Shape of tail feathers

[asra]

JohnI have read the thread on Nicks gyro, and in order to acheive a (centre of gravity) forward of the Rotor thrust line (in a high thrust machine) you simply need a large enougth down loaded Stab that will react to propwash and airspeed in order to balance the slightly nose down movement of a high thrustline.This will raise the nose during power increases, and lower the nose during power decreases.The only penalties of such a design is it could take some time to get it right, there are some drag penalties due to the harder working stab, and a CLT machine will normaly climb better compared to a high thrustline machine.Now lets see if I get some heat about this. [B)]Regards Sam.[][][]

[asra]

CamelA PPO in a High thrust line machine is normally due to icreasing the gyro's airspeed to a point were the machines thrustline pushes its nose down, and inturn also reducing the rotor disc angle to a point were a sudden gust, or pilot over reaction, or sudden power change will result in a forward tumble.The machines C of G being forward of the Rotor Thrust Line just means that if you put power on, the machine's nose will climb, and power off, nose will drop.This is what is called stable in angle of attack.Such a machine is less succeptable to PPO. To further reduce the risk of PPO or PIO we should always employ a Horizontal Stab of suitable size.Regards Sam. [][][]Image Insert: 57.34 KB

[asra]

quote:Originally posted by SamLCamelThe machines C of G being forward of the Rotor Thrust Line just means that if you put power on, the machine's nose will climb, and power off, nose will drop.This is what is called stable in angle of attack.Such a machine is less succeptable to PPO. To further reduce the risk of PPO or PIO we should always employ a Horizontal Stab of suitable size.Regards Sam. [][][]Sam,Maintaining the RTV behind the CG also produces G load stability. This means the gyro will automatically raise its nose in a downdraft and load the rotors. It will also lower its nose in an updraft. This is a stable response and is very important for limiting PIO and its consequences.

[asra]

PeterThat may be so, but relying on that alone in a high thrust line machine is not wise, were as a H.S would be much more effective, it would react quicker to those up and down drafts, if suitablly loaded would move the C of G forward, and help resist any PIO and PPO.Regards Sam.

[asra]

Sam,An HS is ALWAYS necessary. The ONLY way for an HTL machine to maintain the RTV behind the CG is have an adequate stabilizer with a negative incidence. I didn't mean to imply anything else. The stabilizer is the means to locate the RTV properly with respect to the CG, but it is the rotor's reaction to up/down drafts that will cause the nose to move in a stable direction as long as the RTV is behind the CG.

[asra]

It seems to me one important aspect of this whole discussion is being overlooked, i.e. the damping effect of a big paddle sticking out the back and countering up and downdraft effects on whatever is in front of the C of G, as distinct from the stab providing a continuous nose lifting effect by virtue of its AoA.To my simple little mind this means we should consider looking at a plan view of the apparatus - much as we advocate looking at a profile view when we consider the adequacy of fin and rudder.John EvansThink logically and do things well, think laterally and do things better.

[birdy]

I' got a 1"x 1' aluminium stab on the ferel, and it flies perfectly.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

John,The stab provides both static and dynamic stability. Both of these are important, but different. For example: in an HTL machine, an adequate stabilizer with some negative incidence is necessary to lift the nose and maintain the RTV behind the CG. That is static stability -- a balancing of all the moments around the CG such that the gyro will move in the right direction in response to a disturbance, for example pitching nose up in a downdraft. Dynamic stability is the ability of the gyro to return to its original state after a disturbance. That's the damping effect. Once disturbed, by wind or control inputs, a dynamically stable gyro will return to it's steady state after some period of decreasing occillations.It is possible, but not common, for a gyro to be statically stable, but dynamically unstable or neutral. It is not possible to be statically unstable but dynamically stable. The great thing about a horizontal stabilizer is that it accomplishes both goals.

[asra]

Peter You have made me totaly unstable with all that stability talk.[][][]How about a few more Like Airspeed Stability, G- Load Stability, Negative Stability, Neutral Stability, Positive Stability, Power Stability, and Instability. Mate my head's realy spining now. [8D] Only Joking !!!!Your right with what you have stated and I agree, a Horizontal Stab will acheive both. [][][]Regards Sam.

[asra]

Fair enough Peter, the point I was making is that while suprisingly little A of A on the large HS is very effective in balancing out the moments about the C of G, adequate area of HS and moment arm should be looked at in relation to the area of pod in front of the C of G to stabilise in the presence of vertical air movement. Thus it would appear that the induced drag penalty to be paid with a large HS is probably less than with a smaller one requiring greater A of A. Perhaps Cierva knew what he was talking about when it came to tail volume.John EvansThink logically and do things well, think laterally and do things better.

[asra]

Top stuff Sam. Now I just have to print it all off and spend 3 weeks trying to comprehend.I wunder if an inverted H-stab with some diethral, anhedral and a delta shape would give us an advantage of small drag coefficent, good stability in rough air and buy us a beer in the pub.Rob Leith said G'day too.

[asra]

Is it possible then to have a HS that's too big?If the HS is meant to balance out the moment being created for straight and level flight, could a sudden big squirt of prop thrust cause a big enough nose up effect that it starts to unload the lift of the rotors?Would that nose up effect then be dampened by the momentary loss of lift?TedSquare bear at least once a day keeps the blues away.

[asra]

Thanks CamelSay G'Day to Rob !!!You can employ an old method once used to find an aircrafts centre of pressure, and help you confirm the size of your HS being sufficient. I will try and explain how I worked it out on mine.First you need to know were exactly you machines Vertical Centre of Gravity is.This spot is the spot your machine piches and rolls through. Consider it the centre of all movement.Now you will need a high enougth viewing spot to photo your machine from above. eg. I used my garage, or you could use a ladder or 2nd story home balconey.So sit in your machine and get someone to take a photo from above, pref over the centre.Now print onto heavy weight paper, then cut it out following the machines contures as closely as possible.Disregard the the rotor blades.Now mark the C of G in the exact spot on you overhead cutout of your machine.Using a pin , punch a small hole through this spot and balance the cutout holding pin point and allowing cutout to rest on flat pin head.If your cutout is tail heavy, then you HS is large enougth and far enougth back.If your cutout is pod heavy, or front heavy, then your machine requires a larger HS, or requires moving it further back.This test isnt perfect as not all components on your machine are flat, but your HS normally is, so enything forward of it that isnt flat would only help adding to the test result.Hope you can make sense of all this ??????????.Regards Sam. [][][]

[asra]

Sam,Not sure that I would agree with your method here. The piece of paper would only reflect a 2 dimensional aspect whereas in reality the weight and balance of your gyro is 3 dimensional.I'm more than happy for a more detailed explanation of your method if you are prepared to post it for my review.Ted

[asra]

TedThis method is all about surface area and not weight.If your machines paper cut out is tail heavy then it shows that during a vertical gust, up, or down, your machine tail would move in that same vertical direction.This is how your machine should respond during vertical gusts.The cut out takes into account total 2D surface area in front, compared to behind the C of G. The larger of the 2 will weathercock first.Image Insert:[img]http://www.asra.org.au/forum/uploaded/SamL/200574233246_****.gif[/img] 3