# Effect of prerotation on takeoff distance

#### All_In

##### Gold Supporter
PS: I will read the paper as soon as I get a little more time. Maybe that will answer my questions.

#### XXavier

##### Member
Still confused my friend!! Only with a flat bar is all of the lift created by the downdraft! Same with a flat HS it is all Newtonian with no benefit from a camber it works but not as efficient. Bernoulli lift creates a low-pressure area and is more like being sucked up in a tornado.
IF IT IS CAMBERED IT IS NOT ALL DOWNDRAFT LIFT!!!! That my only real problem with your drawing. It needs to show both forces of lift and the quantity would be needed to know how much downdraft there is for any particular blade.

My training in Physic was back in 1970. However, the math I did back then showed that you had to know the cord/camber of a wing and use the Bernoulli's math to add to the Newtonian math and that was the combined lift. It was not Bernoulli vs Newtonian it both.

With a flat bar, the Bernoulli portion of the lift was zero and without adding the Newtonian lift to it you will not come up with the correct answer and vice versa!

Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering the conservation of energy. So both could come up with the correct answer but you have to have both in the calculations somewhere.

Well, in my opinion, the vertical force that allows any heavier-than-air aircraft to fly is the reaction generated by the acceleration of a mass of gas downwards.
You don't need a wing –cambered or not–. Even a washing machine, moving through the air with a convenient orientation and speed, will have enough lift to fly... And all that lift will be due to the downdraft generated by its motion... Of course, that of the washing machine is an extreme example, but it's anyway valid and true...

#### All_In

##### Gold Supporter
Well, in my opinion, the vertical force that allows any heavier-than-air aircraft to fly is the reaction generated by the acceleration of a mass of gas downwards.
You don't need a wing –cambered or not–. Even a washing machine, moving through the air with a convenient orientation and speed, will have enough lift to fly... And all that lift will be due to the downdraft generated by its motion... Of course, that of the washing machine is an extreme example, but it's anyway valid and true...
Only with a flat bar is all of the lift created by the downdraft! Same with a flat HS it is all Newtonian with no benefit from a camber it works but not as efficient. Bernoulli lift creates a low-pressure area and is more like being sucked up in a tornado.
IF IT IS CAMBERED IT IS NOT ALL DOWNDRAFT LIFT!!!!

With a flat bar, the Bernoulli portion of the lift was zero
and without adding the Newtonian downdraft lift (dragging it through the air with power) to it you will not come up with the correct answer and vice versa!

#### All_In

##### Gold Supporter
Sailplanes want all Bernoulli lift if they could because Newtonian is that portion that creates all the drag or I think of it like the super high-pressure area or a hose squirting water on the wing and it is deflected down. The higher the AOA the more it is deflected down and the more drag.
You are ignoring the low-pressure lift areas completely in the drawing and your statement that it's all downdraft is not correct. Unless it is a flat HS/airfoil.

Last edited:

#### Greg Vos

##### Active Member
Sorry to hear that you have fallen into this conceptual trap.

Any airfoil that supports weight of an aircraft will do it by generating a downdraft.
Gliders/sailplanes (with no engines at all, nothing but gravity) make downdraft.
A ram-air chute on a paraglider is a fixed wing (look at the cross-section), and it makes downdraft.
Helicopters make downdraft.
Airplanes make downdraft.
Inverted airplanes make downdraft.
Gyroplanes make downdraft.
Think in the simplest possible terms: you push air down with the wing, and Mr. Newton's equal and opposite reaction is to push the wing up. They all work that way, whether fixed or rotary.

Power is irrelevant. All that will do is determine whether you can add energy to replace that lost to drag, to hold altitude or climb, not produce an upward force to support the weight of the craft. The wing, of whatever description, pushes air down to hold the aircraft up.

What magical force could create lift otherwise?
You have scribed a few entertaining pieces on this forum 😂 I think if we were in a room with a white board and had an opertunity I could explain it a bit differently ....so in your opinion a guy falling with a parachute creates downdraft? Or the guy farting next to you is creating thrust 😉😳

your comment gyroplanes make down draft is like saying a car passing at high speed is making thrust in the opposite direction?
NO
I think it’s best we agree to disagree

Last edited:

#### Greg Vos

##### Active Member
I can't resist the temptation here, so forgive my transgression - -

with all the wonderful stringent requirements that South Africa has for pilots, so much more demanding than the U.S. regulations, it's a wonder that you don't have to know what makes them work to get a gyroplane license.
With respect sir I can’t believe you have a licence 😳 with your understanding of how things fly ...they are preparing your Darwin Award 🤣
Does the little Ferris wheel thing you buy for your kid that He puts it out the car window on the way home from the fair make down draft? ....if it was at the 20 Degree angle that’s been thrown around ?
send me your full details I will CNC cut the plaque for your award on a nice price of billet and send it to you, no cost, 😁
Does a sycamore seed make down draft?
all these comments made in the scope of the issue that the gyroplane will have sufficient downdraft to lift when prerotating and I’m saying NO unless you can alter the pitch (collective)
now I’m TIBA nothing more for me to learn on this one 🍺🍺😜

Last edited:

#### WaspAir

##### Supreme Allied Gyro CFI
....so in your opinion a guy falling with a parachute creates downdraft?
If it's a ram air canopy chute (parafoil), yes. They actually form a wing, and that wing glides (at least one manufacturer claims 6:1, better than many gyros, but 3:1 or better is common). The ram air inflates the cells into an easily recognizable airfoil shape when viewed in cross-section. Under one of these canopies, you're not just falling, but flying like a low-performance glider.

A round canopy emergency chute is another matter, which is why you never see them on paraglider or powered parachute rigs.

#### All_In

##### Gold Supporter
Greg, you are right their use of downdraft is very confusing if you are thinking of the downdraft they are describing being like it's a helicopter. When I 1st came here it took me a while to understand that they are talking about the normal aerodynamics of sycamore seed DEFLECTING the air creating the resistance/drag to slow it's decent. Like a kite. It's not downdraft it the DEFLECTION of any plane being pushed or dragged through the air. Next time you wash your blades take a hose and squirt a high stream of water like it the relative wind on the bottom of the blades and you will see how it is deflecting down and at what angle. Increase the angle of attack and like a protractor, the stream will deflect down toward the ground even more.
When they say downdraft think deflection and in a parachute ram air deflection, it is for very little distance, unlike the water example. Ask how far it is deflected and at what angle to the ground if you wish to get on the same page.

Last edited:

#### Jean Claude

##### Junior Member
John,
Your work table is flat and smooth, and a cylindrical pencil placed on it rolls at the slightest inclination.
Looking now under the microscope, is appears actually bumped and with an indefinable horizontality.
Both visions are correct. But depending on the problem to be solved, the rolling of the pencil or the rubbing of the sheet of paper, they do not have the same relevance.

Greg
By aerodynamicist definition, "lift" is in the direction perpendicular to the direction of movement. Whereas "drag" is parallel.
So, a tennis ball has no lift during its "flight". Just a drag
An infantryman parachutist slows down his fall by drag, not by lift.

#### All_In

##### Gold Supporter
No one here is saying or thinking that a gyro can deflect the relative wind straight down at the ground like a helicopter, are they?

#### Greg Vos

##### Active Member
Greg, you are right their use of downdraft is very confusing if you are thinking of the downdraft they are describing being like it's a helicopter. When I 1st came here it took me a while to understand that they are talking about the normal aerodynamics of sycamore seed DEFLECTING the air creating the resistance/drag to slow it's decent. Like a kite. It's not downdraft it the DEFLECTION of any plane being pushed or dragged through the air. Next time you wash your blades take a hose and squirt a high stream of water like it the relative wind on the bottom of the blades and you will see how it is deflecting down and at what angle. Increase the angle of attack and like a protractor, the stream will deflect down toward the ground even more.
When they say downdraft think deflection and in a parachute ram air deflection, it is for very little distance, unlike the water example. Ask how far it is deflected and at what angle to the ground if you wish to get on the same page.
Greg, you are right their use of downdraft is very confusing if you are thinking of the downdraft they are describing being like it's a helicopter. When I 1st came here it took me a while to understand that they are talking about the normal aerodynamics of sycamore seed DEFLECTING the air creating the resistance/drag to slow it's decent. Like a kite. It's not downdraft it the DEFLECTION of any plane being pushed or dragged through the air. Next time you wash your blades take a hose and squirt a high stream of water like it the relative wind on the bottom of the blades and you will see how it is deflecting down and at what angle. Increase the angle of attack and like a protractor, the stream will deflect down toward the ground even more.
When they say downdraft think deflection and in a parachute ram air deflection, it is for very little distance, unlike the water example. Ask how far it is deflected and at what angle to the ground if you wish to get on the same page.
All In
, your on the money...👍

#### Greg Vos

##### Active Member
No one here is saying or thinking that a gyro can deflect the relative wind straight down at the ground like a helicopter, are they?
they are saying just this! and this is what is confusing me?

They are confusing pitch with angle of incidence? this thread is a huge debate and it seems im the only one who is saying a Gyro cannot be like a helicopter ever, but this is being debated because they are using hypothetical formulas with unrealistic rotor speed to suggest that the (angle of incidence) they use term built in Pitch....

Now I think we are all really on the same page but we use different terminology and that many of the post are lost in translation? one thing I think? it has us all going through past literature, Im resolute in what I have said and even Wasp (who I respect his comments ) is on record 'seconding' that a gyro under pre rotation is in fact helicoptering I have never heard of the term? and )Im current with My helicopter licence ). saying a Gyro produces downdraft in the traditional manner we as rotor-wing (helicopter pilots) understand is very confusing? and inaccurate.

There can be little doubt that some of the highly complicated posts are well thought out and are scribed by very clever people, the same people who do very little flying?

But
, the truth is so simple that every pilot can grasp it in 5 minutes.
1. The wing is a device for deflecting air down - it uses Newton's third law of equal and opposite forces to move the wing up.
2. In deflecting the air down a pressure differential is created above and below the wing - this pushes the wing up.
If you don't agree, think about this: A helicopter produces no lift at all until its wings start pushing air down. Helicopter blades too have an airfoil section! now think its sitting on the ground rotors spinning at flying speed with no collective pitch ...there certainly is no DOWNDRAFT or downdraft worth discussing?

IT'S DEAD ACCURATE, IT'S THAT SIMPLE.

It's far more simple, and accurate, than the wishy-washy half truths banded about by BernoulliistsJ

As a flight Instructor this simple analogy and description in my opinion is sufficient for a GPL? YES we can have PHD"s and mathematicians who can over complicate things?
Look at the thread that was written by the PHD regarding ground effect?? and he was authoritative that it cannot exist in a Gyro-plane? yet how many pilots and instructors swear we have experienced it? I rest my case, I am not smart enough to challenge JC but I do respect his knowledge and I wish I had it.

I also apologize unconditionally if I seem to offend certain people, now its time to chase down some skirt and get a cold beer!

Last edited:

#### Greg Vos

##### Active Member
John,
Your work table is flat and smooth, and a cylindrical pencil placed on it rolls at the slightest inclination.
Looking now under the microscope, is appears actually bumped and with an indefinable horizontality.
Both visions are correct. But depending on the problem to be solved, the rolling of the pencil or the rubbing of the sheet of paper, they do not have the same relevance.

Greg
By aerodynamicist definition, "lift" is in the direction perpendicular to the direction of movement. Whereas "drag" is parallel.
So, a tennis ball has no lift during its "flight". Just a drag
An infantryman parachutist slows down his fall by drag, not by lift.

A gyro flies because of the drag and thrust it does not create downdraft that keeps it aloft? IMO.. we are on opposite sides of the table here? however I am not educated enough to challenge your calculations, but for me a rotor spinning because of air coming up through the bottom is unable to produce downdraft?

#### All_In

##### Gold Supporter
Greg, I understood you and Jon were really saying the same thing in different ways.
I love math because we all get and understand the same answer. The written word not so much.

The math shows most gyroplanes with their rotor blades so high off the ground cannot compress the air to increase the laminar airflow which is what increases the lift anywhere near as much a low wing plane.
However, many experienced pilots report that negligible amount does create a ground effect in many/most gyros.

#### Vance

##### Gyroplane CFI
Staff member
JC why does the rotor spins at the same rpm regardless the gyro airspeed ? (at constant G load I mean)
In The Predator there is a slight increase in RPM as the indicated airspeed increases in straight and level flight.

There is also an increase in rotor rpm in level flight at higher altitudes.

#### Attachments

• ooo.jpg
58.3 KB · Views: 0

#### Greg Vos

##### Active Member
In The Predator there is a slight increase in RPM as the indicated airspeed increases in straight and level flight.

There is also an increase in rotor rpm in level flight at higher altitudes.
Increase in rotor speed at high altitude is normal the air is thinner I see it a lot, at the coast I see say 350 odd at the reef with field elevation of say 5500ft it will go up by at least 100 for a similar gyro and weight ( we are talking level flight)

#### Jean Claude

##### Junior Member
this thread is a huge debate and it seems im the only one who is saying a Gyro cannot be like a helicopter ever, but this is being debated because they are using hypothetical formulas with unrealistic rotor speed to suggest that the (angle of incidence) they use term built in Pitch....
My hypothesis of 540 rpm was unrealistic? the 530 rpm of the R22 was so far?

1. The wing is a device for deflecting air down
OK!
2. In deflecting the air down a pressure differential is created above and below the wing - this pushes the wing up.
Bravo!
If you don't agree, think about this: A helicopter produces no lift at all until its wings start pushing air down. Helicopter blades too have an airfoil section!
Excellent!
now think its sitting on the ground rotors spinning at flying speed with no collective pitch ...there certainly is no DOWNDRAFT or downdraft worth discussing?
Exact!

But a gyro rotor has a positive collective pitch of about 3°. Because of this, it produces a downwash during its pre-launch at the fixed point.
exactly the same way as a helicopter.
The calculations says the rotor axial thrust according to the diameter, the chord, the rpm and the collective pitch setting: 75 kg pour le rotor du ELA à 200 rpm. Not clear?

(Now, I see that Greg has become a keyboard addict!)

#### Greg Vos

##### Active Member
My hypothesis of 540 rpm was unrealistic? the 530 rpm of the R22 was so far?

OK!

Bravo!

Excellent!

Exact!

But a gyro rotor has a positive collective pitch of about 3°. Because of this, it produces a downwash during its pre-launch at the fixed point.
exactly the same way as a helicopter.
The calculations says the rotor axial thrust according to the diameter, the chord, the rpm and the collective pitch setting: 75 kg pour le rotor du ELA à 200 rpm. Not clear?

(Now, I see that Greg has become a keyboard addict!)
A gyro has no collective pitch it has built in incidence angle not to be confused with pitch or with AoA .....you re making me a keyboard addict....now if we scroll back a few pages the tone is changing .... because we agree that most gyros ( I never said this) pre rotate with stick fully fwd (AG as example) and then the blade has fwd angle of attack of say -2 degrees

remember stick fully fwd normally -2, stick Centre +7, stick aft + 19-20 ...😁

#### XXavier

##### Member
In The Predator there is a slight increase in RPM as the indicated airspeed increases in straight and level flight.

There is also an increase in rotor rpm in level flight at higher altitudes.

Perhaps the slight increase in RRPMs with the gyro's airspeed is due to the fact that the disks gets flatter when the airspeed goes up and with a flat disk, there may be more aerodynamic interference between the blades...

The increase in RRPMs with altitude is exactly the same that happens with a FW, altitude, and TAS. In order to keep lift constant, if you fly higher, the ir density drops, and the TAS (in a FW) or the RRPMs (in a gyro's rotor) have to increase in order to keep constant the wing lift (in a FW) or the thrust (in a gyro's rotor)

#### XXavier

##### Member
A gyro has no collective pitch it has built in incidence angle not to be confused with pitch or with AoA .....you re making me a keyboard addict....now if we scroll back a few pages the tone is changing .... because we agree that most gyros ( I never said this) pre rotate with stick fully fwd (AG as example) and then the blade has fwd angle of attack of say -2 degrees

remember stick fully fwd normally -2, stick Centre +7, stick aft + 19-20 ...😁

The AoA of a gyro's blade, at any station, and at any blade azimuth, with the gyro in pre-rotation, (i.e. stopped, and with zero wind) is constant, and independent of the stick's position...

Replies
9
Views
847
Replies
2
Views
261
Replies
8
Views
561
Replies
3
Views
496