Shape of tail feathers

[asra]

Has anyone tried a negative or upside-down air foil on a H stab? My thinking(probably flawed)is that with an increase in AS/prop blast the force generated is increasing pitch. At lower AS /prop blast forces the effect is more neutral. The benefit would be a smaller H stab with less parasitic drag.(possibly).FYI, the longest recorded flight for a chicken was 40 feet.(self generated).

[telf]

PB had one on a single seater to try the same theory.He seemed happy with the result, I believe.I think he said that he used an offcut of Jeff H-S's rotor extrusion.The hstab was mounted below the tail, on the boom, so it wasn't directly in the centre of the prop-blast.The theory was to increase reverse lift at the tail as the airspeed rose.This, in theory, should help to prevent a ppo, by dragging the tail down, as well as a pio.Anyone else tried it?Flying - The knack lies in learning how to throw yourself at the ground and miss. (Douglas Adams-The Hitch-Hiker's Guide to the Galaxy)

[asra]

Camel if you employ an inverted aerofoil as your HS and set it at zero degree's of incidence then it will produce no lift up or down.By placing it in the prop wash it will enable it to react quicker during any vertical gusts.To explain it consider this, during straight and level flight air is being drawn through the prop and straight over the HS.During a vertical gust, up or down the air does enter the prop but also at an angle. The prop will blow the angled gust out at a reduced angle, but it is still angled. Because the prop increases the angled airs speed it will have a more dramatic effect on a stab in the prop wash.Having an inverted stab at zero degree's means if the craft desides to nose over the HS will have air being blown over it at an increased angle af attack increasing its effect to neutralize the desturbance. An inverted HS will also help keep your C of G in front of your Rotor Thrust Vector in the case of a high thrust line machine. The other reason I prefer a HS in the, or close to, the centre of the prop thrust line is if the machine noses up, this is no problem, but if it noses down, then a low HS will be hidden behind the seat, engine, and passenger, lessening its effect to function correctly.A centre mounted HS during a P.P.O will most likely be forced up above the draggy bits into the flowing air enabling it to do its job better.(See pic below)Regards Sam. [][][]Image Insert: 66.77 KBImage Insert: 3.44 KBImage Insert: 61.96 KB

[asra]

Is this good or bad Sam?If C of G is out in front I am guessing here that it will be harder to PPO.Gotta keep it simple for us pest controllers.Great graphics tho'Cam.

[Waddles]

Gidday Birdy & Murray,A bit late on this one, but I tried the vortex generators on the RAF with the doors on. No good. I think it was because they were too close to the point of airflow separation at the rear of the cabin. I think it was Owen Dull who told me that the VGs need to be placed at a point about 2/3 of the distance back from the nose of the cabin. Didn't try that, as I opted to fly with the doors off at all times. Murray has probably sold them by now as I haven't seen them around the hangar for a while. All I know is that on one flight I did in the earlier days, the gyro yawed such that it needed positive rudder input to recover from the side slip. After that, I was dancing on the rudders all the time to stop any potential extreme yaw.I flew with Geoff Jamieson in Tassie earlier this week and noted that his RAF is exactly the same. If you look at a side on picture of a RAF and compare the "sail" area forward of the mast to that rear of the mast, I think it speaks for itself despite the aft moment of the vertical tail feathers.That's my tuppence worth.Regards,Waddles.

[birdy]

Actualy, the wobbl'n bout in yaw of my Raf don't bother me much, its usualy too rough to notice any 'out of trim' fly'n anyway.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

Al,yes they have to be placed well before the point of airflow separation to do any good,and obviosly the airflow will seperate at different places at different speeds.The best idea for the RAF drivers is to place tufts of wool all over the cabin and go for a fly.You will soon see whats happening re seperation.Not only will you be able to cure the RAF tail wagging problem but you may pick up a bit of efficency if you can stop a bit of that suction behind the cabin.I dont think I have seen any modern airliner that didnt have VG's somewere to make the airflow do what they want.PS, birdy you wont have to use tufts of wool on your RAF,just look at where the red dust is starting to make patterns on the side of the gyro and you will see the separation point.M Barker

[birdy]

Are you try'n to tell me my gyros grotty Muz??Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[asra]

Camel.If the CG is below the propeller thrust line as are most Bensen derived gyros (low profile gyro). We can see in figure 4. that the engine induces a nose down pitching moment.To be in trim, the rotor must induce a nose up (positive) pitching moment and for that, the CG must be behind the rotor thrust line. Such a gyro is therefore unstable in angle of attack.Image Insert: 3.52 KBImage Insert: 39.82 KBIf you have a CG which is above the engine thrust as shown in Figure 5, then the moment coming from the engine is a nose up moment and to trim the gyro, the rotor must induce a nose down moment. For that, the CG must be in front of the rotor thrust line. Such a gyro is therefore stable in angle of attack. Image Insert: 3.44 KBImage Insert: 29.91

[asra]

Nice illustrations Sam, that white CLT machine above obviously has the divine seal of approval with that rainbow pointing at it![]However that stuff needs to be viewed in the light of other information, showing that these are not the only factors involved in pitch stability.See the thread on Nick's attempt to find the Holy Grail "EA 81 - Douglas Redrive"John EvansThink logically and do things well, think laterally and do things better.

[Waddles]

Birdy & Murray,Tufting works really well as long as there is some way of recording what the tufts are doing. Fixed video is good if one can be safely fitted.Another somewhat messy solution is to use an electric blower/vac in blower mode, and sprinkle flour or powder into the airlow. Some of the dust will stick to the airframe except where it starts to separate. I wanted to try this in the hangar away from the influence of the wind, but the owner wouldn't let me for some reason!!!Must be up to threepence now.Waddles.

[asra]

Al,In the name of science I wouldnt mind a bit of a mess in the hanger.Even when your gyro is rebuilt into an OZ version of the Sparrow Hawk with Tall Tail you may still have to do something about the aerodynamic instability of the cabin.I think the "Hunting" side to side is the cabin "Stalling" and unstalling in the vertical plane.We will experiment with VG's when its finished. By the way I didnt sell the doors of your RAF,we use them at BBQ's, they hold two family size bags of cornchips and a large dip.M Barker

[asra]

I think the need for a significant AOA on the stabiliser is overstated. I know mine is a monster, but I started off with 2 degrees negative, with provision to adjust back to 1 degree, however I've found I needed modify the mountings to reduce it to what must be very close to zero now. I have yet to put it through the regime recommended by Greg Gremminger, but increasing power raises the nose and reducing power lowers it. I don't have in flight adjustable trim, this would appear to be helpful with Greg's recommendations.While mine appears to be the biggest around, it doesn't quite meet Cierva's recommendation for tail volume.John EvansThink logically and do things well, think laterally and do things better.

[Waddles]

Muz,At least the doors are being out to good use.Anyone know where I can source a couple of metres of 6061 T6 4" X 2" X 3mm wall thickness preferrably internally radiussed? I can get it in 6 mm wall thickness, but that will not work. Thanks.Waddles.

[Tim]

There is a RAF pod and a Dull pod here and both have conventional tails. Both fly normally with the doors on. The Dull pod lost its vortex generaters over time and the owner reported no difference in yaw or pitch stability. I am of the belief that the main trouble is simply not enough tail volume. I don't believe that we are going fast enough for "stall" behind the cabin to affect the airflow over the rudder.Tim McClure