The rocking horse effect is now Cha Cha.

[Chuck_Ellsworth]

I read Ken Armstrongs article in Kitplanes about flying the RAF.

His comment about the Cha Cha effect about sums up the validity of that article when it comes to his understanding of the subject of gyro stability.

By the way I have known Ken for decades, he lives near me and one day I overheard someone discussing his magazine articles in Victoria at the flying club.

Their comment was there are those who understand how to fly aircraft and those who write about it.

His article does nothing to convince me to disagree with the opinion I overheard in Victoria.

Oh, about the subject title.....Don LaFleur referred to the pitch instability of his RAF in his company propaganda letter as the "rocking horse effect" another dumb description by someone who knows zero about flying.

Marketing works wonders, all one needs is lots of spin and neat sounding words.

Chuck E.

[Victor Duarte]

lol chuck !
i read ron's post and saw "cha cha effect" and as i m from france, i thought ..damn another slang technical word.. and looked for it in google... dont laugh please :D,
i didnt know gyro could dance cha cha, is a "tango" version available ;)
you guys have a rough opinion of RAF, but usefull VS the marketed version we have in our magazines, go on.
cheers

[donshoebridge]

I shouldn't be saying this, but I have to inject a little humor into an almost "taboo" topic. So do you think we can convince RAF that all of their new builds have N-numbers that end in "TT"? :)

Sorry, bad joke. Please disregard.

[Hognose]

Nobody was ever hurt by Cha Cha directly but it's those tunes that end (like the Beatles cover of Besame Mucho) "cha cha BOOM" you gotta be careful about.

I recall Ken's glowing article about how great the Mini500 was and how its detractors were all cranks. Magazine lead times being what they are, that issue popped through my mail slot right as Dennis Fetters was absconding to the UAE with all the money from dozens of deposits on the (fortunately never flown) 2-seat version.

cheers

-=K=-

[Hognose]

I just went to the nearest Borders and Barnes and Nobles looking for this mag (I let my subscription run out). Unfortunately, neither had it but they had all kinds of other cool stuff and I'm about $250 poorer without having gotten the damn gyro article.

cheers

-=K=-

[Doug Riley]

Cha cha? Rocking damn horse?? Back when I was a boy in the Bensen era, cutesy euphemisms weren't what they are today. It was called porpoising and it killed you.

The latter part, at least, is still true -- if assorted smoking craters are any evidence.

[banaari]

Q. How do you catch an unstabilised RAF?
A. Buy ten acres and wait...

[Victor Duarte]

wow, that s called shot down in flames john ! i would not be too cynical but have you seen a stable rotorhead, in the sense of positive stability ? unmanned drones are stable.. maybe the answer...
reading all posts about RAF i just learn it has a very bad reputation, or is it just that the manufacturer statements are not in phase with the reality of their products ?
cheers

[Doug Riley]

Victor: The Bensen-style offset gimbal rotorhead* is very stable. It produces control pressures much like those of a fixed-wing airplane (at least if the gyro's airframe is stable). It allows hands-off flight for long periods of time.

The problem with unstable gyros is not in the rotorhead. The problem is with unstable airframes that can (and will) overpower the stability of the rotor and rotorhead during low-G maneuvers.

Your countryman Jean Fourcade was one of the group of very bright people who pointed this out to us some years ago.

*I say "Bensen-style," but it was actually invented and patented by Juan de la Cierva in the early 1930's for his first "direct control" gyros.

[Victor Duarte]

It allows hands-off flight for long periods of time.
thank you doug, i must admit i learned something again.
i meant positive stability in the sense of pitch and roll auto-leveling, like with FW.

i would have the expertise of jean fourcade , sorry fo rmy recurent questions/remarks, i just a little worm ;)

but at the risk of throwing again hair brained ideas, the only real way, on my understanding, to make a rotor head auto-stabilizing, is a gyroscope w/fly by wire or a mechanical one coupled w/trim systems...

kevin, i f remember, you talked here about a stab device Dr Bensen was around...

thank you doug (and kevin)

[Doug Riley]

Victor -- yes, you will need a gyroscope or some electronic "black box" if you want an aircraft that returns to level by itself. FW planes without autopilot will typically stay in a turn or actually tighten the turn once the turn is established. Gyros with gimbal heads are more stable in turns than a typical FW plane, because they have no adverse yaw, and no yaw-roll coupling (= spiral instability).

I would think that you could attach a FW-type autopilot to a gyro if you wanted to. Most of us fly for the fun of working the stick ourselves, though!

[Victor Duarte]

yes , right doug, the pleasure of self-piloting instead of autopilot !
you re a good advocate for the gyro, with arguments..
about spiral instability, is this behaviour shared between tractive/pusher types?
some one posted (dont remenber who) a thread about ideal "proportions" between front/rear fuselage.. again with my words i d say i can't "feel" a "dart-effect" on pushers, more on tractives..
just coming back to the RAF subject, it seems they just "over-sell" stability.
thank you doug

[Doug Riley]

Victor, the FW spiral instability that I mentioned is caused simply by the fixed wings. The wing on the outside of the turn has a higher airspeed and therefore it creates more lift. The plane will tend to stay in a bank once the turn is established even if the aileron input that started the turn is removed. Some planes require OPPOSITE stick pressure to prevent the turn from tightening.

The gyro's wings rotate at a tip speed that is significantly higher than the aircraft's airspeed. Therefore, in a turn, the difference in airspeed between the "outside" blade and the "inside" blade is proportionally less than it is with the FW plane. Furthermore, because of the teeter hinge and gyroscopic precession, the "outside" blade reacts by flapping up or down (depending on the direction of the turn) when it reaches the fore-aft position. As a result, instead of tightening the bank, the rotor disk reacts by flapping back. This adds to the need to provide back pressure on the stick -- but we need to do this anyway in a turn.

Because of the outside-wing/inside-wing airspeed effect, you can often get a FW plane to bank and turn with the rudder alone (even if the plane has no dihedral). Old-time FW pilots, especially, were taught to use rudder as a roll control at slow airspeeds, using just this effect. I see FW pilots who transition to the gyro struggling to use the pedals in the same way on approach in the gyro. They kick furiously at the rudder, wondering why it doesn't produce any instant roll effect! They must learn to use the stick as their only roll control -- the gyro lacks the quick coupling between roll and yaw that a FW plane has.

(Some gyros with lots of fuselage side area do have slip-roll coupling that can be quite dangerous -- that's another story.)

[Victor Duarte]

Some gyros with lots of fuselage side area do have slip-roll coupling that can be quite dangerous -- that's another story.
you have talent for telling, ;) slip-roll, sorry i v a lack of english words... :confused:

sorry i was asking if there was a kind of yaw instability in gyros, i misenderstood, i was reading "flat spin"... ok i mixed everything, sorry

you explained spin very well, and perfectly the need for back stick,
right, for FW roll control, thats the principle of 2 axes airplanes, and if i m not wrong glider pilots use roll-yaw a lot, i had the (scary) occasion to be the passenger in a demo for an ultralight (joker j300) where the pilot-designer made a pass over the runway in crab alternating right-left, to demonstrate low speed capabilities, you know, the kind of manoeuvre the confirmed pilots like to inflict to newbies, like brutal rise-and-stall :D :D

just for entertainment, do you remember a guy whitch demonstrated a technique to go out a deadly spin, i just remember the plane was a CAP 10 and he performed 64 spins, counting them before he went out... courageous and skilled

i see some myths must be burried, i guess it s your intention, thank you doug for your very clear explanations, gyros are safer with a little knowledge, call it information..
thank you

[Doug Riley]

Yes, some amateur-designed gyros have yaw instability. This occurs if there is not enough vertical fin area to overcome the instability caused by cabin and other items located ahead of the aircraft's center of gravity. The gyro behaves like an arrow that tries to fly tail-first. Properly designed gyros do not have this problem.

A "slip" is a condition of flight in which the aircraft moves somewhat sideways through the air. If the aircraft tends to roll when this happens, then it has "slip-roll coupling." Dihedral produces STABLE slip-roll coupling in a FW plane (when the plane slips, it tends to bank AWAY from the direction in which it is slipping; yes, that's how 2-axis FW planes are turned).

Flapping produces stable slip-roll coupling in a rotor, much like a fixed wing with dihedral. However, if the gyro has a lot of cabin side area BELOW its CG, the cabin may over-power the rotor and cause UNstable slip-roll coupling. If this occurs, the gyro will tend to bank TOWARD the direction in which it is slipping (this makes the slip worse and can lead to even more bank, more slip and eventually a crash).

Again, a properly-designed gyro will not have this problem.

[Victor Duarte]

oh yes ! perfectly understandable,really.
if i try to sum up some basic design tips :
clt, enough rudder surface, h stab i suppose (in the proportions of la cierva), a little coning, proper head trim, ideally clt on a middle line in fuselage... an i suppose a huge dose of learning and training, thats logical.. mm i see advantages in tractors...

i hope i dont abuse with anoother question, if so justtell it ;), what about proportions of the mast? i mean height, some say (particularly in helicopters) too much weight concentrated up may induce instability, the contrary a lazy one, is there a rule you experimented in gyros ? i guess there s somthing with the rotor lift and drag ?

THANK you doug, i ll let you after this one, promised ;)

[Hognose]

I would think that you could attach a FW-type autopilot to a gyro if you wanted to. Most of us fly for the fun of working the stick ourselves, though!

You know, I have thought of this, in looking at making an IFR capable gyro. In the FW world, an a/p is somewhere between a great help and mandatory in single-pilot IFR.

But most F/w autopilots work by moving the (necessarily electric) trim on a plane to produce turns. How would you do that on a gyro? The trim systems in use on some of the sleds (i.e. RAF, AAI) aren't going to do it, are they?

dheers

-=K=-

[Victor Duarte]

kevin, i m just working on a swashplate whitch can be controlled by a mechanical device or ready for fly by wire, yes , i tried to simplify the design , i reached my goals but the disadvantage is that there is strictly NO feedback on the stick (that was why i created a thread about this ;) )

a such swashplate, controled by 3 electric motors could be linked to a gyroscope and electricaly trimmed almost like in RC helicos.. then not talking about the yaw control (that could be easy to control) you could have an auto leveling rotor...

let me time to put my drafts in 3D, i ll show you..

cheers

[Hognose]

kevin, i m just working on a swashplate whitch can be controlled by a mechanical device or ready for fly by wire, yes , i tried to simplify the design , i reached my goals but the disadvantage is that there is strictly NO feedback on the stick (that was why i created a thread about this ;) )


This is interesting and would interface nicley with a solid-state AHRS. It also would enable another possibility -- artificial stick feedback. I think the flight computers of Airbus Industrie's machines do this, no? All the hard conceptual work is done for us, heh heh.

In the F-16C the stick does not move perceptibly -- it just detects your pressure on it and transmits that to the a/c -- so there is no feedback whatsoever, except what you get from your body's own kinesthetic system.

As the RAF "rotor stabilator" was explained to me by Dofin (who insisted I really had to talk to Duane for a truly accurate explanation, which I was unable to do because our schedules were incompatible -- Duane was working hard flying sales prospects and I was working hard doing news), one of its benefits was improved stick feedback in the lateral (pitch) axis.

I'l try to find your feedback thread.

cheers

-=K=-

[Victor Duarte]

hi kevin

an AHRS like this ? http://www.xbow.com/Products/productsdetails.aspx?sid=30

a little more about my design, just imagine 3 rods, like hi-fi- rollers :

collective +/-
rear-left +/-
rear-right +/-

these rods are very close to the SWplate, so could be controlled by everything, tubes, pulleys, cables or, better electric motors

i d say that with electric motors, just by changing 3 screws, the coltrol could be better, more precise, more powerful.. but actually, i set it up for manual control...

yes a real feedback would be much more complicated, but ot impossible if you see my rotorhead design, we could mount pressure sensors an drive info to the processor by dynamic wiring but for a homebuilt, it reaches a level of complication not really needed.. i ll give a call to airbus for some tips :D :D

you can find the thread anout feedback there
http://www.rotaryforum.com/forum/showthread.php?t=2461

it was a poll to determine how much pilots need feedbak, it appears that it is a must-have on light gyros...

cheers

[MechFx]

The Thai to English translation for Cha Cha, is Slow Down, Slow Down......

I wonder how that relates to gyro stability......he he...