Gyroplane teeter rotor systems.

[Vance]

These are my opinions based on what I have read and learned about airfoils and gyroplane rotor systems.

What causes an airfoil to stall?

The critical angle of attack is exceeded.

Does airspeed have anything to do with an airfoil stalling?

In my opinion it does not.

How does a rotor blade stall?

A rotor blade stalls progressively from the center to the tip. There is always a stalled region on a gyroplane rotor blade.

Why does the retreating blade stall first?

The retreating stalls first because it is at a higher angle of attack than the advancing blade.

How does too much indicated air speed for the rotor rpm cause the retreating blade to stall?

As the indicated airspeed rises in relation to the rotor rpm a two blade semi rigid rotor system flaps further in order to deal with the dissymmetry of lift. This flapping increases the angle of attack of the retreating blade.

Is the relative wind always from the front of the rotor disk?

No.

Can turbulence cause a rotor blade to stall?

Yes.

 

[Vance]

I would like to know what airspeed VNE to rotor rrpm is achievable in each type of known gyro rotor blades available. Min max values. How would this be done given all the different types of gyro's and their rotor systems?
Also what is the fastest gyro the everyday man can own?
What top airspeed a safely achievable?

I don’t know anywhere that information would be available.

Writing in the most general terms in my opinion anything more than a tip speed ratio of .35 is less efficient and .5 is probably a reasonable speed limit and probably exceeds the power available in any modern gyroplane.

In The Predator at 1,100 pounds and 30 foot 8.5 inch chord blades turning 330 rotor rpm that would be around 150kts. With 160 horsepower I might see 90kts straight and level at 7,500 feet msl.

More rotor rpm within reason allows faster speeds before retreating blade stall become a problem.

If you want to go fast a gyroplane is probably a poor choice for the mission.

Any number of things may limit Vne; it is not just a rotor question.

Vne is set to keep people out of trouble.

 

[C. Beaty]

Correctly designed rotor blades, those with zero residual pitching moments, will result in the stick reaching the forward stop long before catastrophic stall of the retreating blade can occur.
With the stick on the forward stop, a gyro won’t go much faster; an increase of power results in a climb.
If a set of rotorblades results in rearward stick movement as speed increases, head for the ground and remove that rotor.

 

[Jean Claude]

The limit of forward speed that a rotor can reach depends on the ratio between this speed and the blade tip speed (this ratio is called μ).
This μ max ratio is lower when the aerodynamic pitch setting of the blades is higher.

For example with 5 degrees, then μ max is only 0.25, and since this high setting decreases the rpm to 345, the speed limit does not exceed 73 mph.
While if this setting is only 2.5 degrees, then μ max is .425 and since this low setting increases the rpm to 445, the speed limit can reach 158 mph
(Calculated for 24'x7" rotor loaded by 600 lbs, and provided that the engine power allows it )

 

[bryancobb]

I'll bite. During any climbs, two things are taking place. You are gaining height above the ground...and that ground ins passing under you.
Both of those events occur at some rate. The ratio of your gain in height to how much Earth passes beneath you in a measure of time, tells you if you are climbing at Vy or Vx.

Vy is the airspeed that results in the most altitude gained in a sample of time, while not being concerned with how much distance you are covering.

Vx is the airspeed that results in the least amount of Earth passing beneath you, while you are continuously gaining height, and while not being very concerned with how slowly you are gaining.

Neither have anything to do with the "accelerate-climb" or "zoom climb" technique. Vx and Vy are "steady-state" climbs. A short field with trees would make Vx the best choice until clear of obstacles, then speed up to Vy. A Vy takeoff would get you to the trees quicker and you may not be high enough to clear.

I you are going on a cross-country and your takeoff field has no obstacles, just climb at Vy and you will reach your cruise altitude sooner.

 

[Vance]

I'll bite. During any climbs, two things are taking place. You are gaining height above the ground...and that ground ins passing under you.
Both of those events occur at some rate. The ratio of your gain in height to how much Earth passes beneath you in a measure of time, tells you if you are climbing at Vy or Vx.

Vy is the airspeed that results in the most altitude gained in a sample of time, while not being concerned with how much distance you are covering.

Vx is the airspeed that results in the least amount of Earth passing beneath you, while you are continuously gaining height, and while not being very concerned with how slowly you are gaining.

Neither have anything to do with the "accelerate-climb" or "zoom climb" technique. Vx and Vy are "steady-state" climbs. A short field with trees would make Vx the best choice until clear of obstacles, then speed up to Vy. A Vy takeoff would get you to the trees quicker and you may not be high enough to clear.

I you are going on a cross-country and your takeoff field has no obstacles, just climb at Vy and you will reach your cruise altitude sooner.

In my opinion Vx and Vy are indicated air speeds and what is happening in relation to the ground is not relevant to the best indicated air speed.

If I am trying to clear a obstacle I use Vx because that allows for the best angle of climb.

If I just want to get to altitude then I use Vy because that allows for the best rate of climb.

 

[Jean Claude]

I thought the question was about maximum forward speed, not climb speed.

 

[Vance]

I thought the question was about maximum forward speed, not climb speed.

I started the thread because I find a lot of confusion in my clients about how an airfoil stalls and what happens when a rotor blade stalls in a two blade semi rigid rotor.

In my opinion there are a lot of takeoff mishaps because people don’t understand about what causes a rotor to become divergent.

I have seen things posted on the Rotary Wing Forum that are divergent from my basic understanding of airfoils and rotor management and I was hoping for some discussion about that.

The thread drifted quickly from there to VNE and then Vx and Vy.

It appears to be the nature of the Rotary Wing Forum.

I am always grateful for your input Jean Claude. You often bring numerical clarity to an emotional topic.

 

[Aviator168-aircraft]

What top airspeed is safely achievable? 120+mph

15 more miles per house, gyroplanes can compete LSAs.

 

[Martin W.]

These are my opinions based on what I have read and learned about airfoils and gyroplane rotor systems.

What causes an airfoil to stall?

The critical angle of attack is exceeded.

Does airspeed have anything to do with an airfoil stalling?

In my opinion it does not.

How does a rotor blade stall?

A rotor blade stalls progressively from the center to the tip. There is always a stalled region on a gyroplane rotor blade.

Why does the retreating blade stall first?

The retreating stalls first because it is at a higher angle of attack than the advancing blade.

How does too much indicated air speed for the rotor rpm cause the retreating blade to stall?

As the indicated airspeed rises in relation to the rotor rpm a two blade semi rigid rotor system flaps further in order to deal with the dissymmetry of lift. This flapping increases the angle of attack of the retreating blade.

Is the relative wind always from the front of the rotor disk?

No.

Can turbulence cause a rotor blade to stall?

Yes.

You made the statement :
Does airspeed have anything to do with an airfoil stalling?
In my opinion it does not.

I think airspeed is a factor ... (along with the high angle of attack of the retreating blade)
If a rotor has a tip speed of 300 mph and the aircraft is flying at 100 mph ... the advancing blade has 400 mph of air speed and the retreating blade has only 200 mph air speed.

So to me the lower air speed of the blade contributes to the stall .
But I am not an expert.

 

[Smack]

Martin, in my opinion (ha, there are equations, but I don't have them handy):
'stall' is entirely governed by angle of attack (AoA) as such can happen at ANY airspeed,
'lift' is how much force is generated by the airfoil and that is where airspeed (air molecules over the airfoil) comes in.
An airfoil can stall or it can generate so little lift that it falls out of the sky in an un-stalled condition.

 

[XXavier]

Martin, in my opinion (ha, there are equations, but I don't have them handy):
'stall' is entirely governed by angle of attack (AoA) as such can happen at ANY airspeed,
'lift' is how much force is generated by the airfoil and that is where airspeed (air molecules over the airfoil) comes in.
An airfoil can stall or it can generate so little lift that it falls out of the sky in an un-stalled condition.

The term 'stall speed' is sometimes misleading. A wing stalls when the AoA is higher than the critical value and the flow detaches. For a given load, it's true there's a corresponding 'stall speed', but only because below that critical speed the wing has to work with an AoA that is higher than the critical -stall- value for the airfoil... The only culprit is the AoA...

 

[Martin W.]

Thanks guys .... I had loosely equated "stall" with "loss of lift" due to low air flow. Mostly from a fixed wing perspective (I do not fly fixed wing)

For example .... A Cessna has good takeoff into the wind ... turns too soon , inside wing "stalls" first , then aircraft drops to the ground.

 

[Vance]

Thanks guys .... I had loosely equated "stall" with "loss of lift" due to low air flow. Mostly from a fixed wing perspective (I do not fly fixed wing)

For example .... A Cessna has good takeoff into the wind ... turns too soon , inside wing "stalls" first , then aircraft drops to the ground.

The reason I mentioned that airspeed over an airfoil doesn’t effect a stall is because some fixed wing flight instructors emphasize stall speed rather than angle of attack.

Stall/spin accidents still kill a lot of pilots and it typically is because they overshot the centerline and banked to fix it. Speed in a fixed wing is an approximation of angle of attack; as soon as you add some load the angle of attack changes for a given air speed.

This confusion carries over to gyroplanes and your thoughtful reply is a good example of the down side of these conflating concepts Martin.

Based on you post the greatest risk of sailing a blade would be at high speed. At high speed the angle of the disk is less so the retreating blade stalls progressively from the center out as the indicated air speed increases.

The most likely time to sail a blade is with the cyclic well back because the angle of attack of the blades to the relative air flow is the highest and only rotor rpm changes that. Too low a rotor rpm for a given forward speed is how a blade is sailed.

I feel you are falling into another misconception trap with your last post Martin. Wind is a ground reference term. Once the aircraft is flying it is all about indicated air speed rather than the wind direction or the speed over the ground.

A downwind turn is only a problem if you are referencing the ground for speed. This is a particular problem with low time gyroplane pilots because it is so easy to reference the ground. They see they are going faster over the ground and reduce their airspeed trying to climb with the cyclic and reach the point where it takes more power to fly than they have available and they descend into the ground. As training has improved this has become less of a problem although even with a very good flight instructor I found it difficult to focus on airspeed early on when making a downwind turn.

You may be referencing the dangers of uncoordinated flight and the answer is the same, the nose should be pointed into the relative wind rather than where you want to go on the ground.

Keep working at it Martin and the pieces will fall into place. Get good instruction, read everything you can and you will likely have a long, safe and rewarding gyroplane adventure.

 

[kolibri282]

We have had another question regarding AoA of a rotor blade section here:
https://www.rotaryforum.com/threads/retreating-blade-aoa.1145366/

I have cobbled together a small program that uses the formulae in report naca-716 to calculate AoA around the blade disk for a given rotor section station. This is the value uT, which in the case below is 0.3, thus we are calculating the AoA at the 30% radius station.
An angle of attack of 12° is a ball park figure for stall, this, of course depends on the blade profile used. Please keep in mind that the blade sweep angle starts over the tail of the aircraft, thus psi=90° is to starboard and psi=270° is to port for the usual US ccw turning rotor.
The first line of the table gives some geometry values. Line two contains the advance ratio mu (0.15... 0.45). The next three lines contain flight speed in mph, control angle alfaNf and disk back flapping angle a1 for the given advance ratio.

As you can see for larger advance ratios mu, this blade section stall angle is attained over a larger part of the sweep angle psi.
For uT=0.3 and mu=0.25 only stations at 210°, 240° and 270° are stalled, for mu=0.45 stall starts already at a sweep angle of 150° and goes all the way to 330°. Since a stalled section generates much more drag than a non stalled one and generates no lift the rotor will probably at this advance ratio not generate enough lift anymore or the rotational speed will not increase if the stalled region spreads to far outward.

If you want to play around yourself you can download the program from here:
https://www.magentacloud.de/lnk/PABJkanN

password is

alfamax

all lower case


You need octave to run it.
https://www.gnu.org/software/octave/download

Have fun!

uT=​	0.3​	R [m] =​	3.05​	sigma=​	0.0313​
#​	psi​	0.15​	0.25​	0.35​	0.45​
V[mph]​	​	52.52​	84.37​	118.45​	153.79​
alfaNf​	​	17.02​	9.03​	5.57​	3.64​
a1​	​	1.33​	2.16​	2.92​	3.61​
1​	0.0°​	8.39°​	8.35°​	8.74°​	9.61°​
2​	30.0°​	7.79°​	7.28°​	7.22°​	7.66°​
3​	60.0°​	7.70°​	7.14°​	7.02°​	7.38°​
4​	90.0°​	8.15°​	7.85°​	7.90°​	8.28°​
5​	120.0°​	8.95°​	9.14°​	9.56°​	10.16°​
6​	150.0°​	9.84°​	10.58°​	11.48°​	12.50°​
7​	180.0°​	10.56°​	11.67°​	12.87°​	14.15°​
8​	210.0°​	10.97°​	12.21°​	13.44°​	14.67°​
9​	240.0°​	11.05°​	12.36°​	13.67°​	15.03°​
10​	270.0°​	10.81°​	12.18°​	13.78°​	15.62°​
11​	300.0°​	10.21°​	11.42°​	13.07°​	15.17°​
12​	330.0°​	9.32°​	9.97°​	11.11°​	12.74°​
13​	360.0°​	8.39°​	8.35°​	8.74°​	9.61°​