Ground effect in a gyroplane?

I'm struggling to understand gyro teetering rotor physics. Can someone help? I was under the understanding that the advancing rotor teeters up to reduce lift since it is moving at RRPM plus airspeed and the retreating blade teeters down to increase lift as it is moving at RRPM - airspeed.

Is this correct?
 
I'm struggling to understand gyro teetering rotor physics. Can someone help? I was under the understanding that the advancing rotor teeters up to reduce lift since it is moving at RRPM plus airspeed and the retreating blade teeters down to increase lift as it is moving at RRPM - airspeed.

Is this correct?

Yes. Advancing blade teeters up and its AoA is reduced and retreating blade teeters down and its AoA is increased.
 
Yes. Advancing blade teeters up and its AoA is reduced and retreating blade teeters down and its AoA is increased.
Thanks, I have that part right then but I'm missing the AOA part. I'm thinking backwards, advancing blade teeters up and angle from the blade chord line to relative wind also increases. What am I not getting?
 
Coming at this with an outsider's (common sense?) viewpoint:
It seems to me that EVERY pilot should understand ALL the principles of flight, to at least a basic level. It may make practical sense for for the person giving a test to "randomly" select some subset of these to ask the person being tested, but I would think that if that person being tested demonstrates inadequate knowledge or confusion in ANY of these principles, it should be grounds for going back, relearning, and trying again another day/time.

Giving a pass to someone who understands only a minority (3) of the stated principles of flight just seem ludicrous.
 
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Bobby, I found this helpful in visualizing how the AOA of the advancing/rising blade is less than that of the retreating/descending blade:
 

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Bobby, I found this helpful in visualizing how the AOA of the advancing/rising blade is less than that of the retreating/descending blade:
Thanks, I'll have to look at that and try to make sense of what the apparent wind is doing and why.
 
Thanks, I have that part right then but I'm missing the AOA part. I'm thinking backwards, advancing blade teeters up and angle from the blade chord line to relative wind also increases. What am I not getting?

First think of vertical autorotation where there is no dissymmetry of lift and lift is equal on both sides. Hence teetering isn't really needed. Also 2/rev vibrations are largely gone because no teetering is happening and thus no application of law of conservation of angular momentum (ice skater raising her arms "in" to spin faster causing acceleration twice (2/rev), one at 3 O clock, one at 9 O clock)

In this case, angle of attack is high (stalled) close to the root (slow speed due to rotation) and low towards the tip of the blade because of faster speed from rotation. The regions of the disc are 1) stall, 2) driving, 3) driven and between stall and driving region there is an area of equilibrium where it transitions. Also between driving and driven region there is an area of equilibrium where transition happens.
The main reason for angle of attack to be high at the root and low at the tip is rotational speed. In the driving region the twist of the blade also contributes to a more positive angle of attack and helicopter blade designers play funky with this area to gain some advantages for their stated mission and spread the load more evenly across but for our purposes we just let nature do its thing because designing twist or [washout] isn't worth the effort for us.

To get a fuller picture of this, crack open your Rotorcraft Flying Handbook and look at page 3-9 onwards and read that carefully. Fairly simple and clear. Its the helicopter private pilot section but its the same concepts. Teetering 2 blade fixed pitch semi-rigid rotors are the simplest implementation of rotors from the helicopter point of view.


Once you understand and can visualize that in vertical autorotation these 3 regions have different AoA because primarily of rotational speed difference from root to the tip, you are then ready to go to autorotation in forward flight.
In forward flight, the airfoil path of advancing (and teetering up/rising) blade results in relative wind (resultant relative wind) that gives us a smaller angle of attack and airfoil path of retreating (and teetering down/falling) blade results in relative wind that gives us a larger angle of attack. A visual of this is attached in one of the pictures.
Don't need to do all the math and vectors. Just think that when at 3 O clock position the advancing blade starts to rise up (because this is where it sees the fastest wind speed and thus higher lift), the resultant relative wind also starts to come from "more above" because the airfoil has started to add a vertical rise path in its motion in addition to circulating forward in its in-plane path. Similarly, when the retreating blade at 9 O clock starts to go down (because this is where it sees the slowest wind speed and thus least lift), the resultant relative wind also starts to come from "more down". If you can visualize that, you can make sense of it.
Another way to think of this is, as in vertical autorotation you saw that the rotating blade has high angle of attack at the root and low at the tip due to rotational velocity producing different speeds. Well when advancing blade sees "additional speed" due to forward motion it then follows by same logic that it will inevitably see even lower angle of attack compared to its retreating brother which sees the opposite because its tip is now subtracting the forward velocity and slowing down. Awesome nature always looking for symmetry does the job for us.

Now we know from our 9th grade Physics that acceleration is a change in speed or direction (change in velocity). At 3 O' Clock and 9 O' Clock these blades are "both" accelerating due to law of conservation of angular momentum as they teeter up or down (doesn't matter). Their center of gravity is moving inwards slightly. That acceleration twice in one revolution is 2/rev hits you get and is directly the result of equalizing the difference in lift between the blades. You feel that most at 12 O Clock and 6 O Clock because that is how rotational systems work due to gyroscopic precession (90 degrees later effect)

Hope this helps.
 

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It's kind of amazing to me that the FAA hasn't changed or updated that handbook in 22 years, except to slap a FOR GYROPLANE USE ONLY, on the front cover – "pay no attention to all that helicopter stuff here".
Even if they don't see fit to update the actual contents, the least they could do is to take out all the stuff they don't want anyone to look at anymore, and retitle it the Gyroplane Flying Handbook.
 
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Coming at this with an outsider's (common sense?) viewpoint:
It seems to me that EVERY pilot should understand ALL the principles of flight, to at least a basic level. It may make practical sense for for the person giving a test to "randomly" select some subset of these to ask the person being tested, but I would think that if that person being tested demonstrates inadequate knowledge or confusion in ANY of these principles, it should be grounds for going back, relearning, and trying again another day/time.

Giving a pass to someone who understands only a minority (3) of the stated principles of flight just seem ludicrous.
Yes, the standards are written so the examiner can pick three or more. I suspect this was done so examinations don't take days to complete. When you start doing these, you realize not everyone communicates the same way. Lets take an example below about Aeromedical Factors.

Assume the applicant is asked to explain hypoxia. The applicant answers: it is a problem when you fly over 10,000 feet and sport pilots can't fly over 10,000 feet. What should the examiner ask next? Is there anywhere you can fly a gyro plane over 10,000 feet as a sport pilot? What if you were in Colorado with a mountain that was 14000 feet tall. How high could you legally fly? They answer 16,000 feet, because they can fly 2000 feet above the ground. The examiner then asks about oxygen requirements, and the symptoms of hypoxia. Are the symptoms the same for everyone? How do you distinguish hypoxia from carbon monoxide poisoning? (Carbon monoxide is an odourless, colourless gas, and poisoning causes hypoxia). And I would probably stop there. A more thorough examiner might ask what are the 4 types of hypoxia. And if smoking has an impact.

To continue with the example, lets assume the applicant states the symptoms of hypoxia are Cyanosis (blue lips/fingertips), and initial symptoms can often include euphoria and a carefree attitude. --but they can't think of any other symptoms. An examiner might consider the lack of other symptoms as marginal knowledge. --the applicant has the big things, but they could know more. Did the applicant meet the standard or not? It really depends on what the examiner considers an acceptable level of knowledge. Should the examiner go further? And if the first three aeromedical factors the examiner asks about are barely passing knowledge, should the examiner ask about all 8?
 
Yes, the standards are written so the examiner can pick three or more. I suspect this was done so examinations don't take days to complete. When you start doing these, you realize not everyone communicates the same way. Lets take an example below about Aeromedical Factors.

Assume the applicant is asked to explain hypoxia. The applicant answers: it is a problem when you fly over 10,000 feet and sport pilots can't fly over 10,000 feet. What should the examiner ask next? Is there anywhere you can fly a gyro plane over 10,000 feet as a sport pilot? What if you were in Colorado with a mountain that was 14000 feet tall. How high could you legally fly? They answer 16,000 feet, because they can fly 2000 feet above the ground. The examiner then asks about oxygen requirements, and the symptoms of hypoxia. Are the symptoms the same for everyone? How do you distinguish hypoxia from carbon monoxide poisoning? (Carbon monoxide is an odourless, colourless gas, and poisoning causes hypoxia). And I would probably stop there. A more thorough examiner might ask what are the 4 types of hypoxia. And if smoking has an impact.

To continue with the example, lets assume the applicant states the symptoms of hypoxia are Cyanosis (blue lips/fingertips), and initial symptoms can often include euphoria and a carefree attitude. --but they can't think of any other symptoms. An examiner might consider the lack of other symptoms as marginal knowledge. --the applicant has the big things, but they could know more. Did the applicant meet the standard or not? It really depends on what the examiner considers an acceptable level of knowledge. Should the examiner go further? And if the first three aeromedical factors the examiner asks about are barely passing knowledge, should the examiner ask about all 8?

And given that every student pilot should have cracked open the PHAK at least once fully in their preparation for the checkride, they should easily pick up that book, go to Chap 17 and spell out all 4 types of Hypoxia for you. That tells me at least they know where to go find the information. If the candidate has never even seen the PHAK and does not have it with him/her in the checkride, I will hope he has memorized a ton from it.
In your example, if the applicant hits the big things, I'd pass them. But if the first 3 aeromedical factors were barely over the ridge, I'd ask about more for sure.
 
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It's kind of amazing to me that the FAA hasn't changed updated that handbook in 22 years, except to slap a FOR GYROPLANE USE ONLY, on the front cover – "pay no attention to all that helicopter stuff here".
Even if they don't see fit to update the actual contents, the least they could do is to take out all the stuff they don't want anyone to look at anymore, and retitle it the Gyroplane Flying Handbook.

True but there is useful info there and in the new helicopter flying handbook also. We should seek out good clear info anywhere. The concepts are the same. The application and implementation is different but there is no difference at the basic level. In a way there is no difference between an airplane wing and a rotor. One just has to open their mind and start to move the airplane wing in a circle and imagine what happens.

From Helicopter Flying Handbook says the same thing I said above about relative wind on teetering up blade and teetering down blade:

"Advancing Blade
As the relative wind speed of the advancing blade increases, the blade gains lift and begins to flap up. It reaches its maximum upflap velocity at the 3 o’clock position, where the wind velocity is the greatest. This upflap creates a downward flow of air and has the same effect as increasing the induced flow velocity by imposing a downward vertical velocity vector to the relative wind which decreases the AOA

Retreating Blade
As relative wind speed of the retreating blade decreases, the blade loses lift and begins to flap down. It reaches its maximum downflap velocity at the 9 o’clock position, where wind velocity is the least. This downflap creates an upward flow of air and has the same effect as decreasing the induced flow velocity by imposing an upward velocity vertical vector to the relative wind which increases the AOA."
 
G. TASK: AEROMEDICAL FACTORS REFERENCES: FAA-H-8083-25; AIM.

Objective. To determine that the applicant exhibits knowledge of the elements related to aeromedical factors by explaining:

1. The effects of alcohol, drugs, and over-the-counter medications.

2. The symptoms, causes, effects, and corrective actions of at least three (3) of the following— a. hypoxia. b. hyperventilation. c. middle ear and sinus problems. d. spatial disorientation. e. motion sickness. f. carbon monoxide poisoning. g. stress and fatigue. h. dehydration. i. hypothermia.

One of the nice things about the practical test standards is they tell you where to look.

For example the aeromedical information can be found in the AIM section of your Federal Aviation Regulation/Aeronautical Information manual (FAR/AIM).
 
Thanks, I have that part right then but I'm missing the AOA part. I'm thinking backwards, advancing blade teeters up and angle from the blade chord line to relative wind also increases. What am I not getting?
Be careful not to confuse flapping action (rising or dropping of the blade, or teetering in the case of a simple two blade rotor) with pitching action (tilting that chord line up with respect to the relative wind).

If you hold your hand out flat to your side at shoulder height and swing it down, you will feel air against your palm, essentially a huge 90 degree angle of attack. Pushing down increases AOA. Conversely, if you raise that same hand, you have an extreme negative angle of attack as the air strikes the back of your hand. Pushing up decreases AOA.

Pitching action would require tilting your hand thumb up or thumb down, and that's not what we're talking about for teetering.

The next step is considering that the highest teetering speed (not position) is where you get the biggest AOA effect. The advancing blade starts rising from a low point at 6, is climbing fastest at 3, and hits its peak height at 12. Speed makes the difference, not height , because speed makes the air move against your palm or the back of your hand. Just holding your hand high or low does nothing. When the blades are at 3 and 9, they are at the same height but moving quickly in opposite directions. That's when you get the biggest AOA difference to compensate for the biggest dissymmetry in airspeed.
 
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Be careful not to confuse flapping action (rising or dropping of the blade, or teetering in the case of a simple two blade rotor) with pitching action (tilting that chord line up with respect to the relative wind).

If you hold your hand out flat to your side at shoulder height and swing it down, you will feel air against your palm, essentially a huge 90 degree angle of attack. Pushing down increases AOA. Conversely, if you raise that same hand, you have an extreme negative angle of attack as the air strikes the back of your hand. Pushing up decreases AOA.

Pitching action would require tilting your hand thumb up or thumb down, and that's not what we're talking about for teetering.

The next step is considering that the highest teetering speed (not position) is where you get the biggest AOA effect. The advancing blade starts rising from a low point at 6, is climbing fastest at 3, and hits its peak height at 12. Speed makes the difference, not height , because speed makes the air move against your palm or the back of your hand. Just holding your hand high or low does nothing. When the blades are at 3 and 9, they are at the same height but moving quickly in opposite directions. That's when you get the biggest AOA difference to compensate for the biggest dissymmetry in airspeed.
I very much like your example of swinging your arm up and down. I don't know if I have heard it explained that way before. It is a great way for a student to gain insight into why the AOA changes as the rotor flaps.
 
Thanks for your help to all. Will play with the info and see if that helps. Still see the advancing blade rising creating a larger AOA due to relative wind. My sailing days perhaps interfering?
 
True but there is useful info there and in the new helicopter flying handbook also. We should seek out good clear info anywhere. The concepts are the same. The application and implementation is different but there is no difference at the basic level. In a way there is no difference between an airplane wing and a rotor. One just has to open their mind and start to move the airplane wing in a circle and imagine what happens.

From Helicopter Flying Handbook says the same thing I said above about relative wind on teetering up blade and teetering down blade:

"Advancing Blade
As the relative wind speed of the advancing blade increases, the blade gains lift and begins to flap up. It reaches its maximum upflap velocity at the 3 o’clock position, where the wind velocity is the greatest. This upflap creates a downward flow of air and has the same effect as increasing the induced flow velocity by imposing a downward vertical velocity vector to the relative wind which decreases the AOA

Retreating Blade
As relative wind speed of the retreating blade decreases, the blade loses lift and begins to flap down. It reaches its maximum downflap velocity at the 9 o’clock position, where wind velocity is the least. This downflap creates an upward flow of air and has the same effect as decreasing the induced flow velocity by imposing an upward velocity vertical vector to the relative wind which increases the AOA."
No argument there, I was simply saying that they ought to reincorporate all the relevant information into a Gyroplane Flying Handbook, leaving out the stuff that only applies to helicopters, instead of leaving students to sort of guess with the "current" Rotorocraft handbook.
Twenty-two years is WAY too long to just let things slide, especially when we know many knowledge test questions are based on it.
The way it stands now, gyro students might be expected to just ignore everything in the old helicopter section, and even if they don't, they'll have to sift through and figure out for themselves (or be told by someone else) what is actually useful (e.g. advancing vs retreating blades), and what isn't (e.g. anti-torque systems, vortex ring state, ground resonance).
(I actually had a question on ground resonance on my gyroplane knowledge test (PRG)!)
 
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No argument there, I was simply saying that they ought to reincorporate all the relevant information into a Gyroplane Flying Handbook, leaving out the stuff that only applies to helicopters, instead of leaving students to sort of guess with the "current" Rotorocraft handbook.
The way it stands now, gyro students might be expected to just ignore everything in the old helicopter section, and even if they don't, they'll have to sift through and figure out for themselves (or be told by someone else) what is actually useful (e.g. advancing vs retreating blades), and what isn't (e.g. anti-torque systems, vortex ring state, ground resonance).
(I actually had a question on ground resonance on my gyroplane Knowledge Test!)

I hear that there is a new Gyroplane Flying Handbook in the works. I do hope that they take the applicable sections from Helicopter and incorporate that knowledge in. Some of this is more clear in Heli section than gyro section I think. The CFI should be guiding this study for the student and assigning sections to read before ground discussions on the same subject next time. At least we hope.
 
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