horizontal stabs

The angle will vary depending on:

1. How much lift the stab produces under different flight regimes
2. The distance of the stab from the CG (the moment)
3. The distance of the propeller thrust line from the CG
4. The amount of thrust

All of these factors can vary considerably from machine to machine.
 
Nah, Tim .......it's as plain as black & white!

Maybe I'd start with flat plates parallel with the keel & EXPERIMENT !
 
It shouldn't be THAT hard to come up with some hard numbers. Once the VCG is determined and prop thrust has been measured, the pitching moment, if any, is known. Since the moment of the stabilizer is going to be predetermined by the existing airframe, it's a matter of deciding what combination of stabilizer area, airfoil, AOA, and air flow will provide enough lift to compensate for the pitching moment in the worst case. This should be low airspeed and maximum thrust, just what can occur when a go-round is initiated very late in the approach.
 
Determine by iterative flight testing

Determine by iterative flight testing

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 airspeeds

Then, 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.

- Greg Gremminger
 
Thanks for the replys. It is a Bensen with a mac 72. I had intended to build a fiberglass vertical and horizontal one piece stabilizer, and was wondering if 1 or 2 degrees of down incidence would be good to start with. Then I could shim the front or rear at the bottom for final adjustment.
 
gyrogreg said:
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.

- Greg Gremminger

Greg,

Why would it be so difficult with a simple, open machine like a Bensen? The magnitude of the pitching moment can be determined in a pretty straight-forward manner. The position of the HS is given, and the lift generated by various airfoils at various AOA can be looked up. The only unknowns that I can see are the amount of air over the stab -- this depends on its position relative to the prop stream, and the amount and center of parasitic drag. Wouldn't it be possible to make educated guesses about these last two factors and, therefore, a good starting point for the stab design and AOI? Of course it would have to be tested and fine-tuned, but couldn't we get close by calcualtion? Or are there factors that I've missed?
 
Peter - calculating a stab is not very difficult if you have all the necessary data. But we don't. You don't know the airspeed of the prop wash in all the different locations. You don't have the lift curve of a typical gyro stab. It's more complicated than using a known airfoil because the short aspect ratio of a stab is making theoretical estimation of lift curve almost impossible. All this data can be gathered experimentally, but I don't know anyone (other than Doug, maybe) who has bothered going thru the exercise of collecting all this data.

I agree with Greg. Design a large and effective stab, guestimate a starting position and pitch for the stab, and go flying.

Udi
 
With all that said, on a gyro like A1Frankie's Bensen with a mac and my KB-2 with a mac which mounting position would be best to try, a keel mount or high and centered on prop?
 
Timchick said:
With all that said, on a gyro like A1Frankie's Bensen with a mac and my KB-2 with a mac which mounting position would be best to try, a keel mount or high and centered on prop?

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.

Thanks, Greg Gremminger
 
Udi said:
Peter - calculating a stab is not very difficult if you have all the necessary data. But we don't. You don't know the airspeed of the prop wash in all the different locations. You don't have the lift curve of a typical gyro stab. It's more complicated than using a known airfoil because the short aspect ratio of a stab is making theoretical estimation of lift curve almost impossible. All this data can be gathered experimentally, but I don't know anyone (other than Doug, maybe) who has bothered going thru the exercise of collecting all this data.

I agree with Greg. Design a large and effective stab, guestimate a starting position and pitch for the stab, and go flying.

Udi

Udi,

Thanks for the insight. Some problems are more complicated than they appear at first, or second, glance.
 
A flat plate can indeed have good lift and a high stall angle of attack, if the aspect ratio is close to 1- in other words a square shape with width equal to length. I've attached a plot of data compiled with various airfoil shapes, all of them at a low aspect ratio. The flat plate is marked in red.
As you can see, it stalls at around 37 degrees!
 

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Al_Hammer said:
A flat plate can indeed have good lift and a high stall angle of attack, if the aspect ratio is close to 1- in other words a square shape with width equal to length. I've attached a plot of data compiled with various airfoil shapes, all of them at a low aspect ratio. The flat plate is marked in red.
As you can see, it stalls at around 37 degrees!

Al, could you also provide a similar plot for an airfoil shape as well - to compare the relative lift (effectiveness as a HS). Your statement implies that a flat plate is as good as an airfoil. You might also address the effectiveness of tip plates on lift - a neat way to improve effectiveness of a HS while also improving yaw stability.

Thanks, Greg Gremminger
 
Sure, Greg. There's not much a little googling won't find.

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.

Attached is a plot comparing some standard airfoil shapes. As you can see, the flat plate actually stalls later , but the lift slope isn't as good. The effect diminishes rapidly as the aspect ratio starts to increase. A typical horizontal stab has an aspect ratio probably closer to 4 than to 1.
I merely wanted to point out that many factors can influence what the performance will be.
As for tip plates, I'll confess I don't know anything about them, so I'll keep quiet, unless I get lucky and come across something worth posting.
 

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I'm more optimistic than Greg (and maybe than Udi) than HS loads can be predicted with a useful degree of accuracy. I'd use such predictions to guide my first setup of the HS and then refine based on flight tests.

With a 2:1 aspect ratio and an 0012 airfoil, we got a max C.L. of 0.8 at about 14 degrees. This is obviously a much shallower lift-curve slope than Al (correctly) reports for more "normal" airfoil conditions -- as reported by NACA, etc.

Back when the Sport Pilot standards were being worked up, I believe it was Alan Loughry who reported actaul test results showing much higher C.L. possibilities with tip plates -- something like 1.4, using a HS that was otherwise fairly similar to ours.

The airspeed of the prop blast can be measured by poking around with an airspeed indicator in the blast while doing a static run of the engine.

A small, open-frame CLT gyro needs only a nominal amount of negative incidence -- maybe one degree. This is simply to get the rotor thrust line a little behind the CG. A HTL gyro needs more negative incidence.

On the Gyrobee, with a 1-2" HTL thrust offset and a 447 engine making perhaps 250 lb. of static thrust, I found that 3 deg. neg. incidence on a 0012 HS with small tip plates and dimensions of 24" x 48" was little too much, but close. I picked that incidence based on numerical estimation, and by golly, it came out pretty close.
 
Doug Riley said:
The airspeed of the prop blast can be measured by poking around with an airspeed indicator in the blast while doing a static run of the engine.
While you're at it -- go the extra mile and install your stab on strain gauges, and chart a lift curve for your specific stab, immersed inside your specific prop wash.

Better yet -- mount the whole gyro on strain gauges, and adjust your stab "on-line" to cancel-out any HTL moments. Anyone ever tried that?

Udi
 
Al_Hammer said:
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
 
Greg,
Thanks for your explanation. On my Mac I have one of the rock guards. If I mount my HS down on the keel should anything be done to the rock guard or just leave it in place? It is mounted just forward of and under the prop in it's original location.
 
Rock guard

Rock guard

Timchick said:
On my Mac I have one of the rock guards. If I mount my HS down on the keel should anything be done to the rock guard or just leave it in place? It is mounted just forward of and under the prop in it's original location.

Tim, Leaving the rock guard in place should not hurt anything aerodynamically, but it is weiessentially no good for stability because its moment arm to the CG is so short, it is physically small, and it is just a flat plate anyway.

If you have need for a rock guard - fly on rocky runways - and if the guard actually affords some prop chip protection, you might consider leaving it in place. I've always doubted the rock guard actually prevents chips to the prop completely, but it may shelter some portion of the prop from rocks kicked up by the wheels.

Thanks, Greg Gremminger
 
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