Effect of prerotation on takeoff distance

JC why does the rotor spins at the same rpm regardless the gyro airspeed ? (at constant G load I mean)
As the weight lifted is the same whatever the forward speed, so is the lift of the blades. Hence a constant rotation speed gived the same airspeed to the blades.
But as Vance said, the rotor rpm increases a bit when the forward speed is increased.
This is due to the increase of the stalled zone (see a previous post): a little more rpm is needed to compensate for this loss of cL.
 
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Jean with no sarcasm intent, the calculations of a gyro blade spinning at 500+ Rrpm are pure fiction, yes in very tight turn I have done it but then other factors are at play, the blades are conning up they are reducing the surface area and in reality diameter for that moment is smaller All contributing to momentary high Rrpm. But to think you could get it there with a pre spinner and it will produce suffiecient downdraft to lift off simply when pulling the stick back ...
I did err on the R22 because I have not had exposure to one for many years and most of the turbine machines we see around 300-308 Rrpm, so sir you can have that one.
Now the rotor head assembly of a helicopter is very different to that if gyro, The gyro has cheek plate and or a hub bar The manufacturer warns in the PoH to avoid high Rrpm’s and specific caution when approaching in the 500’s
Helicopter has thrust bearings and spherical bearings designed for higher torsion loads
this is why I say your calculations despite impressive are of little use in practical terms.


This thread started out to ascertain if one could reduce distance on take of trajectory By pre rotation higher, I was from the outset sure of the answer and went in to prove it is of of no value to wring the neck of the pre rotator mechanism after 15 pages of exhaustive posts it was agreed ....now your confused with angle of incidence and collective pitch ....possibly you could send me your details and I could bill you for ground school classes 😜
 
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...
You know what a consulting engineer is?

a consulting engineer is a person you give your watch to, and he tells you the time .... read what you have written above and go and understand it ?AoA will change during rotation angle of in incidence will not A pilot can control AoA he cannot control angle of incidence
 
You know what a consulting engineer is?

a consulting engineer is a person you give your watch to, and he tells you the time .... read what you have written above and go and understand it ?AoA will change during rotation angle of in incidence will not A pilot can control AoA he cannot control angle of incidence

The angle of attack of a blade is a function of the inflow velocity Voo, the linear velocity Omega · r and the pitch theta. If Voo is zero (with no wind and the gyro stopped) the angle Alphaa is zero, and the AoA is exactly the same as the pitch. And the pitch is constant.

Captura de pantalla 2020-09-10 a las 23.06.49.png
 
Greg,
I never said that the rotor of an ELA spins in flight at more than 500 rpm. My message expressly indicated that my calculation applied to the Robinson R22.

I never thought a pre-launcher could reach. I said, like Xavier, that it would take more than 500 rpm for the rotor of an ELA to lift 450 kg.
 
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Yes Jm, since then the air flow through its disc increases with the wind speed, the rpm will becomes more speed, and the axial thrust rotor will also be increased.
 
Greg, I will give this one last stab to attempt to straighten out the misunderstandings.

1) Collective pitch on a typical two-blade teetering rotor is fixed, and positive, not zero. The position of the stick is irrelevant. This has nothing to do with incidence. It is a question only of the construction of the blades. We are not talking about the angle of the disc with respect to the ground or with respect to the direction of motion (if any) of the airframe, but the angle of the blade chord with respect to the plane of rotation of the rotor, and that is fixed.
2) If the aircraft is sitting still, with no wind (for simplicity), and prerotation begins, that small positive collective pitch will cause air to be drawn "down" through the disc. That is what others have referred to as "helicoptering" (I attempted to explain their meaning to you to help communication; it's not a term that I typically use). At that time, with air drawn from above the disc to below the disc, the flow is like a that in a helicopter with positive collective pitch. But the gyro is not yet flying.
3) Nobody expects back stick / tilting the rotor aft to produce a takeoff while the rotor is behaving as in (2), and it's confusing to hear your references to that. Stick position doesn't matter at all.
4) In that mode, lift will be produced. It will never be enough to levitate the aircraft. To get to full flight rpm and produce lift sufficient to overcome gravity would require so much applied torque that the friction of the tires on the ground (resisting yawing and giving effect to the wheel brakes' resistance to prop thrust) will be overcome before that condition is reached, and the aircraft will skitter around uncontrolled before any liftoff could be achieved. Again, stick position is irrelevant. There is no tail rotor to overcome torque, so you simply can't get into that condition.
5) In the process of performing a takeoff, beginning with prerotation, a time will be reached at which autorotation begins, and the airflow will be from below the disc to above the disc. Much of the original discussion deals with the timing of this transition from one airlfow mode to the other, and whether reaching a higher rpm in prerotation is beneficial, or whether that extra rpm will be inevitably lost and provide no advantage.
6) In autorotative level flight, a gyroplane rotor produces a down wash, as does the wing on a Cessna 172. Pushing air down pushes the aircraft up.
7) Nobody is asserting that a gyroplane in autorotative level flight produces a wash comparable in intensity (or pattern) to a hovering helicopter. Even a helicopter in forward flight doesn't do that, which is why air taxi is less disturbing to things on the ground than hover taxi. Get your helicopter going above ETL speeds and the disc is a wing, displacing air downwards.
8) Heavier-than-air aircraft of comparable weight are going to be imparting comparable momentum into downward displacing air no matter what kind of wing you have. The major difference between a helicopter rotor and a gyroplane rotor is the direction of airflow with respect to the tilted discs (from the topside to the bottom side on one, and the reverse on the other). Helicopters fly with the disc tilted forward and gyroplanes with the disc tilted back, and "above" and "below" are pertinent to the position of the tilted disc, not the direction of gravity.
 
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Wasp I don’t think anyone disagree with what you say, however I don’t use the term collective in discussions with Gyro and I refer to the angle of incidence as this what the designers build into there rotor systems and not the fixed collective you refer too.

A pilot cannot alter the angle of incidence he can however control the angle of attack. We agree on this?


Im willing to bet that I will now be challenged on this,
 
it has always been difficult for me to understand that the rotor RPM is a function of the G load ... I admit it of course, and I see it in flight.
I find that the rotor dynamics are counter-intuitive like many things in the areas of technology .
every time I think that I have understood something I realise that in fact no !
it is notably difficult for me to understand things dynamically ... anyway We are lucky to have people like you , chuck , jean fourcade, people we are sure that they don't speak with the intention to have the last word in a debate but only for science .... It took me ages to understand the very depth of my ignorance ... I knew I was ignorant , but I did not know how ignorant I was ... it is very important to realise this for an home builder
thanks again mate , and sorry again for the past disputes really


Since I joined this forum in 2007, I've been learning a lot from the best heads here, such as Jean Claude, Beaty, Fourcade, Doug Riley, WaspAir, kolibri282, and many more... The most productive way of learning is, I think, to enter the discussions, even with some boldness –but always in a courteous and friendly manner– in the hope that the things that you may have wrong will be corrected by those who really know. Stubbornness and mental rigidity should be avoided at all times...
 
Wasp I don’t think anyone disagree with what you say, however I don’t use the term collective in discussions with Gyro and I refer to the angle of incidence as this what the designers build into there rotor systems and not the fixed collective you refer too.

A pilot cannot alter the angle of incidence he can however control the angle of attack. We agree on this?


Im willing to bet that I will now be challenged on this,
With respect, I think you have temporarily forgotten the true meaning of "collective"
Weather the blade angle of incidence is movable or fixed, collective simply refers to the two (or more blades) being at the same AOI as the other (others).

wolfy
 
I agree with you it is not easy to choose who you can believe ...
one day I was doing some refresh dual with and instructor I did not know ... I was vertical 1000 ft agl at 120 km/h level flight and the guy pulled the throttle saying ok now land the gyro ( we were above the aerodrome) , then I pushed the stick to keep my airspeed and did an approach at best glidding airspeed and landed on the heli H ..
the guy told me : " you hardly killed us it is a miracle we survived, you should have pulled the stick to charge the rotor before gently push it to avoid the air to go through the rotor from the top stopping it ... only the heaviness of the magni blades saved us with an autogyro rotor you we would have died..."

Not being there, it's difficult to judge, but I perceive a whiff of exaggeration on the part of the instructor. Of course, a sudden, strong pitch-down may unload a rotor, but it has to be very aggressive for that.

On the other hand, a prudent pitch-down in order to keep airspeed and start maneuvering for landing is perfectly safe...
 
I agree with you it is not easy to choose who you can believe ...
one day I was doing some refresh dual with and instructor I did not know ... I was vertical 1000 ft agl at 120 km/h level flight and the guy pulled the throttle saying ok now land the gyro ( we were above the aerodrome) , then I pushed the stick to keep my airspeed and did an approach at best glidding airspeed and landed on the heli H ..
the guy told me : " you hardly killed us it is a miracle we survived, you should have pulled the stick to charge the rotor before gently push it to avoid the air to go through the rotor from the top stopping it ... only the heaviness of the magni blades saved us with an autogyro rotor you we would have died..."
only the heaviness of the magni blades saved us with an autogyro rotor you we would have died..."

your instructor said this? No man please tell us you are just adding some light humor to us all on a Friday ...:giggle: I assume you have now changed instructors?
 
no, what is unbleivable, is that the guy did not think that by pushing the stick the g load could pass under 1 and then reduce the rrpm, he tought that the air could come from up to down and stop the rotor !
People have suggested that same fear on this forum in the past. Some seemed to think that if you fly really fast, with a very low disc angle, you are at risk of pushing the stick just a little too far and suddenly reversing the airflow with catastrophic effects. Of course, this relies on the belief that you can magically keep the fuselage going straight ahead in level flight at extremely high speed while somehow you have enough leverage on the cyclic stick to force the rotor disc to move suddenly below horizontal. It reminds me of the dragons at the edges of ancient maps, only in this case, they are on the graph of the flight envelope.
 
A pilot cannot alter the angle of incidence he can however control the angle of attack. We agree on this?
I have my own terminology preferences, and I am very cautious about saying THE angle of attack for a rotorcraft without qualification. What most people mean by it, I infer, is the apparent disc angle to the aircraft's direction of motion, as if it were a big round fixed wing. Every spanwise station on the blades will have its own angle of attack, because of the different rotational velocity there (complicated by any twist/wash), and it is changing constantly through the rotational cycle, so there are many angles of attack to consider if you look on a smaller scale. Certainly, one can change the disc angle, tilting the tip path plane, and it is often convenient to speak in such broad terms, but it can be misleading when it comes to things such as stall to speak of "the" angle of attack without specifying where on the disc. Retreating blade stall, for example, happens at relatively low disc angles in a gyro, in contrast to the fixed wing expectation.

As an aside, on the gyros I prefer, one has swashplate control and collective pitch is variable, but that isn't the design under discussion on this thread.
 
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 ...😁
- Angle of Attack says its definition very clearly, but you still have to specify the reference of the object under consideration:
If it is the disc, the most convenient reference is the plane traveled by the blade tips. Contrary to what the philistines think, the plane perpendicular to the hub does not coincide, in flight, with the plane of the tips.
ForIf it is the blade sections, the use is not to refer to the line leading edge - trailing edge which is too difficult in practice. The most used by aerodynamicists is the direction of zero lift. The angle is then called "Aerodynamic pich".
Our gyrocopters generally have an "aerodynamic pitch" of +3° giving an axial thrust from the pre-launch .
 
I have my own terminology preferences, and I am very cautious about saying THE angle of attack for a rotorcraft without qualification. What most people mean by it, I infer, is the apparent disc angle to the aircraft's direction of motion, as if it were a big round fixed wing. Every spanwise station on the blades will have its own angle of attack, because of the different rotational velocity there (complicated by any twist/wash), and it is changing constantly through the rotational cycle, so there are many angles of attack to consider if you look on a smaller scale. Certainly, one can change the disc angle, tilting the tip path plane, and it is often convenient to speak in such broad terms, but it can be misleading when it comes to things such as stall to speak of "the" angle of attack without specifying where on the disc. Retreating blade stall, for example, happens at relatively low disc angles in a gyro, in contrast to the fixed wing expectation.

As an aside, on the gyros I prefer, one has swashplate control and collective pitch is variable, but that isn't the design under discussion on this thread.
You mean for every degree of the 360 degree ....yes fundamental stuff in a heli training syllabus ..... wagtendonk explained it so simplistically, but Im sure this thread would confuse him ....

gyro theory and flight like it’s mechanics is so much more simple, ....KISS was not only a great rock band with interesting make up, it’s a part if my every day life.......I refuse to overcomplicate stuff.

Now back to that 0000.2 % downwash that has our knickers in a knot for the first few seconds of prerotator intervention......cos there sure as hell ain’t no downdraft while in autorotation 😉

🍺
 
there sure as hell ain’t no downdraft while in autorotation 😉

🍺
No air sucked from above the disc, but definitely air pushed down by the rotor like any other wing would do in level flight (hoping that we are actually in agreement at last)?
 
This is an interesting thread on the optimal prerotation RRPM for shortest departure. Our gyros can be spun up to flight RRPM (~275-300, depending on pilot weight and DA), and the below videos show take-off rolls <20 feet.

Sport Copter Vortex M912 takeoff at flight RRPM

Our 2-seat M2-915 will have similar if not better STOL performance with its superior power:weight ratio (and improved controls to more quickly/safely disengage the p/r while adding full throttle). The M2 will also have the same excellent off-road capability that the M912 has become renowned for. We're in its final assembly this week, and will be firing it up shortly! An M2 Update with photos and video is soon to follow this month, with related revisions to our website page.

Sport Copter M2
http://www.sportcopter.com/Gyroplanes/M2/tabid/217/Default.aspx
 
No air sucked from above the disc, but definitely air pushed down by the rotor like any other wing would do in level flight (hoping that we are actually in agreement at last)?
We are 🍺
 
This is an interesting thread on the optimal prerotation RRPM for shortest departure. Our gyros can be spun up to flight RRPM (~275-300, depending on pilot weight and DA), and the below videos show take-off rolls <20 feet.

Sport Copter Vortex M912 takeoff at flight RRPM

Our 2-seat M2-915 will have similar if not better STOL performance with its superior power:weight ratio (and improved controls to more quickly/safely disengage the p/r while adding full throttle). The M2 will also have the same excellent off-road capability that the M912 has become renowned for. We're in its final assembly this week, and will be firing it up shortly! An M2 Update with photos and video is soon to follow this month, with related revisions to our website page.

Sport Copter M2
http://www.sportcopter.com/Gyroplanes/M2/tabid/217/Default.aspx
I just love it ....
 
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