RRPM increase in the Auto flare...
Join Date: Nov 2004
Location: USA & UK
Posts: 89
Likes: 0
Received 0 Likes
on
0 Posts
The effect of the flare in autorotation reduces the forward speed, as it does in powered flight, by tilting the Total Rotor Thrust away from the direction of flight. This gives a horizontal component of total rotor thrust, rearwards, to decelerate the helicopter. The other effect of rearward tilt of the rotor disc in the flare is to cause the inflow angle to increase. This gives an increase in the angle of attack, which produces an increase in total rotor thrust. The vertical component of this increased Total Rotor Thrust will, during the flare, increase to a value greater than the weight, and consequently the rate of descent will decrease. The increase in Total Rotor Thrust will increase the coning angle and Coriolis effect will cause an increase in R.R.P.M. Also the increased inflow angle causes the autorotative section of the blade to move outboard, resulting in a substantial increase in Rotor R.P.M. It will be seen that the larger the flare, that is the greater the change of disc attitude, the greater will be the RRPM increase, rate of descent decrease and the more rapid the reduction in forward speed.
How did that sound?
How did that sound?
Join Date: Jan 2003
Location: Abz
Posts: 38
Likes: 0
Received 0 Likes
on
0 Posts
IMHO the coriolis effect caused by the increased coning angle is an insignificantly small component of the rrpm rise.
The main one is the increased angle of attack (resulting from increased air flow up through the disc). The angle between the relative airflow and the total reaction (total rotor thrust if you like) remains roughly constant for a given set of conditions - it represents the lift:drag ratio of the aerofoil. So if the angle of the relative airflow increases, this tilts the total reaction further forward (ie in the direction of blade travel) and this pulls the rotor blade along harder - ie the rrpm increases.
The converse is that in the hover, you are flying in a descending column of air - ie air is going down through the disc and that tilts the total reaction backwards, tending to reduce rrpm unless you have lots of power on!
EC
The main one is the increased angle of attack (resulting from increased air flow up through the disc). The angle between the relative airflow and the total reaction (total rotor thrust if you like) remains roughly constant for a given set of conditions - it represents the lift:drag ratio of the aerofoil. So if the angle of the relative airflow increases, this tilts the total reaction further forward (ie in the direction of blade travel) and this pulls the rotor blade along harder - ie the rrpm increases.
The converse is that in the hover, you are flying in a descending column of air - ie air is going down through the disc and that tilts the total reaction backwards, tending to reduce rrpm unless you have lots of power on!
EC
Join Date: Nov 2003
Posts: 185
Likes: 0
Received 0 Likes
on
0 Posts
For what reasons does the RRPM increase during the auto flare ?
How about...
Student inadvertantly rolling on throttle and *raising collective!!!! Arghhh!!!!
Particularly in R22, this can be very problematic. The correlator, govenor, increased disc loading (Conservation of angular momemtum or Coriolis - - whatever you call it) and low inertia can all work at the same time to bring up that RRPM, and before you can say,"Jack Robinson!" you have an overspeed to explain to maintainance.
I'll admit, it got me once some time ago, with a commercial student who had just joined to continue training with us/me. What it boiled down to was a difference in my timing and technique to what he'd already been taught. Very frustrating...but at the end of the day, as PIC (and instructor) my fault.
Lesson: Guard the collective and throttle like a hawk until you are sure you are familiar with a new student. (Even one with hours of experience.)
Hope this helps someone out there!
cl12pv2s
*Before anyone tries to flame me for this...(Of course I know raising would normally reduce RRPM)...if raising of the collective from flat pitch (full down collective) the correlator will raise RRPM faster than the increased blade pitch will have any effect in reduceing it. (This is demonstrated in the initial pickup and is the reason for the governor to be on then.)
Last edited by cl12pv2s; 29th Oct 2005 at 16:11.
Join Date: Jul 2005
Location: North
Posts: 124
Likes: 0
Received 0 Likes
on
0 Posts
If during the flare the RRPM increases and does so at a rate that the uppper limit is judged likely to be exceeded then a short sharp correction [control] by application of collective can be used to prevent the upper limit from being exceeded. However by carrying out this corrective action and following removal of the collective correction, the resultant RRPM will settle at a lower value than it was at prior to the flare being initiated. Lower RRPM is not desireable so how about use of secondry effect of cyclic control [push forward to achieve a higher ROD and fWD speed] ? Whaddayathink? Of course this is only a viable option at altitude.
Join Date: Feb 2005
Location: South Africa
Posts: 111
Likes: 0
Received 0 Likes
on
0 Posts
Energy
Feeble attempt at scientific explanation, but potential energy (height) is converted to kinetic energy (falling outta sky, aka autorotating), and then back into potential energy in the rotor blades during the flare.
Its all relative...
Its all relative...
Join Date: Jan 2003
Location: Abz
Posts: 38
Likes: 0
Received 0 Likes
on
0 Posts
R1T and SchmitF
Give me a clue what it is you disagree with? Or to put it another way, why not contribute something more interesting than a snipe?
996 - sure, moving the cyclic forward reduces the disc loading and the air flowing up through the disc, so the rrpm will go down but only so long as the reduced g is held - so its only a short term solution and the longer term solution is to increase collective pitch.
EC
Give me a clue what it is you disagree with? Or to put it another way, why not contribute something more interesting than a snipe?
996 - sure, moving the cyclic forward reduces the disc loading and the air flowing up through the disc, so the rrpm will go down but only so long as the reduced g is held - so its only a short term solution and the longer term solution is to increase collective pitch.
EC
Lu, where are you mate? this must be a good giggle for you up there, and it s threads like these that we really miss you.
I can hear "centrifugal force" from here1
Prune fan loves this one too!
Try a search on this one fellas.
I can hear "centrifugal force" from here1
Prune fan loves this one too!
Try a search on this one fellas.
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
Actually, the rpm rises because the rotor thinks the weight of the helicopter went up - the positive G that builds because of the increased thrust is exactly the same as if the aircraft suddenly got heavier. If you pull 1.2 G as you flare, and you do, that is as if you added 20% gross weight, as far as the rotor is concerned. This means the trimmed rotor rpm must be higher, too (rpm goes up as weight goes up, for a given collective pitch position).
Another way to look at the same effect is to realize that the inflow velocity is raised considerably. It rises because we force the rotor to eat more air, which propels the system to higher rpm's.
Another way to look at the same effect is to realize that the inflow velocity is raised considerably. It rises because we force the rotor to eat more air, which propels the system to higher rpm's.
The gospel according to Mike Smith, Heliair. .
Oh, and if you are an instructor, listen to cl12pv2s and watch for those students rolling on the throttle with their 'death grip'. .
Oh, and if you are an instructor, listen to cl12pv2s and watch for those students rolling on the throttle with their 'death grip'. .
Last edited by Johe02; 31st Oct 2005 at 16:27.
Join Date: Oct 2005
Location: Button Moon
Posts: 36
Likes: 0
Received 0 Likes
on
0 Posts
Why not just read priciples of Helicopter flight. Great book which goes into quite a bit of detail. Excellent book for instructors and commercial students. Maybe a little to heavy for primary guys.
I used to explain it in terms of enery management to students to keep it simple. I do agree with Eurochopper and Mike Smith in relation to the airflow. However I feel it is easier to explain to new students in energy terms.
i.e. ; Your coming down in your glide with lots of airspeed and altitude stored. When you get to the flare you start to bleed off that airspeed (energy), and that rate of decent (energy) this energy has to go somewhere it doesn't just disapear. Therefore it goes towards the RPM (causing the RPM rise as it now has more energy than it had before). Blade coning really isn't that large a factor in RPM rise.
Now sure this isn't a scientific explaination with lots of vector and airflow diagrams. However in layman terms it is why the RPM rises in a flare and it is far easier to grasp when you are actually doing the auto. I have never flared and thought of a vector diagram, however maybe I'm abnormal. I'm always thinking about how much energy I have left to use to keep me unbroken.
I used to explain it in terms of enery management to students to keep it simple. I do agree with Eurochopper and Mike Smith in relation to the airflow. However I feel it is easier to explain to new students in energy terms.
i.e. ; Your coming down in your glide with lots of airspeed and altitude stored. When you get to the flare you start to bleed off that airspeed (energy), and that rate of decent (energy) this energy has to go somewhere it doesn't just disapear. Therefore it goes towards the RPM (causing the RPM rise as it now has more energy than it had before). Blade coning really isn't that large a factor in RPM rise.
Now sure this isn't a scientific explaination with lots of vector and airflow diagrams. However in layman terms it is why the RPM rises in a flare and it is far easier to grasp when you are actually doing the auto. I have never flared and thought of a vector diagram, however maybe I'm abnormal. I'm always thinking about how much energy I have left to use to keep me unbroken.