Gimball offset question

[rtfm]

Hi,
Every post I have found on the forum dealing with gimbal offset assumes the following as "given":
The teeter tower, prerotator ring gear and horisontal torque tube form a unit at the top of the mast. In other words, the gimball offset (usually about 1") is situated as close to the top of the mast as possible.

Question:
What would be the implications if the torque tube, prerotator gear ring and offset gimbal were situated at the bottom of the mast, leaving only the teeter towers up high at the top of the mast? This would require, of course, a rotating mast, but let us not get caught up in this side-issue. I'm really interested in possible issues surrounding the placement of the prerotator ring, torque tube and gimball at the bottom of the mast.

Would the standard rule-of-thumb of 1" gimbal offset need to be increased? Seems to me that there is some underlying relative angle between gimbal bolt offset/something? which would need to be maintained.

Comments? Ideas?

Regards,
Duncan

[SamL]

I believe the next generation Cater Copter will employ such a design.
Check out there web site and promo video.

[Bob]

Do you mean anchoring the tetering block to the top of the mast and spining the entire mast by having the berring at the bottom ?
if so I'ed say the berrings would never handle the torque because that is a extreamily long lever arm....
now with the berrings that alow the rotor to spin at the top of the mast there is only a few inches of torque put down by the rotor. and they can handle that.... if we then move all the rest of the goodies down to the bottom of the mast say the torque tubes and pre rotator stuff and just have a drive shaft up to the rotor from the pre rotator sprocket, you'ed have a trimdious amount of leverage to over come by the controls... if the entire mast is tilting to the right left and foward and back...
for instance if you were flying along and pulled up you'ed be trying to move the entire mast back by rotateing it at its base
which doesn't sound too hard if you consider your controls should be ableto lift the entire craft anyway... but going in the oppisit direction you'ed be fighting the length of the mast pluss the force of lift by the rotors
I dunno ...I suppose it probly couldbe done , with the right leverage and good strong components but I don't see any benifits other than getting a few pounds of weight lower and you introduce many other possable problems into the mix....
think of your gyro setting there with the rotor slowly spinning and no hand on the stick... the rotor falls off to one side of the center pivot point and slams against the stops at a 8 to 12degree angleto the rest of the machine moveing the center pivot point of the rotors a good foot or more to the side ...would it alow the rotors to touch the ground ?
....its an intrequeing idea, but WHY ? there has to be some benifit to it or its just useless ! <grin>
....
I seriously doubt you could spin the entire mast with the teter block mounted to the top of the mast and the berrings below
if you did You'ed have to build in some sort of stops half way up the mast so as to not over stress the berrings
but You could build a gimbled system that would be strong enough to handle the torque and limit its travel as well but I am sure it would be purdy heavy, but you would in effect have all your controls to the mast behind the pilots seat and just a spinning tube and a rotor above as a mast....

might be worth thinking about Duncan ! I'm sure it could be built, but I don't see any benifits to it that make it worth it
besides easy to streamline ! <grin>
....
and Hay there Sam !!!!
...
Bob......

I just thought of something ... with the moveing mast like that how would you Haing test it ? if your mast can move almost 2 feet foward and back ???
your RTV would be all over the place ! i dunno if that would hinder or help actually ! in theory it should still work
... its a weird concept ! but may have its place !!!!
keep thinkin' Duncan ! <GRIN>

[Doug Riley]

Duncan, the correct offset is a function of the rotor blowback or flapping angle. That's the angle between the tip-path plane and a plane that is perpendicular to the mechanical axis of the spindle. This angle is zero in a vertical descent and, with a given rotor and aircraft, it gets larger as your airspeed increases.

The nominally correct offset of the pitch pivot will lie on an extension downward of the tip-path-plane axis. For instance, if the blowback angle of your rotor at cruise is 3 degrees, then the pitch pivot will lie along a line that passes through the teeter bolt and is inclined aft 3 degrees relative to the spindle axis.

In practice, we use extra offset and balance the controls with a spring.

A control system that tilted 4, 5 or 6 feet of mast would have unacceptably high control pressures. Remember that you're shoving the entire rotor mass around edgewise in a direct tilt-spindle system.

Laying out the airframe moments so that the rotor thrust line is behind the aircraft's CM, and then adding a down-loaded HS aft, essentially turns the entire airframe into a big gimbal head. The HS is an aerodynamic version of the trim spring; the aft displacement of the rotor thrustline is the offset.

[Bob]

Duncan....
I know this is unrelated but i was wondering how the Razorback is comeing along ? last I hurd you were having the cabin made for you
and I was wondering if that ever took place or not ?
and also if you solved the wing movement problem or CG movement
which ever you desided to go with ...?

I noticed on Homebuilt airplanes.com that a big reason that people shy away from rotor craft is their low top end speed
as most helos are below the 175mph speed limit
so I mentioned that you were designing a fast GYRO and that gyros might be an alternitive to the other wize slow Helo's
though I don't know the fastest speed a Gyro has traveled at
I do know that most gyros are flying well below the 100mph mark
, if i remember right you are shooting for 225mph and I hope you reach that magic number ! so you can prove it can be done !
I have a feeling you'll get alot of inquiries if you do !<Grin>
...
Bob......

[rtfm]

Hi,
Doug, thanks for the explanation. Extremely succinct and useful.

Bob - the Razorback has been chacked by a number of aircraft engineers, who raised a number of issues - mainly to do with stresses and CG placement. I've done quite extensive redesign of some key elements of the airframe, and have decided that before I commit serious development money into the project I would be wise to build a small (ie single seat) version of the gyro and test each of the innovations separately.

To this end I have designed an extremely simple all composite gyro which will weigh in at under 100kg (220lbs) ready to fly. I will use this as a proof-of-concept gyro and test all the new things out on it. For example, the variable pitch rotor head, the Residual Rotor Powertrain, and the elevators. Wings are completely optional, and can also be moved back and forwards while the gyro is on the ground. I am aiming to get the gyro in the air in a few months - actually, as quiickly as finances allow. Which might be longer than I expect, unfortunately.

Very soon now I hope to start posting photos of the airframe, and then progress reports as work progresses.

Thanks for asking.

Kind regards,
Duncan

[rtfm]

Originally Posted by Doug Riley

Duncan, the correct offset is a function of the rotor blowback or flapping angle. That's the angle between the tip-path plane and a plane that is perpendicular to the mechanical axis of the spindle. This angle is zero in a vertical descent and, with a given rotor and aircraft, it gets larger as your airspeed increases.

Hi Doug,
"Flapping angle". I know the looseness of some gyro terms has been discussed in the past, but it seems that "flapping" is one of the most misleading. You qualify this term with the synonymous "blowback", which is helpful. To be absolutely sure that I understand your explanation correctly, allow me to pahaphrase...

"The correct offset is a function of the rearward tilt of the tip path plane of the rotor in flight with respect to the teeter bolt". Am I correct?

Second you say that "it [rearward rotor tilt] gets larger as your airspeed increases". I'm not sure I understand this. As airspeed increases, in order to maintain S&L flight, the stick has to be moved forward, decreasing the angle at which the rotor tip plane makes with the airflow. Can you explain this further?

The nominally correct offset of the pitch pivot will lie on an extension downward of the tip-path-plane axis. For instance, if the blowback angle of your rotor at cruise is 3 degrees, then the pitch pivot will lie along a line that passes through the teeter bolt and is inclined aft 3 degrees relative to the spindle axis.

I was under the impression that the nominal blowback angle was 9 degrees, not three. This is why most masts tilt backwards 9 degrees. However, you go on to say:

In practice, we use extra offset and balance the controls with a spring.

which seems to indicate I have something wrong with my understanding. Are you saying that the extra 6 degrees of blowback angle is simple there to provide (with the assistance of the spring) tactile feedback to the controls?

Part of my question is academic, but there is also a practical dimension to it. Specifically, since the Razorback will be using tail elevators for pitch control (as in the early Littlewing's) the angle between the teeter bolt and the tail will be less than in a configuration which employs a fully articulated rotor head. The question is - how much less? 9 degrees? 6 degrees?

Laying out the airframe moments so that the rotor thrust line is behind the aircraft's CM, and then adding a down-loaded HS aft, essentially turns the entire airframe into a big gimbal head. The HS is an aerodynamic version of the trim spring; the aft displacement of the rotor thrustline is the offset.

Are you suggesting this as a design principle? Again, I was under the impression that a downward-loaded HS was only required to balance out the overtuning moment caused by a LTL pusher configuration. In a tractor with the thrustline at or near the CM of the gyro, I would have thought the horisontal stab would need to be set at 0 degrees. What is the theory here? Or do I have to build in a ground adjustable HS incidence in order to determine the optimum angle of incidence?

Thanks in advance for your help,
Kind regards,
Duncan

[Doug Riley]

[QUOTE=rtfm;155737]Hi Doug,

"The correct offset is a function of the rearward tilt of the tip path plane of the rotor in flight with respect to the teeter bolt". Am I correct?

Sorry, no. Blowback is the angle that the tip-path plane makes to the spindle (= the rotor shaft) .

Second you say that "it [rearward rotor tilt] gets larger as your airspeed increases". I'm not sure I understand this. As airspeed increases, in order to maintain S&L flight, the stick has to be moved forward, decreasing the angle at which the rotor tip plane makes with the airflow. Can you explain this further?

The blowback angle gets larger automatically. We can't stop that happening with movements of the stick. It's the angle of attack (AOA) of the rotor (angle of tip-path plane to the airstream) that must get shallower as we go faster to avoid climbing. To accomplish this, we tip the spindle forward by moving the stick forward. If spindle tilt is measured relative to the airstream, then Spindle Tilt + Blowback = AOA. Some of the forward movement of the stick is simply to compensate for increased blowback.

I was under the impression that the nominal blowback angle was 9 degrees, not three.

No, the normal SPINDLE angle to the airstream is 9 degrees. This, plus blowback of around 2 deg., makes the normal rotor AOA about 11 degrees.

Part of my question is academic, but there is also a practical dimension to it. Specifically, since the Razorback will be using tail elevators for pitch control (as in the early Littlewing's) the angle between the teeter bolt and the tail will be less than in a configuration which employs a fully articulated rotor head. The question is - how much less? 9 degrees? 6 degrees?

Best use conventional terminology. The "spindle" is the axle on which the rotor turns. The "teeter bolt" is the hinge pin on which the rotor hub rocks back and forth. Your gyro should be set up so that the spindle is at the same old 7-9 degrees relative to the aircraft datum.


Are you suggesting this as a design principle?

Yes. It's the same principle as the down-loaded HS on a FW plane. Even on a perfect CLT gyro, a slight HS down-load moves the CG forward of the rotor thrustline, which produces positive static stability instead of merely neutral static stabiity.

[Udi]

Originally Posted by rtfm

..."The correct offset is a function of the rearward tilt of the tip path plane of the rotor in flight with respect to the teeter bolt". Am I correct?

The relationship that is important here is between the RTV and the rotorhead pitch pivot. I think the drawings I attached below will make this easier to understand.

Second you say that "it [rearward rotor tilt] gets larger as your airspeed increases". I'm not sure I understand this. As airspeed increases, in order to maintain S&L flight, the stick has to be moved forward, decreasing the angle at which the rotor tip plane makes with the airflow. Can you explain this further?

Due to dissymmetry of lift, the rotor "blows back" in order to maintain equal lift between the upwind blade and the downwind blade. This means that the tip path plane is tilted back in relation to the rotor head. Yes, stick forward tilts the whole rotor head forward, and the tip path plane with it, but the angle between the tip path plane and the rotor head - and thus the angle between the RTV and the pitch pivot - is a function of airspeed, not stick position.

I was under the impression that the nominal blowback angle was 9 degrees, not three. This is why most masts tilt backwards 9 degrees.

The angle of the rotor is 9 deg. at about 50 mph. The blow back angle is about 2.

Part of my question is academic, but there is also a practical dimension to it. Specifically, since the Razorback will be using tail elevators for pitch control (as in the early Littlewing's) the angle between the teeter bolt and the tail will be less than in a configuration which employs a fully articulated rotor head. The question is - how much less? 9 degrees? 6 degrees?

I would start with 9 degrees. I would also set the stabilizer at 0 degrees and provide plenty of elevator power to control the gyro in pitch.

Udi
p.s. I just saw that Doug has already answered your questions - oh well. Maybe my drawings will be useful.

[rtfm]

Hi,
Hey, you guys are great. Thanks for clearing up what is essentially a rather complex set of relationships. I'll print out both of your replies, and ponder them in the workshop till I'm satisfied that I have a good grasp of the principles involved.

Thanks.

Duncan