Shape of tail feathers

[asra]

pwendellQuotedA longer moment will make a tail of the same area and airfoil more effective than that same tail with a short moment. A larger tail with an airfoil fully immersed in the prop stream will be very powerful, however, even with a shorter moment.That's all well and good as long as there is a prop stream. How well do these tall tails handle, with engine failure? I havn't flown a tall tail, but the short moment arm allways concern me.Ross B

[asra]

Ross,I've done quite a few engine idle landings in AAI modified RAFs and had plenty of rudder authority. I think the highest x-wind I tried this in was about 12mph directly accross the runway. The AAI tall tail has LOTS of area and is a true airfol which provides it with plenty of lift.

[asra]

With due respect I would regard an idling approach as vastly different from a real engine out landing [I've had a few ] in my old VW, and even though my tail was bigger than the usual Bensen, and was in fact a very thin airfoil, the rudder was much less effective.Nick,Thanks for asking about progress on the tractor, even though I haven't got the floor in it yet, I have sat in it and gone brrrmmm brmmm. I am a little distracted at the moment, the Red Monster is AOG as Jack is making me a 3 ' hub bar and modifying my towers to take the usual cotton reels. She's pretty heavy [ alright so am I ] rotor RPM has been up to about 420 at Cooma, a little lower here.Also we've just been up to visit Lloyd and Marj for the " Queen's Birthday" medicals.John EvansThink logically and do things well, think laterally and do things better.

[asra]

John,That maybe, but I'm not about to shut off a perfectly good engine in flight to test your hypothesis The AAI mod RAF runs a Subaru engine, as you know, which idles at around 800-900 rpm and does not produce very much thrust at idle. I do know folks who have put them down safely with no engine and no one has complained about a serious lack of rudder authority. Obviously the rudder will have to be used more aggressively with no engine. All designs have their trade-offs.

[birdy]

I'v never had a problem with the authority of a tall tail, reckon it'd make a machine more slippery coz it'd be shorter, but I'd still need the long tail boom, to keep me rotors off the ground when land'n short.Ignorance is bliss............but only till you realise you were.You can always get the answer you want, if you ask enough experts.

[NiqueNaque]

John,Saw them, and Jack's new two seater project for Lloyd, Jack might be an 'olde bloke', but he's bloody gifted, Bob tells me he can cut a perfect cube on a lathe! How the .... do you do that?? (I know.. just don't tell everyone).Your posts were there in the middle of being fitted up and it would seem that Jack is worried about losing some hair so he wants to get you out of it as quickly as he can tomorrow..If I'm in your neck of the woods, very rarely as it is, I'll drop in for a peek.Cheers,Nick.

[asra]

You'd be welcome Nick.When we were there a few days ago the subject of the 2 seat standards came up, it seems in a pusher the engine mounts have to withstand 15 Gz forward, I wonder how many actually comply?Any budding 2 seat builders need to ensure that they understand the specs, or their pride and joy will be registered as a single seater[:0][!] They are on here for you to download.[]John EvansThink logically and do things well, think laterally and do things better.

[asra]

There's a very interesting thread going on within the American forum at this moment regarding size of horizontal stab's, and I've taken the liberty of obtaining the following quotes from Greg Gremminger. These comments I believe will encourage more positive thinking amongst us on the initial purpose of this thread. Greg Gremminger"It would be fairly difficult to compute the HS parameters for any gyro. As mentioned, many gyro configuration and HS factors go into the HS configuration.However, it is fairly easy to tune a HS through iterative flight testing. The first step is to flight test the "Static Power Stability". Do this by flying level at best rate of climb airsped (or about 55 mph). Add power and, without cyclic pitch input, the airspeed (in the climb) should remain within about 10 mph of the original airspeed. Verify the airspeed remains within 10 mph of the original airspeed with full power applied. Then, do this same thing - starting at higher airspeed - 60 mph? Then a higher airspeed. It is better to have no airspeed change with power. You certainly want to avoid a trimmed airspeed INCREASE with INCREASED POWER - this indicates the CG is moving aft relative to the RTV at higher airspeeds - more buntover prone!What this does is verifies the HS is properly balancing two things: - Propeller thrustline offset- Airframe pitch changes from airframe aerodynamic effects at different airspeedsThen, you should verify that this configuration is both statically airspeed and G-Load stable:Static Airspeed stability: Trimmed at any and all different airspeeds, verify that it requires a forward stick position AND pressure to increase airspeed. Verify that it requires aft stick position AND pressure to decrease airspeed. do this at all allowable airspeeds - not too fast though, you don't know yet if it is DYNAMICALY stable (PIO resistant). This test verifies that the HS is properly downloaded to move the CG forward of the (Rotor Thrust Vector) RTV and balances it.Static G-Load stability: Trimmed at any and all different airspeeds, verify it requires aft stick pressure AND position to maintain the original airspeed in a banking spiral - about 30 degrees. Aft stick position and pressure to maintain the original airspeed in a bank - higher than 1G - verifies that the CG is properly forward of the RTV. This G-Load static stability is what minimizes the potential for a buntover.Once you have configured the HS for Static Power Stability, especially if your propeller thrustline is a bit higher than the CG, the other two static stabilities are probably proper - but test them, it doesn't take but a couple of minutes.What can you adjust on the HS? You can change it's size, it's location aft, it's location in the propstream, it's airfoil shape (airfoil is almost twice as effective as a flat plate!), and it's mounted angle of incidence. Generally, to mostly ensure very good results on a moderately high prop thrustline, start with as large HS as you can with a clean symetrical airfoil shape - more effective and less drag. Mount it as far back as possible on the tail. Mount it on the keel to start - probably will not have to change that vertical position. Allow for adjustments to the mounted angle of incidence. (Keel mounted HSs are bst to have a little dihedral to avoid scraping the ground in a crosswind landing.)For most Rotax Bensen configurations with 60 inch props - a bit high prop thrustline, a HS stab similar to the Air Command 2-place tandem HS will reqire only level angle of incidence mounting. Start with that, then see how trimmed airspeed responds to changes in power. You should be able to fine tune the angle of incidence to minimize airspeed changes with power.Standard Mac Bensens with smaller props and lower prop thrustlines and no enclosures have very little prop offset or airframe aerodynamic pitching moments to require a large HS for the static Power stability. However, you should still install a large effective HS - statically tuned for most prop offsets on Bensen similar configurations - providing both static stability and the all-important DYNAMIC stability. Without a HS, it would be very difficult for any aircraft to be dynamically stable enough to be strongly resistant to PIO.Tim, although there are benefits to a HS centered on the prop on a tall tail - mostly reduction of torque effects - my preference is mounting the HS on the keel so it is just on the fringe of the propwash and mostly in the free airstream. The HS does need to feel the effects of both the propwash (proportional to prop thrust), and the effects of the free airstream (proportional to any aierodynamic pitching moments on the airframe as a result of airspeed).If the HS is totally immersed in the propwash, the propwash multiplies the effectiveness of the HS - when the propwash is provided! The problem is that the effectiveness of the HS is much reduced when the propwash is not so high - idle power, engine off, etc. This tends to make the stability margins (both static and dynamic) vary greatly as a result of power applied. It also tends to make flight nose attitude and airspeed a large function of pwoer applied. It can be difficult to find the right balance between the HS balancing the prop offset while also balancing the aerodynamic airspeed pitching moments on the airframe. Ideally, neither power changes or airspeed changes should change the relative locations of the CG and RTV so the pitch sensitivities and stability characteristics remain the same at all power and airspeed conditions.I believe the best is to start with a large and effective HS mounted on the keel. Allow for adjustments to the mounted angle of incidence. It might be nice to allow for raising the HS a small bit to bite off more propwash - but this might be necessary only if you have a very large prop thrustline offset to start with - requiring the HS to use more of the propwash to balance the nose-down pitching moment from the high prop offset.Also, a centered HS may have more blocking and disturbing stuff in front of it - disturbing it's effectiveness, especially when propwash is not accelerating the air over the HS. Also, be aware, that many people assume a centered HS gets the most accelerated air from the prop. Actually, the most accelerated airflow in the propwash is about 2/3 the way out from the prop hub. So, a HS mounted about 1/3 up into the lower prop semicircle would have the most amplification (effectiveness enhancement) from propwash - but only when the engine is producing significant propwash!It is my belief you will find the most "harmonized" control characteristics - flies more like a good airplane - when the HS is using only enough propwash to statically balance the nose-down pitching moment of the high prop thrustline. Nose-down pitching moments from power or flight airspeed are destabilizing - they position the CG more aft relative to the RTV. Any moment that tends to lower the nose in flight is undesireable, reduces the stability characteristics and possibly makes the machine more buntover and PIO prone.A HS mounted on the keel, just at the outer edge of the propwash, does get some down-lift from the propwash from the slightly expanding propwash cone. So even a keel mounted HS can balance a small amount of prop thrustline offset. Also, often on a slightly high prop thrustline configuration, even a HS mounted level on the keel will assume a slight downward AOA in flight because the high prop thrustline and nose-down airframe moments cause the airframe to fly slightly nosedown and provide a slight nose down AOA to the HS. So, a level mounted HS on the keel, or just at the bottom of the prop arc, can provide the HS down-lift that improves airspeed and G-Load stability. A lot said - but just mount a good large airfoil shaped HS as far aft as possible, level on the keel to start. It is likely very little tuning of the HS AOI or vertical positioning will be necessary to achieve the flight stability criteria described above.One more point - the more nose-down pitching moments you have from either a very high prop thrustline or a lot of nose-down drag

and moments, the more balancing download you need on the HS. Nose down moments from large windscreens add to the load on the rotor. The HS down-load required to balance the nose-down moments also add to the load on the rotor. On a very highly offset prop thrustline and/or very large sloping windscreens, together with the required HS balancing down-loads, can increase the load on the rotor as much as several hundred pounds - like carrying around a lot of extra weight. For this reason, reasonable prop offsets and reasonable enclosure moments, and a good long tail boom are much preferred to reduce this extra "baggage" load on the prop! The further aft mounting of the Hs reduces the actual down-load require to balance the nose-down moments - less induced load on the rotor.Quote:Originally Posted by Al_Hammer"Conventional airfoils at conventional aspect ratios and low Mach numbers have lift curve slopes of approximately 0.10 per degree or 5.73 per radian. So, at 12 degrees AoA, Coefficient of lift would be about 1.2 for a NACA 0012 or similar airfoil."Thanks Al, As the plots show, at shallow AOAs, about 4 degrees or less, a typical airfoil shape has a lift coeffeficient several multiples of what the flat plate does. Since an effective HS is typically operating at very small AOA, we can expect the airfoil shape HS IS very effective and is operating in this shallow AOA range. For a flat plate HS, the pitching motion to achieve the same lift coefficient would be much higher amplitude. This higher pitching amplitude - in dynamic response to a disturbance - would build higher pitching inertias before reversal of direction - poor dynamic stability response - possibly PIO prone or at least stimulating possible over reaction control inputs from a less proficient pilot!I believe we would solve a lot of stability problems on some of the very worst gyro configurations by simply maximizing the effectiveness of an installed HS - large area, airfoil shape, mounted far aft. Mounted AOI and propwash immersion could then be tuned with static flight testing as described above.Tip Plates: I'm no expert on this either, but I'm not shy about expressing my impressions. Generally, tip plates serve to increase the effective aspect ration (span) of the HS - improved effectiveness and efficiency. The tip plates block the tip airflow circulation around the tip of the HS (wing), converting some of the tip vortex drag into thrust. (Airfoil shaped tip plates would also more effectively convert the tip circulation into forward thrust, where flat plate tips would not!) Tip plates on a gyro make a lot of sense - improve HS effectiveness and resultant stability, and yaw stability. The fully immersed VS stabilizing effectiveness suffers from the same issue that a heavily immersed HS does - it's stabilizing effect varies with power. The tip plates, if mounted at the edge of the propwash, react to airspeed more than propwash (power). A lot of rotorcraft are now employing tip plates on HS - even older certificated gyroplanes did so - 18A, J2!Thanks, Greg Gremminger Promoting gyroplane safety through demonstration and education I know there's a lot to think about in what Greg has stated but I'm very interested in hearing your comments.Ted

[Brian]

Ted, just thought I would mention that I posted photos of my gyro back on page 3.I'm not techy enough to give a educated comment on stabs but I know that winglets 'work' in keeping a gyro from tail wagging. I'm am going to try a bigger 'flat plate' stab to see what diferences might be.I dont doubt that an airfoil shape should be better for a stab.Just my opinion.Brian

[asra]

Brian, Thanks for the feedback. John had earlier raised the potential about the suitable fairings surrounding the mast and landing gear. I wondered why not a front Stab as well (since we looking over the top of the 9 dots).Greg pointed out "Also, a centered HS may have more blocking and disturbing stuff in front of it - disturbing it's effectiveness, especially when propwash is not accelerating the air over the HS. Also, be aware, that many people assume a centered HS gets the most accelerated air from the prop. Actually, the most accelerated airflow in the propwash is about 2/3 the way out from the prop hub. So, a HS mounted about 1/3 up into the lower prop semicircle would have the most amplification (effectiveness enhancement) from propwash - but only when the engine is producing significant propwash!".This would lead me to think that the current design of the Butterfly tail is only partly effective. I agree with your thinking about the winglets, provided they are in the free air flowpath.Ted

[asra]

Gentlemen,I note from the many photo's available both here and on the American forum that most gyro's have a flat plate type stab. Paul's Firebird has a reversed airfoil shape, Birdy flies one with a hinged plate. What other types have we in circulation?Greg has also stated "Generally, tip plates serve to increase the effective aspect ration (span) of the HS - improved effectiveness and efficiency. The tip plates block the tip airflow circulation around the tip of the HS (wing), converting some of the tip vortex drag into thrust. (Airfoil shaped tip plates would also more effectively convert the tip circulation into forward thrust, where flat plate tips would not!) Tip plates on a gyro make a lot of sense - improve HS effectiveness and resultant stability, and yaw stability. The fully immersed VS stabilizing effectiveness suffers from the same issue that a heavily immersed HS does - it's stabilizing effect varies with power. The tip plates, if mounted at the edge of the propwash, react to airspeed more than propwash (power). A lot of rotorcraft are now employing tip plates on HS - even older certificated gyroplanes did so - 18A, J2!"What would happen if the tip plates were allowed to move with imputs from the pilot and the horz. stab was fixed?Ted

[asra]

I'm a bit late replying to a few comments about Tall Tails a bit further back on this thread. People seem to think that because a tail is 6 inches or so from the prop it has no leaver arm etc etc, absolute garbage.On some machines we are talking about a rudder around 6 feet tall,do you really think this is going to be inefective ?, even with engine stopped in a cross wind landing the amount of rudder used is very small,somthing which I demonstrated recently during an incident with this machine.I think someone questioned how efective the horizontal stab on a Tall Tail would be in a engine failure?,who cares!! if your engine failed you are only going one way,DOWN,and PIO is no longer an issue.M BarkerImage Insert: 28.1 KB

[Greg Mitchell]

Ted,Your comment:- "This would lead me to think that the current design of the Butterfly tail is only partly effective."....So I guess Tall Tails and T tails are all only partly effective given your comments. Where will you put your tail and HS Ted?Mitch.www.thebutterflyllc.com

[Brian]

Murray, I agree that a tall tail has plenty of rudder authority[ or should have] . I think Ted has raised an interesting question ie where the stab ,particularly on a tall tail is placed. On a conventional tail, I'm not so sure that where the stab is placed, is a particular concern though.One point to consider Ted, is the weight of whatever tail you decide on, I would guess and mayby Murray or someone else could tell us the weight of a tall tail. The tall tail on Murrays gyro looks to me as if it might not be as big or airfoil shaped as the Sparrow Hawk or the AAI mod for the RAF.Another thing to consider is how long you want your gyro to be, a 2 seater like Murrays gyro really has to have a close fitted tall tail or else its going to be one very long gyro.My gyro was built around being able to be fitted into the back of my tray back [Wheel base the prime consideration,Brian

[Tim]

quote:Originally posted by murray.barkerPeople seem to think that because a tail is 6 inches or so from the prop it has no leaver arm etc etc, absolute garbage.Murray is correct here, the lever arm is measured from the C of G and not from the propeller. In the case of the gyro in the picture, it would be somewhere between the two seats.Tim McClure