Roll trimm

[birdy]

hmmm ... I guess Chuck is right, I suppose the RTV between the two sets of blades IS different. ... that must be occurring for the adjustment to be needed. But (to me anyway) it does not really explain why this happens?
I understand why they fly differect coz they have a different loading, but why sideways?

 

[Mark E]

Birdy, I promise to stay off your other thread to avoid clogging it up! :tape:

I will follow it carefully. (it's just like a university here, except the lecturers are a LOT more cryptic!)

But I hope you will do me the favor of looking over here occasionally to answer a few questions (and many thanks for your answers)

I think we are the only ones left over here anyway.:bored:

I understand why they fly different coz they have a different loading, but why sideways?

The different RTV is the answer, as Chuck says - and no doubt it relates to the characteristic of each blade set (different coning angle?) but that does not explain exactly why it is happening, or exactly what is happening ....

1. Different coning angle may be the full answer? (just cant picture how,)

2. Difference in spanwise flex? (ie twist) With the longer blade the outer tip would be at a slightly higher AOA than inboard section? (longer blade, is it the same dimensions(?) .... there will be a little more upward deflection (talking AOA here, not coning) at the tips with the longer blade (especially the advancing blade?) So does that have an effect on relative efficiency - the advancing blade then has a little less "wing" at the optimum AOA for lift, so there is a right shift of RTV? (again, for right side being advancing blade)

 

[Mark E]

Coning angle may be the answer,

Coning is basically dictated by the weight and drag of the machine (?) and the lift and flex of the rotor (I guess)

But, with the rotor in the North South position, the longer blade in front will catch significantly more air on it's underside (it is bigger, AND going a little slower in terms of rrpm) than did the short blade (from the 80 mph forward speed, not talking about rrpm)

This will give it a little bump up in it's path, so it ends up flying a smidgen higher on it's retreating path.

(Conjecture here)... when the same blade then reaches the South position, the (new, relative to the old short blades) upward push it gets there is not as significant, because coning puts it at a less acute angle to the (same 80 mph) airstream hitting it's underside (and it is also copping turbulent air) ...... so it does not get the same degree of little upward push on the advancing path, it gets a lesser push, and flies a slightly lower relative path, closer to the old arc of the shorter blades.

End result - the RTV tilts a smidgen to the right.


:twitch:

 

[birdy]

Mark, mate, im still tryn to compute wot iv learned here meself, so no good me sayn your rite or rong, coz im not sure meself.
But if you want my oppinion, i recon your close to the mark.

Things are still abit backwards in my mind wen i consider the rite roll of the machine with longer blades, coz iv now gota take into account wot load the prespinner cable is and in wot direction, and the fact that this mor'n i was playn abit with it, and found that at 0 AS the roll pressure rite is greatest, and it eases off as AS increases, then starts to roll left at bout 80kts indicated. Itll fly streight hands off at bout 65kts, a speed i rarely do in the ferel.
But first, i gota get CB's stuff sorted in me thick head.

 

[Mark E]

Thanks Birdy

... that's just what was called for - some experimental data! .....but that's thrown a spanner (or three) in the works ... the RTV changes from one side to the other with AS! (not surprising, I know, but it's be nice to know the specifics as to why edit : CB explanation below!)

Question is .... does it also happen with the short blades? same direction, just a question of degree with varying AS?

..wrecked my theories anyway .... though different components of those forces must (may?) be involved at different AS ...


and then THIS from CB! hwellfrom your RTV thread) (but at least it is specific)

The bottom line is that a coned rotor tilts a bit to the right, you must hold a tad of left stick pressure.

AND ...!

JC’s point was that airflow through the rotor disc is not uniform; more air flowing through the front half of the disc than the rear half, also contributing to the leftward tilt. That effect is more pronounced at low speed, as speed increases, flow through the disc becomes more uniform.

I was 100% thinking it was the other way, that the disk naturally wants to tilt left, then higher AS would make it tilt right.. (edit - maybe I was on the right track, cept for the initial left tilt, and if the rear half of the disk gets cleaner air at speed, it all makes some sense!) ... But anyway CB's info matches your experience - may be the same effect but at different stages of AS with the shorter blades? ... ie, back to trim differences. (?)

 

[kolibri282]

Reading through this very intersting thread I feel that there is room for yet another idea as to why the two sets of blades behave differently. I would therefore like to offer this idea for discussion:
It has been known very early in the developement of gyros that the long and slender blades are twisted by the aerodynamic forces acting on the blades. An early attempt to quantify the effect can be found here:

naca.central.cranfield.ac.uk/reports/1937/naca-report-591.pdf

Formula 31 of this report gives the difference in lateral disk tilt angle b1 if cyclic twist is taken into account. Using the measured values for the Kellet KD-1 gyro of this report at an adavance ratio of mu=0.2 the lateral disk tilt is o.4 degrees larger if periodic twist is considered. So my idea is: the difference in lateral disk tilt for the two sets of blades is due to a difference in perodic twist of the blades.
One of the parameters that has a major effect on blade twist is the torsional rigidity of the blades. This one is very easy to determine. Clamp the blade at one end and attach a batten of about 1x1.5 inch cross section to the other end of the blade (make sure you don't damage the surface!) The batten should be 6 feet long and the tip of the blade should be attached midway between the two ends of the batten. The lower part of the blade should touch the upper face of the batten (the batten is underneath the blade.) The whole thing then looks like an upper case T with the blade beeing the vertical part of the T and the batten beeing the horizontel upper dash. Next support the batten at the quarter chord point of the blade by an upright pole so that the blade tip can't droop. The pole should have a sharp wooden point and support the batten, so as not to damage your blade (that is why I proposed to attach the batten to the lower part of the blade). Now you attach a small rope to a can of beans (weighing 1 lb) and hang the can from the batten near the leading edge of the blade. Move the can away from the blade until the tip of the batten has traveld 2" downward. If the 1lb can is not enough use a family size can...;-) If you repeat the procedure with the other blade you get a fair idea whether the torsional stiffness of the two blades is vastly different. With everything else beeing equal a blade that's 10% longer should show 10% more travel of the batten for the same position of your can of beans.
The other possible source of a difference in blade twist would be a difference in aerodynamic twisting moment for one of the two sets of blades. That one would be much harder to measure. You could attach a camera to the mast of your gyro to take a picture of the blade when it is in 0 degree position (i.e. over the tail of your gyro) if you have three markers on the upper part of the blade it might be possible to calculate the blade twist using the QUEST algorithm for determination of position and attitude of a body. This algorithm is available from Thor I. Fossens website in the GNC toolbox. Obviously you'd have to go to some length to measue the twist angle in flight so you might want to start with the torsional rigidity.

I'm looking forward to the comments from the group.

Cheers,

Juergen

 

[C. Beaty]

Rotorblades with a pronounced pitching moment coefficient do have a major impact on rotor behavior. With negative pitching moment, the advancing blade depitches more than the retreating blade, suppressing cyclic flapping.

With an offset gimbal rotorhead, the rotor thrust vector stays more to the aft of the rotorhead pitch pivot, requiring a stiffer spring for trim.

Bensen wood rotorblades had excess TE reflex that compounded cyclic flapping and generally required no trim spring at all for flight at ~50 mph.

It’s all very dependant upon torsional stiffness and magnitude of pitching moment.

If I understand correctly, Birdy’s blades are of extruded aluminum (thick wall section) and of NACA 8H12 profile that has nearly zero pitching moment coefficient.

The subject of that NACA report, the Kellett KD-1 used rotorblades of Gö 606 profile that had quite a large negative moment coefficient and most of the torsional stiffness came from a step tapered steel tube spar. Later on, upswept TE tabs were fitted to the outer 1/3 of each blade to balance the nose down moment.

 

[kolibri282]

At 0.005 the pitching moment coefficient of the 8H12 is indeed an order of magnitude smaller than the -0.056 of the Gö606 but cm0 is not the only source of blade twist. The aerodynamic center of the 8H12 is at 0.278 chord, which is a good 10% from the usual 0.25. So the next question would be: are both blades balanced chordwise to have their center of gravity at 0.278 c? (btw the aerodynamic center of the Gö 606 is at 0.242 c while the center of gravity of the KD-1 blade was at 0.28, so the TE flaps would not eliminate the cyclic twist of the KD-1 blade) Chordwise balance should be quite easy to measure as well.

 

[Mark E]

I think CB has a good story below to explain a decreasing tilt to the right. (which is what Birdy describes)

It doesn't however address the original issue of the difference in roll effect between the shorter and longer rotors..

and that may be as simple as any difference in the individual symmetry of the two different sets - be it twist (as Juergen suggests), weight, profile etc. But if it is a consistent issue it is worth thinking about a bit more, just out of interest.


But some more info would be good if you have it Birdy?,
1. What are the roll characteristics of the shorter set at each of the speeds listed above?

2. And at 0 AS, what is the cause of the roll to the right (at idle? - lack of airflow trim? .. is there some rotor head trim built in)

3. And to what extent does that occur with the shorter blades.

I don't see what the pre-rotator cable could do, other than add friction.

CB's explanation below of the effect of coning is intriguing - as we have an 'imperfection' (coning, due to the flex of the rotors edit - builtin?) to some extent cancelling the tendency of the rotor disk (if it was on a flat plane) to lean left (very neat really!) I don't know why he didn't say that on the first page, Birdy (he does in a way in post #22, and so does JC in #26, but not so clear!) maybe our stumbling around in the dark helped! (Or maybe he just thought we would not understand it at that stage)


Quote CB (RTV thread post #11)


Because of coning, the blade in front, the 12 o’clock blade, catches a bit more wind than does the 6 o’clock blade. There are other reasons as mentioned by JC but that’s even more complex.

Now you already know that on a rotor with central flap hinges, displacement lags force by 90º. It shouldn’t be much of a stretch to understand that if a blade takes an upward whack at 12 o’clock, maximum upward travel occurs 90º later, at 9 o’clock.

As you also know, cyclic flapping relative to the spindle axis equals cyclic feathering relative to the tip plane axis. That’s what equalizes lift between 12 o’clock and 6 o’clock blades.

The bottom line is that a coned rotor tilts a bit to the right, depending upon coning angle and airspeed so to go straight, you must hold a tad of left stick pressure.

JC’s point was that airflow through the rotor disc is not uniform; more air flowing through the front half of the disc than the rear half, also contributing to the leftward tilt. That effect is more pronounced at low speed, as speed increases, flow through the disc becomes more uniform.

 

[birdy]

Geez Grim, now your leaven me behind.

I recon iv got it now tho, iv hada bit of 'thinkn time' and my heads got it......... i hope.
Ill post a SCG explaination on the other thread and see wot CB says, coz this threads getn off line abit.

BTW,
1; there is NO ROLL tendancy with the shorter blades at any speed.
2; not sure, i gota sit init for a bit tmorrow, so ill be ' thinkn' agin. [ but i recon its gota be sumthn to do with the prespinner.]
3: it dont.

 

[Mark E]

Man - that keeps it interesting.

Man - that keeps it interesting.

BTW,
1; there is NO ROLL tendancy with the shorter blades at any speed.
2; not sure, i gota sit init for a bit tmorrow, so ill be ' thinkn' agin. [ but i recon its gota be sumthn to do with the prespinner.]
3: it dont.

Man - that keeps it interesting. (for me anyway)

1. Bones, re NO ROLL effect with shorter blades vs right roll at 0 or low AS with longer blades. - Same experience? Or different? (if you happen to look in here!)

2. Big right roll effect with longer blades, at 0 AS, and none with short ones. There has to be a simple explanation for that. (I just don't see it yet, is all).

3. Ooops! Edit - scrap this one

Again. Many thanks.

.

 

[Mark E]

a simple man's rotary flight

a simple man's rotary flight

well, I guess I'll never know, but it has been fun thinking about it.

Gone here to state my thesis:

http://www.rotaryforum.com/forum/showthread.php?p=325520#post325520