Rat siezed the opportunity to slap Don on the rump, point at Bandy and shout "HE did it!", and run off into...

If you have a trailing edge stalling airfoil, or unwanted separation, and want a lower stall speed and/or possibly better stall handling (if VGs placed correctly!), or lower climb or cruise drag (depending when the parasitic separation occurs); YES. Otherwise, no. They are a very effective boundary layer control mechanism, so if your boundary layer needs more controlling... of course, you should probably wooltuft the whole aeroplane and get some vids of the airflows first...

Here's what my performance estimate gives with Prop #10- it's a bit disappointing (118kt Vh), I think because the slipstream velocity figures I used in the first one were too low. HOWEVER! note that retracting the gear gives 128kt Vh. I realise that retracting U/C with sensible-sized wheels into a small aeroplane is a pain... perhaps a viable alternative would be to fit the spats with doors, so that the wheels are entirely enclosed in cruise? When this was done to the first BD-4 in Oz, the cruise gain was startling; and failure to retract the (balsa) doors was not a major catastrophe... There should be a tad more to come from the prop, though I'm not sure it's much without your TOR becoming silly. Perhaps reducing the diameter will give you 120kt+ Vh, as is... Rough Performance Estimates #2.pdf Rough Performance Estimates #2.pdf Rough Performance Estimates #2.pdf

accomplished berry dancers and able to hold our saki... what is that smoke?" The Rat dropped his J, and looked around in mock surprise. "No, fleabrain!" said Acki, "Not the reefer, the back of the tent... uuuurgh" as he was trampled by the Dandenong safety boots. Turbo grabbed a glass of scotch, threw it on the flames, and flew backwards as it exploded. "Ah!" said Hy, who had just entered behind the departing boots. "Old Suntory? Fuggen' OW!" as the Rat displayed the graceful agility he wasn't famous for, and tried to leap over Hy but kicked him in the head instead. Ben looked in and saw the Rat spasming on the floor; Turbo about to be engulfed in flames; Hy kneeling and clutching his head, and a hole in the wall where Rat's stock-in-trade had departed. "OW" said...

Back again! Based on the idea of trading off takeoff to gain top speed, I removed propellor that wasn't doing so much for top speed - i.e. outboards - and added propellor in the area that's stalled under static conditions, and working at top speed. The result looks like something you'd find inside a frozen lolly, or a bit like some WW2 german bomber props; starting from the tip, it tapers out from 2" to 3.25" at 50% span, then more steeply to 4.8" @ 19% span. I also changed to a quasi-linear twist distribution, which did not very much (but simplifies analysis a bit). This lot gives prop #9, which loses about 1/3 of the takeoff thrust, but extends the redline (donk) speed to ~109kts, and is still giving useful thrust at 130kts. The analysis showed that the tips were a bit lazy, so I added some tip advance (overall twist still linear, so it added all the way in, but by smaller increments). This gives prop #10, which loses out to prop #9 up to ~112kt, but extends the donk redline to ~115kt, and so the top-end thrust. Based on your drag curves, will this give you the Vh you're after? ps spent time rejigging analysis for extended speed range, more useful output; that, and life, got in the way :o)... still planning to fidget with stability stuff... Prop #8-10.pdf Prop #8-10.pdf Prop #8-10.pdf

I like my trusty Thrusters...

Um - without splitting hairs, if NACA weren't doing an april fool's exercise, then the magnitude and intensity of the vortices is shown pretty well, whichever wingtip they were off.

But... but.. LiPO batteries are perfectly safe as long as you leave the pin in, just like grenades, ce ne pas?

Mutha of a mover" said the square-looking truck. "Ooooh!" giggled Great Wall, who was dragging a very small piece of ferris wheel towards the dump, "it's One Big Mutha!". The rest of the descendants fell about laughing, when Peter Bilt said...

you could say I'm always ahead of the aircraft - not hard in a Lightwing... NOW!"; and she hauled back on the stick, blushed, said "oops" and thrust it fowards while booting the rudder; but Ben was back in the "boot" behind the seats, swigging from a bottle of Scotch, and the extra inertia defeated Madge's attempt to recover the aeroplane. Ssan and Hy were at the top of the half-disassembled ferris wheel when Ssan looked behind Hy and said "Oh ***k", and dropped his end of the pipe, which hauled Hy hither. Just then the Lightwing hit the ferris, which spun around a half-turn and flipped the aeroplane back onto the runway, right way up, facing fowards. There was a graceful spray of descendants from the ferris frame...

you're showing your age... "stealth" means, like a Thorp T-18, it has a negligible radar return; "hypersonic" is the noise a direct-drive kfm 107 makes when the prop starts to slip... and radar can't hear sound waves...

When the models run out of fuel, they should easier to catch than the... ahem...

Model Aircraft Association of Australia, Aaaargh!

MH370 landed on a disused strip on a peninsula with an early bedtime, and are being politically re-educated. The hints and signals that have lead to the current search were actually a flight of pot-bellied pigs on amphetamines...

no, "quality" has an "l" in it... The common quality is, we are all aircraft nuts. My post was meant to underscore the point that, once one has mainlined on flight, going back to a substitute such as models doesn't cut it for long...

left nipple, which was also cad plated. However, back in the Lightwing, confusion reigned, as the wake turbulence from the Gulfstleam forced the Lightwing into an uncontrollabe roll. "Roll with it baby" said Madge, who though Ben wanted...

they'd forgotten about the GulfStleam, which had continued to climb to over 45,000 ft before all on board passed out. It was now a mindless robot bent on destruction - rather like Ratty when he ran out of beer - and was approaching the Lightwing... fast! meantimes, Ben Tley reached out his hand and grasped...

running shoes to a good grip on the grass, and they were off! Like Ratty's socks at the end of a hot day ...

SEE! Motorbikes are the way, who needs trucks?

is the MAAA about to get 10,000 new members?:faint:I think not...