Dominator measurement needed

Capt'n Gator

Junior Member
Joined
Sep 25, 2012
Messages
655
Location
Goldsboro, NC
Aircraft
Dominator Single 582BH N401JC
Total Flight Time
200+
Hi fellow Dominator owners. I need a measurement on a Dom single. With the rotor blades all the way tilted back, how much room is there between the bottom of the rotor blade and the support which runs from the mast to the top of the rudder? In other words, how close does the rotor come from touching that top rudder support in the area around the prop.
 
I'm in the same situation. It's like anything Dominator related is a tight kept secret. I'm looking for the assy instructions for a Dom tail.
 
I am trying to get you an answer. It may take a few days. Russ Smith here in NC has a live dominator in captivity. I will call and ask him to measure.
 
For the Dominator; I do not know!
For most Gyros the answer is, "There is No clearance at all."

https://www.youtube.com/watch?v=DJ4EpCphp18

With the Cyclic fully back and the Rotor at the Teeter Stop limits, The rotor WILL strike the tail.
This is what is mistakenly called "Rotor Flapping". In helicopters, it is called "Mast Bumping."
In gyros, It is really "Teeter Stop Bumping" and has nothing to do with anything "Flapping" other than the flapping caused by the bumping.
Rotor Blades do not "Flap" because the Rotor Speed is too low. They Flap because the Teeter Stops are being Bumped.

Under normal flight aerodynamics, it will not occur.
The problem is when the Rotor Speed is not sufficient to absorb the excessive incoming airflow.
The Right Advancing Blade climbs in the higher airflow and the Left Retreating Blade descends in the lower airflow.
This causes the roll to the left that Juan de la Cierva cured with the invention of the Teetering hinge.

The initial assumption would be that the rotor disk would tilt to left. Not so.
The Advancing blade reaches its highest point in the front and then become the retreating blade and begins descending.
This is why the Rotor tilts Backwards.

The Teeter Stop Bumping commonly happens in the takeoff phase with low rotor speed and high ground speed.
Pre-Spinning the Rotor up to sufficient RPM before the takeoff roll will limit the amount of teetering and prevent Rotor Strike.

It also can occur in normal flight with normal rotor speed coupled with high airspeed.
The faster you go, the further the rotor disk teeters backward.
At some speed, the rotor will start bumping the teeter stops.
This is what determines the Vne (Never Exceed) Airspeed of the Gyro.

It would not surprise me at all that the Dominator Rotor is capable of striking the top of the Rudder.
It's what they do!
It is also why Rotor Speed management is important.
 
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A general rule of thumb is with full aft tilt of the head and full aft teeter of the rotor, and with the blade pulled lightly down, there should be no less that one inch of clearance.

If it is possible for the rotor to geometrically strike the tail or prop then centrifugal teeter limiters should be considered if not down right required to help prevent a low rpm strike.
 
Capt'n Gator;n1122269 said:
Hi fellow Dominator owners. I need a measurement on a Dom single. With the rotor blades all the way tilted back, how much room is there between the bottom of the rotor blade and the support which runs from the mast to the top of the rudder? In other words, how close does the rotor come from touching that top rudder support in the area around the prop.

When rotor isn't rotating at 150+ RPM then it will hit the tail/upper tail support. This is why prerotation starts with stick kept full forward. After RRPM reaches 100-120 RPM stick should be slowly pulled back so that it is full back at 150-160 RPM. When rotor is rotating at 150+ RPM then with stick full aft there should be at least 3-4" clearance.
 
Rotorblades22'-28' Dragon Wings - all-aluminum, bonded skins
Length10'
Height8'1"
Width5'6"


From The RFD site
 
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So Wayne, I took some measurements this morning on my Dominator single. I have 24' Dragon Wings. With the stick full back and the blades at rest, my rotors actually sit about 3 to 4 inches BELOW my tail. See pic below. I spoke to a couple of very experienced gyro pilots who said this was not unusual and was also true of their two place Dominator. It seems to me that an adjustment of the teeter stops so the blade would clear the tail would result in reduced flair control during landing. One more reason correct rotor/stick management on the ground is so critical, as has been drilled into me by my instructor.

Russ
 

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My single Dominator has 5-6" clearance between the blades and the top of the tail. Stick full back on the stop and blades back on the teeter stop.
 
Back to Dom roots: a piece of original drawing. dom tail clrnce.jpg
 
Bill (uncle Willie) your explanation about rotor flap is the best one ever,very clear, easy to understand, and to the point .

Great job of writing.Thank you!
 
OK everyone, I appreciate all the input. If everyone will look at post #11 by Twistair. Now if someone could give me the measurement between the rudder stabilizer bar running to the mast and the bottom of the rotor right there above where it says 3/4" prop clearance, I need to know how much clearance there is between the rotor blade and that rudder upper stabilizer bar right above the prop tip area. Thanks.
 
Capt'n Gator;n1122496 said:
OK everyone, I appreciate all the input. If everyone will look at post #11 by Twistair. Now if someone could give me the measurement between the rudder stabilizer bar running to the mast and the bottom of the rotor right there above where it says 3/4" prop clearance, I need to know how much clearance there is between the rotor blade and that rudder upper stabilizer bar right above the prop tip area. Thanks.

If I am understanding your question correctly Wayne, it looks like one inch of clearance is required from the rotor bad to the bolt.

The bolt appears to me to be the high point of the upper rudder brace.
 
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Uncle Willie;n1122345 said:
For the Dominator; I do not know!
For most Gyros the answer is, "There is No clearance at all."

https://www.youtube.com/watch?v=DJ4EpCphp18

With the Cyclic fully back and the Rotor at the Teeter Stop limits, The rotor WILL strike the tail.
This is what is mistakenly called "Rotor Flapping". In helicopters, it is called "Mast Bumping."
In gyros, It is really "Teeter Stop Bumping" and has nothing to do with anything "Flapping" other than the flapping caused by the bumping.
Rotor Blades do not "Flap" because the Rotor Speed is too low. They Flap because the Teeter Stops are being Bumped.

Under normal flight aerodynamics, it will not occur.
The problem is when the Rotor Speed is not sufficient to absorb the excessive incoming airflow.
The Right Advancing Blade climbs in the higher airflow and the Left Retreating Blade descends in the lower airflow.
This causes the roll to the left that Juan de la Cierva cured with the invention of the Teetering hinge.

The initial assumption would be that the rotor disk would tilt to left. Not so.
The Advancing blade reaches its highest point in the front and then become the retreating blade and begins descending.
This is why the Rotor tilts Backwards.

The Teeter Stop Bumping commonly happens in the takeoff phase with low rotor speed and high ground speed.
Pre-Spinning the Rotor up to sufficient RPM before the takeoff roll will limit the amount of teetering and prevent Rotor Strike.

It also can occur in normal flight with normal rotor speed coupled with high airspeed.
The faster you go, the further the rotor disk teeters backward.
At some speed, the rotor will start bumping the teeter stops.
This is what determines the Vne (Never Exceed) Airspeed of the Gyro.

It would not surprise me at all that the Dominator Rotor is capable of striking the top of the Rudder.
It's what they do!
It is also why Rotor Speed management is important.


It is my understanding that rotor "flap" is when the critical angle of attack of the rotor blade airfoil is exceeded and one of the blades stalls.

I feel this happens when too much forward speed is combined with insufficient rotor rpm.

It is not unusual for the rotor to hit the stops in this situation and hitting the stops makes it easier to feel rotor flap in the cyclic.

In my experience in most gyroplanes I have flown the rotor will hit the tail when the rotor blades flap on takeoff and hitting the stops is the result of the rotor flap rather than the cause of the tail strike.

I have not flown a gyroplane where the rotor hub bar hits the teeter stops past VNE.

I have flown gyroplanes where the rotor becomes less manageable as the speed increases.

I suspect the VNE in most gyroplanes is set by the test pilot over stability issues rather than the rotor hitting the teeter stops.
 

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On my Tandem Dominator with the rotor stopped and aligned fore & aft; the rotor blade does clear the top of the tail with approx. 4" of separation. That being said, keep in mind that I have a set of Centrifugal Teeter limits/stops installed. If the limits were removed, the blade would com below the top of the tail. At 150 RRPM, I can move the stick to the full rear position and continue to gain RRPM for the takeoff run. At 150 RRPM there is enough centripetal force to make the rotor rigid and probably even some coning angle to also to get the clearance needed. However, if hit with a really hard gust of wind; the rotor flapping/teeter angle might increase enough to impact the tail. At 200 RRPM, I can go to full power; and not be real concerned about flapping the rotor into the tail.
 
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The first thing is to define what IS rotor flap. ???
It is relatively easy to recognize that something is very wrong in the take-off roll and abort the takeoff.
But what really happens.

The books like to show the graphic of the blades assuming a wild "S" shape with the forward blade bent way Up and pointing at the sky with the Aft Blade bent way down and hitting the Tail.
It is unreasonable to ever expect any blade to be able to assume anything close to that shape without exceeding the yield limits of the materials.

Let's assume a worse case scenario.
The rotor is only pre-rotated to ~50rpm and the gyro is way overpowered.
In the calm air and stationary pre-flight position, The rotor finds its center and aligns with the rotating mast.
The stick is forward and the 2/rev vibrations do not exist. All feels smooth.

The takeoff roll begins and the stick is pulled most, but not all of the way back.
As the airspeed increases rapidly, the rotor picks up speed slowly but not enough to maintain its center.
The advancing blade gets the benefit of the additional airspeed and climbs in the improved lift conditions
The retreating blade is deprived of its airspeed and lift and descends as it loses the battle with the high lift blade on the other side of the rotor.

The pilot begins to notice the 2/rev vibrations caused by the teeter hinge being centered whenever the blades are broadside to the fuselage, and teetered aft when the Blades are in their fore and aft position.
The CG of the Rotor goes out of alignment with the center of rotation every time the blades are fore and aft.
This occurs twice per revolution and is felt in the control sick as a vibration that increases as the teeter angle increases.

At some point, the teeter angle reaches its limits and starts to bump the Teeter stops.
The central hub can no longer teeter but the blades want to go even further.
If the stick still has any travel remaining, the hub bumping the stop is going to force the stick backward with great force.
In many gyros, this is also the limit where the rudder starts to get struck.
In either event, the wise pilot will recognize that something is seriously wrong and will wisely drop power and push forward.
The claim that a "Rotor Flap" event occurred would be reasonable.

Did something Flap? It depends on how you look at it.
When the aft blade bumped the Teeter stop, the rotor hub stopped rotating.
The aft blade continued to bend down, compressing the Teeter stop material and bending the mast somewhat and flexing downward to some degree.
The forward blade continued upwards, flexing the mast backward also and bending along its length to some degree
So when the rotor hit the teeter stops it did bend out of its normal coned shape to some degree and that shape looked like an "S".
Nothing like the shape seen in the books, but a highly exaggerated description would describe it as an "S" shape, however slight.

Teeter stops are at ~3° of travel.
At normal airspeeds combined with reasonable Rotor speeds the Teeter angle is quire small; in the 1/2° degree range.
The teeter angle is barely visible in the Photo in Vance's post #15
There is plenty of clearance from the rudder.

The problem only comes into play when the airspeed is high and the rotor speed is low.
The rudder is only in jeopardy when the stick is so far back that forward flight is not possible.
This is why that if the stick is held in the extreme rear position after lift off, the speed quickly decreases and the gyro descends to the ground.
If the situation is recognized quickly and If the nose is lowered before the airspeed decays very far, a recovery can be made.
Lowering the nose does NOT mean pointing it at the ground. It means lowering it from its excessive high attitude to the proper attitude for climb-out.

At the Vne extreme, the 2/rev vibrations cause by the extreme teetering angle start to become severe enough to cause concerns.
The engineering department would have warned the test pilot of the theoretical Vne limits and the test pilot would have reported a practical value safely below the Teeter stop limit.
So Yes! You should not expect to hit the Teeter stops even if slightly above Vne and the 2/rev shaking will cause most pilots to NOT push the envelope any further.

The Bumping causes the "Flap." There is no Flap event without the Teeter Stop being Bumped.
I think we are actually in agreement, just looking at the phenomenon from different perspectives.
 
Uncle Willie;n1122533 said:
The first thing is to define what IS rotor flap. ???
It is relatively easy to recognize that something is very wrong in the take-off roll and abort the takeoff.
But what really happens.

The books like to show the graphic of the blades assuming a wild "S" shape with the forward blade bent way Up and pointing at the sky with the Aft Blade bent way down and hitting the Tail.
It is unreasonable to ever expect any blade to be able to assume anything close to that shape without exceeding the yield limits of the materials.

Let's assume a worse case scenario.
The rotor is only pre-rotated to ~50rpm and the gyro is way overpowered.
In the calm air and stationary pre-flight position, The rotor finds its center and aligns with the rotating mast.
The stick is forward and the 2/rev vibrations do not exist. All feels smooth.

The takeoff roll begins and the stick is pulled most, but not all of the way back.
As the airspeed increases rapidly, the rotor picks up speed slowly but not enough to maintain its center.
The advancing blade gets the benefit of the additional airspeed and climbs in the improved lift conditions
The retreating blade is deprived of its airspeed and lift and descends as it loses the battle with the high lift blade on the other side of the rotor.

The pilot begins to notice the 2/rev vibrations caused by the teeter hinge being centered whenever the blades are broadside to the fuselage, and teetered aft when the Blades are in their fore and aft position.
The CG of the Rotor goes out of alignment with the center of rotation every time the blades are fore and aft.
This occurs twice per revolution and is felt in the control sick as a vibration that increases as the teeter angle increases.

At some point, the teeter angle reaches its limits and starts to bump the Teeter stops.
The central hub can no longer teeter but the blades want to go even further.
If the stick still has any travel remaining, the hub bumping the stop is going to force the stick backward with great force.
In many gyros, this is also the limit where the rudder starts to get struck.
In either event, the wise pilot will recognize that something is seriously wrong and will wisely drop power and push forward.
The claim that a "Rotor Flap" event occurred would be reasonable.

Did something Flap? It depends on how you look at it.
When the aft blade bumped the Teeter stop, the rotor hub stopped rotating.
The aft blade continued to bend down, compressing the Teeter stop material and bending the mast somewhat and flexing downward to some degree.
The forward blade continued upwards, flexing the mast backward also and bending along its length to some degree
So when the rotor hit the teeter stops it did bend out of its normal coned shape to some degree and that shape looked like an "S".
Nothing like the shape seen in the books, but a highly exaggerated description would describe it as an "S" shape, however slight.

Teeter stops are at ~3° of travel.
At normal airspeeds combined with reasonable Rotor speeds the Teeter angle is quire small; in the 1/2° degree range.
The teeter angle is barely visible in the Photo in Vance's post #15
There is plenty of clearance from the rudder.

The problem only comes into play when the airspeed is high and the rotor speed is low.
The rudder is only in jeopardy when the stick is so far back that forward flight is not possible.
This is why that if the stick is held in the extreme rear position after lift off, the speed quickly decreases and the gyro descends to the ground.
If the situation is recognized quickly and If the nose is lowered before the airspeed decays very far, a recovery can be made.
Lowering the nose does NOT mean pointing it at the ground. It means lowering it from its excessive high attitude to the proper attitude for climb-out.

At the Vne extreme, the 2/rev vibrations cause by the extreme teetering angle start to become severe enough to cause concerns.
The engineering department would have warned the test pilot of the theoretical Vne limits and the test pilot would have reported a practical value safely below the Teeter stop limit.
So Yes! You should not expect to hit the Teeter stops even if slightly above Vne and the 2/rev shaking will cause most pilots to NOT push the envelope any further.

The Bumping causes the "Flap." There is no Flap event without the Teeter Stop being Bumped.
I think we are actually in agreement, just looking at the phenomenon from different perspectives.


I respectfully disagree Bill.

My definition of rotor blade flap is the way a flapping hinge manages the dissymmetry of lift. In a two blade semi rigid fixed pitch rotor system the advancing blade flaps up pivoting on the teeter hinge reducing the angle of attack (reducing lift) and the retreating blade flaps down increasing its angle of attack and increasing the lift until they reach a symmetry of lift.

When the cyclic is full back on The Predator the rotor disk angle is 16 degrees; well beyond the critical angle of attack for most airfoils. If the blades are not rotating fast enough the critical angle of attack is exceeded by the retreating blade and it stalls and the advancing blade pushes the retreating blade down into the rudder.

I have never hit the teeter stops in high speed flight.

As near as I can calculate The Predator at gross weight sees about six degrees of flapping at 135kts and that is not near the flapping stops and well past any reasonable VNE.
 
I do not see the disagreement???

My definition of rotor blade flap is the way a flapping hinge manages the dissymmetry of lift. In a two blade semi rigid fixed pitch rotor system the advancing blade flaps up pivoting on the teeter hinge reducing the angle of attack (reducing lift) and the retreating blade flaps down increasing its angle of attack and increasing the lift until they reach a symmetry of lift.

I have no problem with that. Flapping = Teetering. Advancing blade goes UP, retreating DOWN. Lift become balances by the physics of the motion.

When the cyclic is full back on The Predator the rotor disk angle is 16 degrees; well beyond the critical angle of attack for most airfoils. If the blades are not rotating fast enough the critical angle of attack is exceeded by the retreating blade and it stalls and the advancing blade pushes the retreating blade down into the rudder.

Some portion of the retreating blade is always in a stall condition. the higher the ratio of airspeed to rotor rpm becomes, the more of the retreating blade is in stall.
The stall point is proportional, The entire blade does not suddenly stall all at once.
As the stall point move out the teetering angle increases.
Depending on the position of the cyclic stick, either the rudder or the teeter stop will make contact first.
Does it matter which happens first??? Unless you are saying that unless the rotors strike the airframe, it is not Flapping.
I will define Flapping as Teetering beyond any flyable limits.

I have never hit the teeter stops in high speed flight.
As near as I can calculate The Predator at gross weight sees about six degrees of flapping at 135kts and that is not near the flapping stops and well past any reasonable VNE.
I would not expect that to happen. The Vne is quoted low enough that Teeter stop bumping would be beyond reasonable limits.
It might happen in the 150-175kt range but you would be foolish to try it.

My earlier post contains an error.
Teeter Stop limits are in the 10° range, Not near 3°. My Error.

Despite the highly exaggerated illustration describing Blade Flap, The text in the FAA Handbook actually has it mostly correct
From FAA Rotorcraft flying Handbook FAA-H-8083-21, Page 20-1
BLADE FLAP
On a gyroplane with a semi-rigid, teeter-head rotor system, blade flap may develop if too much airflow passes through the rotor system while it is operating at low r.p.m. This is most often the result of taxiing too fast for a given rotor speed. Unequal lift acting on the advancing and retreating blades can cause the blades to teeter to the maximum allowed by the rotor head design. The blades then hit the teeter stops, creating a vibration that may be felt in the cyclic control. The frequency of the vibration corresponds to the speed of the rotor, with the blades hitting the stops twice during each revolution. If the flapping is not controlled, the situation can grow worse as the blades begin to flex and bend. Because the system is operating at low r.p.m., there is not enough centrifugal force acting on the blades to keep them rigid. The shock of hitting the teeter stops combined with uneven lift along the length of the blade causes an undulation to begin, which can increase in severity if allowed to progress. In extreme cases, a rotor blade may strike the ground or propeller.


Again, Other than some minor differences in perspective, I think we are in agreement.
Or at least, I will not argue the point.
The end results are the same. Control the Rotor!
 
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