Stability of the V-22 and other side-by-side rotorcraft

[Dave_Jackson]

The same craft. http://www.unicopter.com/Thinking.gif Different conclusions.


Focke-Achgelis Fa223
http://www.luftarchiv.de/hubschrauber/focke_1.jpg

Sud Est Aviation (Aerospatial) SE-3000
http://www.lasercamera.de/xml/content/OF00000000400003/1/74/462741.jpg

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Hendrick Focke, the inventor of the FA 223, said in the Jan/1947 issue of the 'American Helicopter' magazine;

"The Fa223 is statically and dynamically completely stable around all axes other than the longitudinal one. At traveling speed of 140 to 150 km/h all controls can be released, because longitudinal instability disappears at about 120 km/h. Then this aircraft behaves just like a normal airplane and is automatically stable."
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Jean Boulet, the test pilot of the SE3000, said some time later;

"Despite the first successes of the Focke aircraft, the two lateral rotors formula is not satisfactory. It shows a good longitudinal stability, because the empenage is well out of the rotor's stream, but in return the formula has defects concerning the lateral stability:

- Dynamic instability of roll while hovering in ground effect; the ground effect increases on the side toward which the aircraft is leans, tends to push the aircraft to the other side.

- Static instability of roll in translation at low speed; when a turn is engaged, at a speed lower than the climbing speed, the increase in the relative speed of the external hub produces an increase in lift which leads to increase in initial tilt.

It will be necessary to wait for the era of automatic pilots which will artificially fix these defects, to see reappear the lateral rotor's formula with the Bell XV 15."
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Translated by Claude Dawson from the French publication 'History of the Helicopter'by Jean Boulet.

[Matthew Parsons]

Jean Boulet's words are less convincing at first glance. In a ground effect hover, a disturbance that causes an uncommanded roll will result in placing one rotor closer to the ground. If that does in fact increase that rotor's lift due to added ground effect, then that rotor will create a restoring moment. That makes the roll statically stable. How that same effect could lead to dynamic instability is not obvious to me.

His second point sounds more like a spiral stability concern. I'm not convinced that the effect described creates an undesirable flying condition.

Both his observations have to do with low speed roll channel. I'm curious if it's a control power issue. It seems to me that it would be very easy to create very strong lateral control power with this configuration.

My $.02

Matthew.

[Dave_Jackson]

- Dynamic instability of roll while hovering in ground effect; the ground effect increases on the side toward which the aircraft is leans, tends to push the aircraft to the other side.


Mathew,

Your questioning of Jean Boulet's statement above to 'dynamic' stability sounds valid. However, his statement does suggest the possibility of 'static' instability on rotorcraft with long fuselages.

This instability may be an insignificant problem for current helicopters, but on future transport helicopters the ratio of payload to empty weight will increases. This will result in the ratio of the fuselage's side X-sectional area to the rotor thrust ratio increasing also. In turn, this means that the lateral area of the fuselage will present a larger area for the downwash in ground effect to push against.



For an example, consider the situation of a helicopter that is transitioning to Starboard. When out of ground effect, the cyclic will be to the right and the rotor disk(s) will be lower on the Starboard side.

As the craft descends it will come into ground effect. There will be a higher 'pressure' under the Starboard half of the disk area then there is under the Port half. This will require the pilot to apply an ever increasing cyclic to the right, which might, or might not, provide static stability.

However, at a low enough elevation, the thrust against the fuselage may apply a stronger force to Port than the lateral component of the disk thrust is applying to Starboard. The craft then starts transitioning to Port. What is the pilot to do, since more right cyclic will only exasperate the situation?


The following sketch is borrowed from Sikorsky's presentation to the Jan/06 AHS VLADC. It is displayed solely for the purpose of advancing rotorcraft technology.
http://www.unicopter.com/Temporary/HeavyLiftRotorcraft.gif


$.02 more into the pot.

[Matthew Parsons]

Dave,
With one rotor being closer to the ground, that implies the rotor discs are staying fixed with respect to the fuselage. With that in mind, there shouldn't be much thrust (downwash?) against the fuselage. I believe this is where you're going with the lateral X-sectional area. Shouldn't be a problem for control power, but as with all helicopters, increasing any X-sectional area can come with a handling penalty. I doubt you'd get into a negative return situation where more right cyclic decreases your right lateral velocity.


If the discs are tilting then the port disc will contribute thrust in the desired direction without adding any drag.


Cheers,
Matthew.

[Graviman]

Initial thought is that the FA223 had outboard retreating while the SE-3000 had outboard advancing - this explains the yaw roll coupling. Dynamic instability in ground effect simply refers to the fact that the pilot has to provide the damping to the "aerodynamic spring" - you find this with collectives too, as the pilot has to work relatively hard for height control near the ground.


It will be necessary to wait for the era of automatic pilots which will artificially fix these defects...


Lockheed did it mechanically in the CL475...

I imaginge the Sikorsky presented sketch was for similar payloads. It is definately worth 1000 words on why coaxial is far more convenient for unprepared landing sites...

Mart

[Dave_Jackson]

The following excerpt is from an excellent article by Mr Roger Conner in the latest issue of the 'Journal of the American Helicopter Society'. However, it should be noted that the reference for this excerpt is Jean Boulet's book.

"The twin lateral configuration had inherent handling difficulties that the Germans had hidden well - specifically that in crosswinds, or while maneuvering, one rotor would move through effective transitional lift while the other had not, causing a low-speed lateral instability"

I suspect that, the risk of Boulet's first objection [instability of roll while hovering in ground effect] will be greatest for the Coaxial configuration and this risk will diminish as the two rotors are located further from the fuselage. The risk of Boulet's second objection [instability of roll in translation at low speed] will be greatest for widely separated rotor configurations and this risk will diminish as the two rotors get closer together.

Mathew,
You may well be correct, or perhaps, it will be necessary to conduct actual testing to arrive at definitive answers.

______________________

This thread is probably;
:ok: for the very few :8
:yuk: for the very many :bored:

:)

[Dave_Jackson]

Mart,


Initial thought is that the FA223 had outboard retreating while the SE-3000 had outboard advancing - this explains the yaw roll coupling.Initial action - look at pictures in first posting.

I imaginge the Sikorsky presented sketch was for similar payloads. All of the above configurations are based on Boeing 737-700 criteria.It is definately worth 1000 words on why coaxial is far more convenient for unprepared landing sites..Challenging words. :) With little humility, may I 'sell' the Interleaving configuration? :eek:
It is definatly worth 50 words on why the Interleaving is far more convenient for unprepared landing sites....

Coaxial rotors with a disk area of 522 sq-ft. fits in a square of 26 x 26 = 666 sq-ft.
Interleaving rotors with a disk area of 522 sq-ft. fits in a square of 31 x 20 = 620 sq-ft.

The interleaving has greater ground clearance under its rotors, due to the dihedral.

And there's more.........

[Graviman]

Initial action - look at pictures in first posting.

With the level of motion blurring, and general lack of resolution i didn't want to draw any conclusions, Dave. I take it you know for a fact that both were outboard retreating?

Your concept is outboard advancing isn't it? Conceptually outboard advancing in this config should produce correct sense roll/yaw coupling. Perhaps Jean Boulet was complaining about too much of a good thing.

...[instability of roll while hovering in ground effect] will be greatest for the Coaxial configuration... ...[instability of roll in translation at low speed] will be greatest for widely separated rotor configurations...

OK, this makes sense. Effectively you tune the basic config to give just the right roll/yaw and sideslip/yaw coupling. Of couse there will be variations with payload and g load, so you will always need the tail stabilisation (Nick's comment about the importance of horizontal stabilisers is food for thought).

Coaxial rotors with a disk area of 522 sq-ft. fits in a square of 26 x 26 = 666 sq-ft.
Interleaving rotors with a disk area of 522 sq-ft. fits in a square of 31 x 20 = 620 sq-ft.

Hmmm, good point well made. Including the necessary horizontal stabiliser (and possible pusher prop) would likely alter these figures to coaxial 26 x 31, interleaving 31 x 31. Say 806 sq-ft vs 961 sq-ft.

General: How do the drag figures stack up? I realise your concept would have aerodynamic pylons. I also raised the concern a while back about the need for a more complex drivetrain, which in practice just adds weight/cost.

Must admit though, apart from longitudinal stability, you've put up a pretty good arguement. In my defence ;) i can only say that gyro stabilisation delivers this performance, regardless of conditions.

Mart

[Dave_Jackson]

Mart,

Kaman tried outside forward and then went back to the breaststroke. The only concern that I know of that relates to the direction of rotation is the pitch-torque coupling. The inside forward on the K-max causes the nose to lower when the power is decreased. The side-by-side helicopters above have a slight dihedral in their two rotors and they rotate outside forward. Therefore, they also will pitch nose down also when the power is decreased.

Both my concept craft have outside forward. The large interleaving 'Nemesis' has dihedral between the rotors and it rotates outside forward. This should contribute to stability for a large transport craft. The small intermeshing 'UniCopter has anhedral between the rotors and it rotates outside forward. However, the anhedral is less than previous intermeshing helicopters, the large preconing should give the combined disk dihedral during slow flight, and the craft is intended to be agile.
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I agree that gyros will help a pilot. The gyro will also help an auto-pilot. However, IMO it is not a substitute for good aerodynamics.

Nick may correct me, if he is reading this thread.
It has been said that the horizontal stabilizers on the Blackhawk were made variable when the rotor had to be lowered so that the helicopter could be transported by airplane. This changed the aerodynamics of the craft.

[NickLappos]

Dave,
The need for a stabilator on the Black Hawk is directly tied to the canted tail rotor. The canted tail rotor is a fall-out of the aft cg arrangement of the aircraft. The aft cg is a fall out of the requirement to fit the C-130.

Let me explain. The canted tail rotor provides about 500 lbs of vertical force in a hover, which is lift. This lets the CG of the H-60 be very far aft (FORWARD CG is 2" AFT of the mast!!). This means the cabin can be forward of the mast and the fuel aft, so that the aircraft can be low and narrow. No fuel below the cabin, nor in sponsons outside, since both fuselage arrangements would bulge the fuselage and not let the 21,000 lb H-60 fit into a C-130 cabin.

OK, but why does a canted tail rotor make the stabilator necessary? Because the canted tail lifts the back end only when there is a need for antitorque. If the aircraft is near zero torque, the tail rotor lifts nothing, and at zero torque, it actually produces a slight down force. In a quick stop, where the MR torque is near zero, the tail rotor has no lift, so the equivilent CG shift for an H-60 is about 5 more inches aft. This chews up any forward cyclic margin in that maneuver, and leaves nothing to counter the horizontal tail download that the aircraft sees when the aircraft slows below about 40 knots and the main rotor wake falls on the tail in said quick stop. Before the H-60 had a stabilator, the pilots could run out of forward stick in an aft CG quick stop. When the stabilator was added, and it moves to the trailing edge down position, it actually provides some up lift (or at least has no downlift) to help keep from running out of forward stick.

So, like the guy who swallowed a fly, the H-60 has a stabilator because it has to fit into a C-130, because it has an aft CG, because it has a canted tail. Whew!

[Graviman]

I agree that gyros will help a pilot... However, IMO it is not a substitute for good aerodynamics.

Agreed, but it does ensure cyclic position always controls pitch/roll directly, with no scope for instability. The machine does have to be designed right first though, with Hiller showing how putting a gyro in as an afterthought does not make for good controlability. I can definately see where you are coming from with the Nemesis and Unicopter designs. (Is Nemesis to be your final undoing then? :} )

Considering the remaining potential concerns with longitudinal stability, i would keep gyro control augmentation as an option. The Lockheed system struck me as both mechanically simple and very effective. Consideration of this up front will probably short cut many of the control difficulties that have affected fixed wing as their performance envelope was expanded. A gyro won't get confused in cloud or induce oscillations in adverse conditions (don't be shy - we've all done it :O ). Of course it won't indicate when control is becoming marginal either, so careful instrumentation is recommended (i prefer DC accelerometers).

Absolutely i'm not saying to retrofit it because of an initial oversight, or mission requirement alteration during development.

When the stabilator was added, and it moves to the trailing edge down position, it actually provides some up lift (or at least has no downlift) to help keep from running out of forward stick.

Interesting story about the development of the H60, Nick. Am i correct in understanding that the main rotor would have still had sufficient authority to keep nose down in a quick stop manouvre without the stick limitation? I still marvel at that photo of the Lockheed XH-51A hovering with a guy hanging from an outrigger, and the pilot waving - with both hands!

Mart

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General interest:

The Lockheed CL-475 (http://www.nasm.si.edu/research/aero/aircraft/lockheed_cl475.htm) and the first Lockheed XH-51A are now stored at the Army Aviation Museum in Fort Rucker, Alabama (Ref: WarbirdTech vol 27).
CL-475 was a light heli with entirely mechanical gyro augmentation, while the XH-51 (later 286) was a medium heli with hydraulic augmentation of the same system - capable of loops as well as stable hover!

[Dave_Jackson]

Nick,

Thanks for the clarification and detailed description about the reason for the movable HS. As always :ok:.


Mart,

The name was changed from 'Igornemesis' to 'Nemesis' since the former is a little too provocative. Also, the name 'Youngnemesis' doesn't play to well. :O

This may provide an additional source of information about the gyro for you. It is cut from a larger quotation by Rene Mouille, while he is discussing the French Alouette I. ".... I was working on this gyroscopic stabilization system, more intricate then the Bell's because we had three-bladed rotors instead of two. (I was surprised and amused to note that Lockheed offered the same solution, fifteen years later with their first three-bladed helicopter with a rigid rotor.) .... "

[NickLappos]

Mart,
You asked, "Am i correct in understanding that the main rotor would have still had sufficient authority to keep nose down in a quick stop manouvre without the stick limitation?"

Actually, the limitation of control is the stick stop, which is usually set by the maximum geometric angle the head can make for the blade, or by the maximum moment that the system can carry when the blade is at that angle. In other words, both control stick travel and rotor capability, at the same time, limit the authority.

Nick

[Graviman]

In other words, both control stick travel and rotor capability, at the same time, limit the authority.

Ah OK. Thanks for clearing me up on that, Nick.

Mart