Tweet
Hello everyone, I know its a bit late as I have already built my tail but I was just wondering if there was a standard or minimum HS area,Rudder area and Vertical fin offset and what affect moment arm length has on the ratio if any???Peter Adams
-
-
Peter,Adrian Stoffels should be able to answer your enquiry on this one, probably more correctly than as follows:SamL advocates that the side elevational area of the gyroplane in front of the mast must be less than the area behind the mast, and the moment of those areas must be greater for the tail than it is for the front.What I mean by this is that if you have a tail area of 1 square metre with area centre located 2 metres behind the mast and, say, a pod with a surface area of 1.5 sq. metres in front of the mast with an effective centre of 1.5 metres you have a problem.The tail moment is 2 x 1 = 2The Pod moment is 1.5 x 1.5 = 2.25This means a cross wind can 'weathercock' your nose downwind, very bad, this *may* have caused Ken Rehler's fatal incident published about a month ago.You want your tail control to be unequivocable and absolute, ideally it should weathercock you into the wind, if yours does not fit the moment then move it further out until it does, if it ends up falling into the path of the rotor then you may have to scrap your existing tail and move it in closer and make it very much bigger. The same principal can be applied to a horizontal stabiliser, only you look at the plan of your machine and determine the stabiliser surface area based on it. If you want a real thumper of a tail, have a chat with John Evans, 'John' on this forum.On an area for area basis, a combination of a fin and rudder is more effective than a fully flying tail, this is because it can create a much greater coefficient of lift than can a flying tail for the same deflection, and if your rudder cables break, a fin is more likely to stabilise the aircraft on the right track than will a flying tail.Hope this helps,Nick
-
Thanks Nick,That gives me some figures to calculate with as I plan to build another tail later, the first one is ok but its a bit heavy. I made it from 9mm ply, the next one will be aluminium sheet.My moment arm is quite long and tail offset is only 6.5 degrees, i was told 10degree offset was about standard.My machine was a lot of guess work as I built it from scratch on modified plans to accomadate my engine and had only seen a RAF and one other Benson style gyro in person before I started building.Peter Adams
Comment
-
Here is a bit more to think about Pete.Comes from Doug Riely ex the NCLT Dichotomy thread US Rotaryforum....................................... ..........................Sure, the Cierva formula is empirical, but experience shows that it's actually pretty conservative for small gyros. It's a couple orders of magnitude better than telling people to take down a bedroom door and strap IT on to see what happens... or, even worse, having people test the RAF-2000 with some irrelevant gizmo like a magic mast, thinking THAT might do the trick.Here's how the Cierva formula would apply to an old short-tail Air Command.Rotor Diameter = 23 feetBlade Chord = 7.5" = 0.625 ft.Tail moment arm (to 1/4 chord of the factory HS, with short tail tube) = 4.5 ft.Rotor volume = 23 x 23 x .625 = 331 cu. ft.10% of rotor volume = 33.1 cu. ft. For 4.5 ft. arm, HS area = 7.35 sq. ft.12% of rotor volume = 39.7 cu. ft. For 4.5 ft. arm, HS area = 8.8 sq. ft.These are very conservative numbers, but they are in the upper end of the range that works to provide good stability in practice. The Gyrobee, which is not very different from a small Air Command, shows very solid AOA, airspeed, and power stability with a tail volume of 30 cu. ft., for example. It obtains this volume with a 6 sq. ft. HS on a 5 ft. lever arm. I think we agree that the original Air Command HS -- the one that bolts to the rudder -- make handling nicer but is too small. Its area is about 4.25 sq. ft., or 20 cu. ft. volume. Aren't we bracketing the acceptable values fairly well here already?The Dominator has, I think, a tail volume of only about 5% of rotor volume. It has its stability enhanced to (mostly) acceptable levels by means of two "tricks:" First, the prop thrustline is a few inches below the CG, adding power stability and keeping the rotor thrust line ahead of the CG while power is up. Second, the HS is deeply immersed in the propwash, giving it an airspeed of over 100 mph at WOT. Still, the Dom. has a couple of flight characteristics that could be improved with more HS area -- especially power stability and power-off pitch behavior. We could (and probably should) come up with more elaborate formulas based on a target position for the rotor thrustline relative to the CG, and taking into account airframe mass, moment of inertia, height of rotor above CG, HS immersion and maybe other factors. Also, tail volume of X obtained by using a short lever arm and large HS is not really the same as the same tail volume obtained with a long lever arm and smaller HS (the latter provides better damping). I'm impressed with how well the Cierva formula works on small, direct-cyclic pusher gyros, however, even though it originally applied to large tractor gyros, some of which used elevators.The one thing the Cierva formula definitely WON'T do is tell you what to do about pitching moments caused by body pods or high thrustlines. HS power to deal with these items has to be figured out separately. The pitching characteristics of a body pod really ought to be tested in isolation, on a truck boom rig, for example. In the case of HTL, the moment is easily quantified.Testing vs. a priori numerical design is not an either-or proposition. Asking someone to choose only one is like asking him to choose between the left wheels and the right wheels on his car. My point is that your first cut for testing purposes can be sized numerically, and you'll have an excellent chance of getting it in the ballpark. You can, and may have to, tweak the design based on test results. By the same token, you can be quite sure that some home-brewed design you see at a flyin with a HS volume of 3% of rotor volume doesn't have enough HS. This may lead to other relevant questions about the designer's knowhow.
Cheers,Mitch.www.thebutterflyllc.comA thinker sees his own actions as experiments and questions--as attempts to find out something. Success and failure are for him answers above all. - Friedrich Nietzsche
Comment
-
Thanks Mitch,Might have to read that 2 or 3 times to fully understand it but it seems to say a longer set of blades of wider chord need a bigger HS area or longer moment arm.I'll have to do a few measurements and see what I come up with..Peter Adams
Comment
-
Mitch,Can you explain the "rotor volume" calculation for me? On face value, the chord measurement would be a function of the blade area (e.g. 23ft + 23ft x .625ft = 28.75ft) whereas the swept area of the blades would be a function of the square of the diameter (i.e. 23ft x 23ft x .786). I don't understand the "blade volume" calculation.RegardsFred
Comment
-
G'Day Peter,Yeah it's a bit like that. I have to read stuff several times to absorb and wrap my head around it.Morning Fred,Do a search ROTOR VOLUME FORMULA on the US Forum and 4 threads will come up with information that may help.Cheers,Mitch.www.thebutterflyllc.comA thinker sees his own actions as experiments and questions--as attempts to find out something. Success and failure are for him answers above all. - Friedrich Nietzsche
Comment
-
Here comes the cat....I have been flying with a largish pod with large windscreen, average size rudder, have to say smallish HS on two gyro configerations for over a decade. My current gyros have a stock EA81 and other other a 65 hp Hirth. Before that I was flying on a 532 powered air-command with a modest HS. I fly several times a week sometimes in the worst of weathers, dropping into friends' properties for a cuppa. My point is that I have never experienced any evidence of my gyro wanting to weathercock 180 degrees from an unexpected gust. Why? Airspeed, airspeed, airspeed. I just don't allow it to have its own way. Simply don't flair 10 foot in the sky and wash off the airspeed regardless what gyro configeration you fly. Look at the evidence. Regardless what you fly, you can crash if you get the numbers wrong, even CLT correct gyros such as butterflys or dominators. Our forum and the US forum has recent examples to back this up. I agree a stable machine is desirable but I believe too many people hang their hats totally on arranging automatic stability of the machine rather than on developing skills and good airmanship habits. A CLT machine, huge HS and tail is not a panacea for all situations and pilot skills still heads the list of requirements in my book.Pete
Comment
-
Peter I can see you've got "Chopper Chick " on your mind again.Adverse YawWhat must be considerd is that a smallish rudder will work, and work quite well, regardless of weather, but only while thrust is present, and you are controlling your machine.The interesting bit comes when you have an engine failure, and the thrust that helped you point your gyro in the direction you required is lost.I beleive this is the situation that could bite a pilot.A very current example of this is the death of Ken Rehler were his rudder volume seemed to be insufficient, and may have contributed to his death.I was never any good at maths, so formulas, and ratio's just dont appeal to me. I prefer the "Old cut out method" to work out if a machine will, or will not be effected by adverse yaw.CLT & HTL tail volume comparison.A CLT machine will not be effected any different to a HTL machine when considering tail volume, they will both be effected in the same way.A CLT machine will not be effected any different to a HTL machine when considering HS volume, they will also be effected in the same way.But it will make a big difference when you add thrust or power, as the HTL machine will want to pitch nose down.The CLT will machine will have minimal to no effect in pitch, as the thrust line is acting through the vertical C of G, bla, bla, bla.Remember both the HS and rudder help your machine point, track and follow your blades regardless of were you fly them. Just my two bob's worth !!!!!Regards Sam.
Comment
-
Hello Sam,What is the "old cut out method" ?Sorry I'm new to all this stuff..Peter Adams
Comment
-
G'Day Peter.We have discussed the "Cut out method" before on another thread.Click on the attached link and it will take you there.Hope it all makes sense.http://www.asra.org.au/forum/topic.a...hpage=7Regards Sam.
Comment
-
Hi Sam. Good comments. My feelings are that any gyro pilot worth his salt in an engine out situation lowers the nose anyway keeping airspeed and stability. I am sure most of us flying the Broken Hill configerations with pods attached have experienced engine outs with no bad experiences of the gyro wanting to reverse direction. You just don't allow it to happen by maintining good control of your aircraft with sufficient airspeed. The gyro deaths I have had some involvement in down these parts have been the direct result of incompetence, not gyro configeration.Pete
Comment
Comment