Stick position

[NiqueNaque]

Mr Wendell,Are you sure about that?Let me walk myself through it..If you are in trim for straight and level flight, with a CLT machine and no horizontal stab the RTV will pass through the COG.To make the nose go up you pull the stick back,the rotor follows, the RTV is now forward of the COG, pulling the nose up. To make the nose go down you *gently* push the stick forward and the RTV goes behind the COG, pulling the tail up.So if we are flying along, minding our own business and along comes a nasty updraft, under my arrangement, you are flying along straight and level with your RTV on the COG, you do nothing, but the aircraft rises in the updraft and continues to fly along. You strike a downdraft, and do the same thing again, you sink but continue to fly s&l.If you strike vertical shear where the front half of the rotor is in lift and the back isn't, then your instinct will be to push the stick forward, maybe too quickly, you do this and nose down, you are now in the updraft and your RTV is behind the COG pulling the tail up, decreasing your angle of attack to the air stream and reducing the lift, hence maintaining s/l flight... tick tick tick... thinking.. hmm..You approach and pass through the downdraft, your instinctive reaction is to stick back, pulling the nose up, increasing the lift and continuing in the s/l path... hmm..Add a tail stabiliser:You enter the updraft shear and the nose rises due to the stab adding lag to the tail, if the RTV is in front of the COG the situation is exasserbated, if it is behind it is not, the same applies to the downdraft.So, Mr Wendell is right when there is a stab present, not so sure when there isn't one.Add a pod...The area of the pod opposes the effect of the stab.. please considerYou can work it out.Cheers,Nick.

[Greg Mitchell]

Nick,This "I'll note it is not good practice according to Greg Grimminger to take the RTV behind the COG as this may cause a bunt over." is not correctly quoted.The GofG forward of the RTV by whatever design application applied is a stablity requirement.Also, "You enter the updraft shear and the nose rises due to the stab adding lag to the tail, if the RTV is in front of the COG the situation is exasserbated, if it is behind it is not, the same applies to the downdraft." When I have experienced a sudden up draft in Butterfly, the nose points down toward the relative airflow, maily because the HS is doing its job. This is not a dramatic or alarming airframe reaction and because I understand this to be an attitude change which is in the right direction, I have quickly become accustomed to this in my very light gyro.So given that you have a stabbed gyro with sufficient surface area on a reasonable moment arm and your flying along S & L, you enter the up draft, how does the nose rise? Why would the RTV be ahead of the C of G ? No I can't work it out! You will need to explain your theory in a little more detail to help me understand what it is you are thinking.Cheers,Mitch.www.thebutterflyllc.com

[Tim]

quote:Originally posted by Waddles Even with the B747, over zealous use of the controls will be manifested by the onset of stall buffet caused by the inability of the airfoil to react instantly to the rapid control input. In this situation, wing flexing is alarming if one happens to be looking there at the right time.Allan has brought up a very good point here. A gyroplane rotor can be induced to act in a similar way.When astute gyronauts realise that the total control movement of the rotor head is approx 9 degrees each direction from centre, they assume that if they had more movement, they would be able to do more advanced manoeuvers. Technically a gyro cannot even land on a 10 degree slope without "flying over". The danger of increasing the head movement is the possibility of inducing blade stall through too much pitch change (stick movement) without the rotors being able to keep up. It bahaves exactly as Allan said - "the onset of stall buffet caused by the inability of the airfoil to react instantly to the rapid control input. In this situation, wing flexing is alarming". In a gyro this is called Precession stall and the rotor becomes uncontrolable as the stall propagates along the blade. Some suggest that the controls then become reversed but I would suggest that it is more like a stalled plane - forward stick to take angle of attack off the wing and un-stall it. Also similar to a stalled propeller causing lack of thrust at a critical time - you have to throttle back to regain thrust.With rotor blades I would think that you would have to move the stick back some (if you could) and hope that there is enough "fly" left in the unstalled section of the rotor to get a response.The good news is that it is fairly hard to get into that situation because the rotor responds very quickly, generally as quick as most pilots move the stick, which means that they are getting nowhere near stalling the blades.Tim McClure

[NiqueNaque]

Greg,In a good design at s/l flight the RTV should not induce pitching moments unless the joystick is moved away from its neutral point by the pilot. A pitching moment is generated by the RTV moving forward or aft of the COG. If the Gyro flies s/l with the RTV offset from the COG by default then the pitching moment that is generated by the offset x the RTV has to be balanced by something such as a horizontal stabiliser, eg, the stabiliser that Paul Bruty was always referring to with a bias of about 2 degrees downward, ie, the stabiliser is always trying to pull the nose up / push the tail down due to its angle of attack in s/l flight. This would follow because the RTV in this instance is behind the COG trying to pull the tail up / push the nose down, so hopefully the stab cancels the effect of the RTV x offset to COG.Hope this helps,Nick.

[Greg Mitchell]

Sorry Nick, not sure how that addressed my questions. Let's look at this, simply as an excersive."If you strike vertical shear where the front half of the rotor is in lift and the back isn't, then your instinct will be to push the stick forward,(WHY? MY GYRO NOSES DOWN MOMENTARILY INTO THE RELATIVE AIRFLOW, AS i BELIEVE IT SHOULD, I HAVE NO INSTINCT OR DESIRE TO PUSH THE STICK FORWARD AND NOSE HER OVER EVEN MORE) maybe too quickly, you do this and nose down, you are now in the updraft ( NOW I'M IN THE UP DRAFT MY C OF G IS FORWARD OF THE RTV AND IF I HAVENT SHAVED THE POWER BACK, I'M CLIMBING) and your RTV is behind the COG pulling the tail up, decreasing your angle of attack to the air stream and reducing the lift, hence maintaining s/l flight... tick tick tick... thinking.. hmm.."Help me out here Nick, Confused. Jump in any time Ted, this is for your enjoyment and comment.Tim,We land into rising ground..7 to 10 degrees (1st third of runway 32 Cranbourne). What do I need to know about this 'flying over' business?Cheers,Mitch. www.thebutterflyllc.com

[asra]

Nick,The RTV must be behind the CoG for the gyro to be positively stable. One of the purposes of a horizontal stabilizer is to maintain the RTV behind CoG in a machine with a high thrustline.In a HTL machine with no stab, there is a static nose down pitching moment in powered flight. The RTV will move in front of the CG to balance that moment, creating an unstable condition. (If the RTV didn't move forward, the machine would simply bunt over, but it doesn't.) With the RTV in front of the CG, a downdraft, for example, which decreases the AOA on the rotor will cause the nose drop, which will further decrease the AOA of the rotor, etc. This is an unstable condition.If the same machine is equipped with an adequate horizontal stabilizer with a negative incidence (it provides up lift), the stabilizer will produce a nose up pitching moment in forward flight which will counter the nose down pitching moment produced by the high thrustline offset. This will move the RTV behind the CG, putting the aircraft in a pitch stable configuration. In the same downdraft, for example, when the rotor AOA is decreased, this machine will lift its nose and increase rotor AOA. The stab also provides dynamic stability, but that's a subject for another post.It's really that simple.BTW My name's Peter. No Mr. needed.Peter Wendell

[NiqueNaque]

oh,Bugger me, you guys are talking about an unstable machine to begin with.. one that needs a horizontal stab, there's my error..Cheers,Nick.

[Tim]

quote:Originally posted by Greg MitchellWe land into rising ground..7 to 10 degrees (1st third of runway 32 Cranbourne). What do I need to know about this 'flying over' business?G'day Mitch. If you fly in towards 9 degree rising ground to land it is most likely that the wind will be "following" the ground surface and the landing should be normal. If however you land just short of the rise, the wind is more likely to be horizontal while the gyro may still be on a 9 degree angle. If this is the case, full forward stick will still leave you with a 9 degree angle of attack on the rotors with respect to the wind, which could definately cause a fly over.This is generally only a problem with a head wind or a high landing speed, such that touch down occurs while the blades are still at flying speed. Most gust fly over landings can be avoided by doing a full flare landing and "killing" the lift in the blades, but this is often impossible with a head wind. If any doubt exists, then land on level ground if possible and get the stick full forward and centre, where the rotors have zero lateral pull in any direction.Peter, I think you made a typo in your statement: - "horizontal stabilizer with a negative incidence (it provides up lift)"I think you meant "(it provides downforce)" Tim McClure

[Greg Mitchell]

Hi Tim,Great explanation Tim. I can visualize this now and see where there could be a problem. Essentially when I first soloed I was doing aircraft type approaches into the first section of rising ground on the runway and found this helpfull in nil wind. These days all my landings are simulated power out approaches and I aim for the middle of the strip which has a flat wallowed section and still allows plenty of runway for touch and goes. Thanks again.Mitch.www.thebutterflyllc.com

[asra]

quote:Originally posted by TimOriginally posted by Greg MitchellPeter, I think you made a typo in your statement: - "horizontal stabilizer with a negative incidence (it provides up lift)"I think you meant "(it provides downforce)" Tim McClureTim,Down force on the tail, up lift on the nose -- same thing, but you put it in the clearer way.Thanks.

[asra]

How far behind the COG can the RTV be in S & L flight? Are we talking of the need to be just behind or a matter of inches?From Tim's point relating to landing into a rising slope then I assume the stronger the wind the further down the slope one needs to be. Warwick airstip has a major disadvantage particularily during the winter months when the South easters are blowing, in so far as the trees on the southern side of the 09 end create a windshear about 40 feet above your touch down point. I'd be interested to know if anyone has any experience in this matter? In my early fixed wing training (cherokee 160) it caused a lot of heart ache.Ted

[Greg Mitchell]

Hi Ted,Dont know if this is just semantics but it helps me. Much of what I have read and how I visualise is in terms of the C of G being FORWARD or AHEAD of the RTV. A neg incidence HS on a HTL craft can be used to create down force on the tail, forces the nose up and ensures the C of G is ahead of the RTV. A LTL craft uses power augmentation to push the nose up keeping the C of G forward of the RTV. In the case of the Butterfly the HS which is in-line with the rear keel is essentially employed as a pitch damper. It is my understanding that up to a point the further forward and more readily the C of G is held ahead of the RTV, the better.Tim bought up an interesting point some time back, that if Butterfly actually flew slighly nose down, then even an inline keel HS would in fact be presenting a slight negative angle of attack, thereby having a down force on the tail which in effect is trying to push the nose up and keep the C of G forward of the RTV again.I'm sure I'll get jumped on if I've got it wrong. Like you, I'm still trying to learn as much as possible and get my head around a heap of stuff.How I visualise Tim's explanation is this. 18 degree total travel fore and aft head movement. Full forward, disk is flat 0 degrees. Normal S & L 9 degrees. Full back stick 18 degrees.On flat ground, lift can be dumped full forward stick. At the begining of and at the top of the slope the wind is more likely to be horizontal to the flat ground and in the middle of the slope wind more likely to be slipping down the slope.On a nine degree slope, either at the bottom or near the pinnicale, full forward stick, may still present nine degrees of disk to the air flow.(if the gyro is on 9 degree slope). So I would guess, (1) find a flat section of the strip or failing that, (2) land in the middle area of the slope. Is this on track Tim?Mitchwww.thebutterflyllc.com

[asra]

Mitch, I follow your explanation easily. What I'm chasing however is what is the general distance for the RTV to be behind the COG in S & L flight? Are we talking just behind or can it be a matter of inches? I understand that this may change with individual gyros given the unlimited setups possible and the differences in Pilot weight etc. The Size of the Horizontal stab and its angle of incidence is then mainly driven by the need to maintain the RTV in a position behind the COG. How far forward can it be safely positioned for a climb? One answer is no greater than 9 degrees as you wouldn't be able to pull the stick back further than that providing that the RTV and COG were neutral.

[Greg Mitchell]

Ted,I believe it can be as much as "a couple of inches". However, I have put this question to Larry for specific numbers concerning Butterfly/Monarch in climb and S & L. "One answer is no greater than 9 degrees as you wouldn't be able to pull the stick back further than that providing that the RTV and COG were neutral."During climb out in Butterfly, at 35-40 knots, the nose seems to be riding high and some forward stick is required to maintain best L/D airspeed, during climb at full throttle. Having a thrustline which is slightly under the C of G, probably accounts for the nose high attitude? I would suspect that the C of G would be held at the most forward position ahead of the RTV at this time. Again, I may be off base here. I'm not sure what you mean by the RTV and C of G being neutral. When the RTV is on the CofG the gyro has neutral stability.I'll get back to you when I have some figures from Larry.Mitch.www.thebutterflyllc.com

[asra]

Mitch, I meant that when the RTV and the COG bisect each other. So that neither one was in front or behind.Ted