Need stab - request for info.

[EI-GYRO]

Hi Folks,

We will have a RAF2000 going onto the Irish register soon.
Can anyone tell me who is currently producing/selling horizontal
stabilisers, preferably in the U.S.?.

Best regards

Fergus Kavanagh.

Dublin
Ireland.

[mikejh]

Hello Fergus

I've looked into these too and have found two:

American Autogyro have a STABLITY FIX KIT for the RAF2000 - scroll down their price list :
http://www.americanautogyro.com/Pricing/AAI%20Price%20List.htm


Secondly Paul Bruty in Australia...

Firebird Gyros Australia - Paul Bruty
http://www.firebirdgyros.com/

Paul has set up a cottage industry himself, making stablisers
(tailplanes) for RAF2000s. His are prettier and have dihedral :

Paul Bruty's HStab for RAF2000s
http://www.retro-composites.com/

Hope this helps - lots to worry about in terms of pitch stability it seems with some Gyros..

Mike H
London

[Timchick]

Wasn't someone in New Zealand making stabs for RAFs?

[KenSandyEggo]

Paul's ex-partner, I believe.

[Alan_Loughrey]

Hi Fergus,
I make a pitch adjustable stab for the RAF2000. Price US$999 delivered.

[LARRYEBOYER]

Alan, does that stab produce negative lift? I understand that is a good thing for a RAF.The stab I am using has a flat surface on top and a curved airfoil on the bottom. It produces down lift.

[Alan_Loughrey]

Larry,
To achieve positive static stability all general aviation aircraft require "down lift" on their stabs. The Mk6 produces 75 lb (34 kg) at 60 knots at 10 degrees AOA. This should be enough to prevent PPO. Interestingly - the RAF2000 flies 10 mph faster at cruise rpm despite the added aerodynamic weight.

[EI-GYRO]

Hi Alan, Passed your info to Jim Henry.
I gather he has contacted you.
Thanks.

Fergus.

[Aussie_Paul]

...to ask a question or two. Why would it go faster?

Alan and I are in dissagreement on a couple of things. I have tried the above stab on one of our modified closer to CLT and a 10" extended keel Rafs. I tried for 2 days to find an angle of incidence that would work. The mast was in the most upright position to have the keel level at cruise.

The test I conducted was,

Stick locked at 60 mph cruise, reduce power to idle and the nose would keep going down until I was uncomfortable with the stick locked speed of 80 mph and increasing.

We spoke to Alan about removing the rear abrupt reflex section from the stab. Alans experience with this stab is on his mates stock Raf, ie. no mods. They have the mast tilted back as far as it will go. Alan may be able to tell us what the keel angle is in s/level flight, but it would have to be the difference in degrees between the number 1 and number 4 positions of the Rafs adjustable mast. I will measure it today but it is probably around 8 degrees.

We made the decision to remove the the rear abrupt reflex section. After a couple of adjustments to the angle of incidence, the same test produced an increase to a stable approx 67 mph.

I have a standard Raf here as well as one of Alans stabs, so later in the year I will test it in the same configuration that Alan and his mate are using, and then when we have conducted the mods I will try it again.

I am heading West Aus. next Monday for a couple of weeks training in a modified Raf and a Magni. I also have to help a guy put one of Alans stabs on his stock Raf and test it for him. I will set that one up in Alans mates configuration and see it it agrees with Alan and his mates results.

I am finding it hard to believe that such a radical reflexed trailing edge can be beneficial.

Also, that the same stab that Alan says works on a stock Raf, will not work on a modified Raf!!!!

I guess, being the doubting Thomas that I am, until I conduct all the tests myself I have doubts!!!

Aussie Paul.:)

[Alan_Loughrey]

Paul: I thought it best answer your questions and relive your doubt by starting with the basics. I’ll toss in a few examples and generally take the opportunity to explain what little I know about gyroplane stability and function of the horizontal stabiliser.

Nearly all general aviation aircraft achieve Positive Static and Dynamic Stability by means of the following general arrangement;
1. The CG is always forward of the center of lift.
2. An horizontal stabiliser airload applies a balancing and damping force.
When the aircraft is slower than its trimmed speed the stabiliser’s aerodynamic force diminishes causing the aircraft to rotate nose down. As the aircraft gathers speed, the stabiliser’s aerodynamic force increases raising the aircraft nose up. Within 1 - 3 cycles the aircraft will be flying at its trimmed speed and attitude.

Gyroplanes are no different in this regard and require the same arrangement for their Positive Static and Dynamic stability.

The distance between the CG and the Center of Lift on general aviation planes is called the Static Margin.
Static Margin is fixed wing terminology and refers to a percentage of the main wing chord.
5.0% - 10.0% of the chord is the normal range. E.g. A two seat 1200 lb fixed wing aircraft that has a 50” wing chord will have a static margin somewhere between 2.5” - 5.0”

There is no corresponding gyroplane terminology, but I will call the horizontal distance between the RTV and the CG - the “Static Margin”.

To be continued.

[Aussie_Paul]

Fine Alan but that is not the question. I have quite a reasonable understanding of the aerodynamics.

The question is why would a drag creating severe reflex, on the h/stab, make a machine go faster? It is the same as reflex on a rotor I would have thought, adds to stability because of the drag.

With the mast in position one, the keel, the datum popint for a Raf, and therefore the prop thrust line will be pointing nose down approx. 6 or more degrees. That must be adding increased parasitic, or as it seems to be called these days, form drag to the whole aircraft. That Raf cabin does not like to be nose down. How can it go faster?

I am looking forward to the Raf test that I can conduct next week. I will set it up in position 1 (most nose down) conduct the tests and then fit your stab and conduct the same tests again.

If they wouldn't mind, I would like Chuck B, Al, Udi, Doug etc to get involved for me. There is obviously something that I am missing here. :o

Regards, Aussie Paul.:)

[Alan_Loughrey]

Paul, patience please, this is a long story.

Rotor disc Loads.
Paul: Take the example of a HTL machine without a stabiliser. When the propeller thrust generates nose down moments around the CG, an opposing force with equal moments must be present to prevent the gyroplane tumbling in the sky. A negative static margin provides the arm and the rotor provides the thrust that generates these moments. The load on the rotor is considerable.

An approximation of rotor loads:
RAF 2000. Prop thrust 440 lbs x offset 10.6” = 4664 inch/lbs of moment. If the negative static margin was (8.0”) then the rotor load would be 4664 / 8.0 = 583 lbs. This load is in addition to the AUW of the gyroplane and the total thrust the rotor has to provide would be approximately
1300 lb + 583 lbs = 1883 lbs.

The greater the length of negative static margin the less the load on the rotor, but greater instability will be experienced. Remember - for positive static stability we require a positive static margin.

What is open for debate here is the actual length of the negative static margin? Is it 8.0” or 12.0”or…? We know, through calculation, that when the passenger departs the RAF 2000 the CG moves rearward approximately 4.0” This rearward movement increases the negative static margin and aircraft becomes very pitch unstable. I think it is normal to carry weights as a substitute passenger.

For a graphic illustration of the forces at play; fly the stab-less stock RAF2000 at cruise velocity - cut the power and experience the violent jerk as the plane zooms skyward. Unfettered by the nose down prop thrust and 583 lbs of rotor load, the machine’s inertia along with the rotor’s trimmed high AOA take the gyroplane into a rapid climb . You think; what stress and strain is this sharp climb is putting on the rotor blades? “No worries” (Aussie expression) - nothing has changed, that same rotor load has been there all along…

How can we remove 583 lbs of rotor load without cutting the throttle?
To be continued.

[Aussie_Paul]

Alan, when flown solo the thrust line to C of M of a Raf reduces from 10+" at MTOW to I think it was around the 7" mark solo

Dr Stewart Houston gave me his figures some time ago. The exact figures are not important though.

The stock Raf is actually more pitch stable flown solo. The problem I have found is the rrpm is slower and the ability to over control is greater, thus giving the feeling of instability.

Aussie Paul.:)

[Alan_Loughrey]

Paul; Dr Stewart Houston - I looked up his figures. Dual CG 10.2” below the thrust line. Solo 7.2” below and 3.0” to the rear.

The Solo 7.2” figure is not quite right for our purposes, because (I suspect) it represents the lightest of pilots and nearly zero fuel. I will use 8.5”, to represent a 190-lb passenger steeping out of a fully fueled machine.

These numbers are graphically displayed in the sketch below. The Solo CG is a further 2.75” aft of the RTV than the Dual CG. Burn off most of your fuel and the CG moves a further 2.0” aft!

A question remains; how far has the RTV moved closer to the Solo CG due to the 17% reduction of prop thrust pitching moments? If “x” was 8.0”, a 17% reduction is a RTV shift rearward of 1.3”.

In approximation; the negative static margin has increased by (2.75”-1.3”) =1.45”. That is a substantial and detrimental shift.

Positive static stability - “is an initial tendency toward its original equilibrium position after a disturbance?” (Greg Gremminger), and is achieved by proper arrangement of the CG, center of lift and horizontal stabiliser. (See first sketch in this thread) With this set up, static stability is not effected by more or less rotor thrust.

However, when the RTV is forward of the CG, pitch control is made easier by a steady supply of rotor thrust. Unsteady rotor thrust resulting from low RRPM’s along with the unknown length of “x” makes it difficult (for the pilot or the theorist) to guesstimate or calculate a more exact increase in negative static margin when the stock and stab-less RAF 2000 is flown solo.

Glad you raised this point Paul - it’s the first time I have applied myself to the detail.

[Aussie_Paul]

Alan, I have always admired you for your theory, BUT I like Birdys postcript.

The reality of experience trumps theoretical beliefs.

You still have not andwered my original question. Why would the machine go faster?

Anyway, I have everythong is need to conduct the relevant tests as I have always done, to prove the practical side.

Alan. it is not much point you and I debating it, because we dissagree. You are the theory man and I am the practical test pilot.

Regards. Aussie Paul.:)

[SamL]

Paul, looking at the H.S above, it seems it has quite a curved upper surface.
This curved upper surface can help cancel some of the down force being created.
Also having such a small and highly angled trailing edge (greater than 10 degrees) will only cause drag.
The idea is to deflect the airflow upward, and that should be done by the whole stab. Having an inverted aerofoil section, alows us to angle it beond 10 degrees, if required, and delay flow seperation. The aerofoil section also makes the H.S more efficient.

Sam.

[Aussie_Paul]

I agree Sam. IMHO, with the very large thrust to CoM offset of a stock Raf I would not incorporate that large drag producing reflex, also I would have more of an upside down airfoil. The closer to CLT, the more the stab can have a symmetrical airfoil. I have proven through testing I have conducted, that a 2" higher thrust line to the Com, as well as having a negatively loaded (pushing the tail down) H/stab works the best.

It was by a bit of luck that I found this out, and after I flew the Magni I knew then why Hybrid was so stable. It was working using the same pitch stable principle that the Magni designer uses so well.

My attempt at CLT with the RAF kit that I made into Hybrid was only successful to within a couple of inches depending on the load, so I had to set the AoA of the stab to provide a continuous downward force.

A completely neutral h/stab is like car steering with a quarter of a turn slop. If you drive on either side of the road camber it is quite easy to steer because it is only pulling one way, but in the center of the road camber you are always chasing through the quarter of a turn slop from going one way and then the other.

That is the only simple way I can explain it that most people would understand.:o

Off to Western Australia tomorrow forseveral weeks training. One in a modified Raf with Alans stab without the reflex, and some in a Magni conducting cross country instruction. Eat your hearts out forum people!!!!:D

Aussie Paul. :)

[Alan_Loughrey]

Paul,
I was explaining in a previous post how prop thrust pitching moments burden the rotor with a load that is in addition to the AUW of the gyroplane.

On a HTL gyroplane - nearly all of the additional rotor loads can be shifted to a horizontal stabiliser. A powerful stab situated on a long arm (keel) can apply a lot of moment, as much as we need to oppose the prop thrust nose down pitching moment, and more.

In the sketch below I have rounded off some of the numbers used in previous examples.
Prop thrust 450 lbs x offset 10” = 4500”lbs of moment around the CG
Stab Load 72 lbs x keel 63” = 4500”lbs of moment around the CG (opposing prop mom.)
4500”lbs / 8.0” = 560 lbs of additional rotor load.
The rotor load has decreased because the stab is in place from 1860lbs to 1372lbs.
An approximate reduction of 488 lbs.

Without a stabiliser we had negative static stability. With the stabiliser we have neutral stability and positive dynamic stability. Things are improving.

Next time: Positive static stability.

[Alan_Loughrey]

Paul,
To achieve positive static stability - the rule is;
The Center of Lift (CoL) must be aft of the CG..

The CoL is located at the intersection of the RTV and a horizontal line running through the CG.
HTL machines have to locate their rotor head in a position that runs the RTV forward of the CoL, but two fortuitous events move it to the rear when the horizontal stabiliser is affixed.

1) Due to the reduced rotor load, the rotor disc angle of attack is less and the RTV swings closer to the CG. (approx 1.5”)
2) The stabiliser airload (72 lb) will rotate the fuselage (including the CG) approx 3.5 degrees and move the CG 3.5” forward. About 1.0” forward per degree of rotation.

Sometimes these gains are insufficient and/or the propeller has reduced efficiency because its thrust is no longer horizontal. If so the next step is to move the rotorhead aft. (The RAF 2000 rotorhead has provision for 10” of longitudinal adjustment.)

In the sketch below the CoL is placed 1.0” aft of CG by moving the rotorhead and this gives a static margin of 1.0”.

The static margin also applies a moment which is 1300 lbs x 1.0” = 1300”lbs.
This adds a another load to the stabiliser; 1300”lbs / 62” = 21 lbs. The stabiliser now carries 72 lb to resist nose down prop thrust moments plus 21 lbs for a total of 93lbs.
Rotor thrust has reduced from 1860 lbs to (1300 +72+21) 1393 lbs - a saving of 467 lbs.

Drag reduction; 467 lbs divided by the rotor L/D slope 6:1.
467 / 6 = 78 lbs less drag.
Stabiliser drag, using the same L/D slope 93 / 6 = 15 lbs extra drag.
Overall reduction in drag = 78 - 15 = 63 lbs.

An RAF 2000 will go 10 mph faster for the same cruise engine RPM when setup as per the sketch. The photo below shows a stabiliser with 10.75 sqft planform, including the fixed trailing edge flap. The flap (and fins) have been added to an older symmetrical foil design and gives us the downforce needed. The CL is unknown. This stab may produce more downforce than the 93 lb mentioned in my example.

Trimmed at 65 mph, straight and level (hands off stick) the gyroplane will climb at 65mph with added power and glide down at 65 mph with engine at idle. When full power is applied during a vertical decent, the nose nods down briefly and then rotates up as the gyroplane gathers forward speed.

Next time: Errors and omissions, plus hunting the “x”.