RPM increasing in AUTO

[brilliant206pilot]

who one knows why RPM increase in autorotation during deceleration ?can any one explain that !!!

[DS_HMMR]

probably adrenaline.

[SBuzzkill]

Conservation of angular momentum.

[akscott60]

Conservation of angular momentum.

 

Bingo.

[Trans Lift]

As you flare (decel), you increase the coning angle on the disk due to blade loading. As the coning angle increases this brings the center of mass of each blade closer to the hub causing the blades to accelerate (just like an ice-skater spins faster when they tighten up). Thats it in a nutshell. I'm sure someone can elaborate more!

Edited November 6, 2012 by Trans Lift

[500F]

also known as coriolis force...

[Trans Lift]

/\ Yup, what he said!

[Flying Pig]

1.2%€=9€%>.¥ = (6.2 x7x) = RPM increase/ decrease. come on guys... this is flight school 101! geez!

[iChris]

who one knows why RPM increase in autorotation during deceleration ?can any one explain that !!!

 

 

A few things happen when you move the cyclic aft and change the attitude of the rotor disk with relation to the relative wind.

 

1. The attitude change inclines the total resultant force vector of the rotor disk to the rear and slows forward speed; moreover, a transfer of kinetic energy, due to the reduction in air speed, is being fed into the rotors.

 

2. The angle of attack on all blades is increased because of the changing air inflow.

 

3. Total rotor lifting force is increased and the rate of descent is reduced.

 

4. The RPM also increases when the total aerodynamic force vector is lengthened.

 

5. The RPM will increase because of the lengthening of the total aerodynamic force vector, which is acting forward of the axis of rotation, causing an acceleration of the rotor blades. This was all the result of the attitude change that brought about the change in air inflow that increased the angle of attack and total aerodynamic force.

 

As you can see from the figure below, an increase in collective at this stage would also lengthen total aerodynamic force. However, it would also increase drag, by a greater degree, and lengthen the drag vector. That would tilt the total aerodynamic force vector aft, thereby decreasing rotor RPM.

 

Decreasing collective at this stage would shorten the total aerodynamic force and shorten the drag vector, by a greater degree. This would tilt the aerodynamic force vector forward, thereby increasing rotor RPM.

 

It’s all a play on the blade region stuff you learned about in autorotation force vector class. Driven, Driving, Equilibrium, Stall. The above describes how the driving region (autorotative region) controls rotor RPM by counterbalancing the drag of the other regions. The Coriolis effect doesn’t really explain the major forces driving and controlling rotor RPM or how the rotor can move from the autorotation state to the windmill brake state.

 

Edited November 11, 2012 by iChris

[Mikemv]

CFIs should be teaching this (^) in aerodynamics/autorotation classes. Randy Rowles went thru this for the FAASTeam presentation on Saturday prior to Heli Success in Las Vegas.

[nightsta1ker]

As you flare (decel), you increase the coning angle on the disk due to blade loading. As the coning angle increases this brings the center of mass of each blade closer to the hub causing the blades to accelerate (just like an ice-skater spins faster when they tighten up). Thats it in a nutshell. I'm sure someone can elaborate more!

 

Sorry to burst the bubble but coriolis force only accounts for a few extra rotations per minute. Not a very significant increase at all. In an R22 at operating RPM, a 0-5 degree increase in coning angle is only going to give you approx. 0.38% RPM change. That amounts to about 2 extra rotations per minute.

 

What iChris demonstrated above is the real meat and potatoes of increase of RPM in the flare portion of the auto.

[d10]

Because the pilot didn't pull in enough collective.

 

Also what iChris said.

[DS_HMMR]

I"m telling ya: its the helicopter getting nervous because it knows the pilot is about to do something stupid.

[Trans Lift]

Yeah his was the meat and potatoes but I didn't have the interest in going into that sort of detail. It was time for beer. Should have just said nothing I guess

Edited November 8, 2012 by Trans Lift

[nightsta1ker]

Yeah his was the meat and potatoes but I didn't have the interest in going into that sort of detail. It was time for beer. Should have just said nothing I guess

 

Made that mistake more than a few times myself

[SBuzzkill]

This definitely cleared up a misconception that I had. Time to look back over aerodynamics.

[crashed_05]

Chris took the words out of my mouth. Look up this subject in Principles of Helicopter Flight...Wagnendonk explains it pretty well, also.

 

...and now for a beer.

Edited November 9, 2012 by crashed_05

[ridethisbike]

My instrument ground instructor showed me something pretty cool regarding this last week. He had a wind tunnel app on his iPhone that showed where the low pressure was on the airfoil. When you drew the airfoil in a way that resembled autorotative flight, the low pressure moved forward to the leading edge and got stronger.

 

I have since been unable to find an android app or a free one on the internet that does such things. Granted I didn't look very hard either.

[iChris]

 

My instrument ground instructor showed me something pretty cool regarding this last week. He had a wind tunnel app on his iPhone that showed where the low pressure was on the airfoil.

 

 

 

http://youtu.be/NoSrsqsY8gA

[nightsta1ker]

That's awesome! Downloading now.

[Spirit of '69]

does anyone have a clear method of teaching the difference between left and right turn 180 autos? has anyone had students favor one side or the other?

Edited November 10, 2012 by Spirit of '69

[nightsta1ker]

It's because of gyroscopic precession, During roll into a left turn, the pilot will have to correct for a nose down tendency in order to maintain altitude. This correction is required because precession causes a nose down tendency and because the tilted disk produces less vertical lift to counteract gravity. Conversely, during a roll into a right turn, precession will cause a nose up tendency while the tilted disk will produce less vertical lift. Pilot input required to maintain altitude is significantly different during a right turn than during a left turn, because gyroscopic precession acts in opposite directions for each. This becomes obvious during 180 autos when the turn is steep and maintaining rotor RPM is in focus. I think the preference usually stems from familiarization. If a student frequently does left turns, they get comfortable with left turns. When they go to do a right turn, it throws them off because they are not prepared for the differences. Variety during training and changing it up frequently, as well as understanding and teaching the differences, will help students become comfortable doing right and left turns in the auto.

[ridethisbike]

It's because of gyroscopic precession, During roll into a left turn, the pilot will have to correct for a nose down tendency in order to maintain altitude. This correction is required because precession causes a nose down tendency and because the tilted disk produces less vertical lift to counteract gravity. Conversely, during a roll into a right turn, precession will cause a nose up tendency while the tilted disk will produce less vertical lift. Pilot input required to maintain altitude is significantly different during a right turn than during a left turn, because gyroscopic precession acts in opposite directions for each. This becomes obvious during 180 autos when the turn is steep and maintaining rotor RPM is in focus. I think the preference usually stems from familiarization. If a student frequently does left turns, they get comfortable with left turns. When they go to do a right turn, it throws them off because they are not prepared for the differences. Variety during training and changing it up frequently, as well as understanding and teaching the differences, will help students become comfortable doing right and left turns in the auto.

 

Does this also correlate to the nose pitch up when rolling OUT of the right 180? I noticed I had to always input a little forward cyclic when rolling out so my airspeed didn't drop off when rolling out of the right turn.

 

I always thought it was me unconsciously pulling aft when rolling out and having to consciously counteract that...

[iChris]

does anyone have a clear method of teaching the difference between left and right turn 180 autos? has anyone had students favor one side or the other?

 

As stated in nightsta1ker post above, the pitch-roll coupling seems to give pilots the most problems. This is especially true in the 180-degree autorotation to the left (Counterclockwise rotor). The pitch-roll coupling results from the way the rotor flaps when the pilot rolls the helicopter.

 

When the helicopter starts its roll to the left, lateral cyclic reduces blade pitch over the nose and increases blade pitch over the tail boom. This unbalances the lift forces and the rotor flaps down on the left side, causing the fuselage to follow with a slight nose-down pitch due to the initial reduction in pitch over the nose. A short time later, lift of the fore and aft blades come back into balance as the helicopter reaches a steady rate of roll.

 

When the helicopter starts its roll to the right, lateral cyclic increases blade pitch over the nose and reduces blade pitch over the tail boom. This unbalances the lift forces and the rotor flaps down on the right side, causing the fuselage to follow with a slight nose-up pitch due to the initial increased pitch over the nose. A short time later, lift of the fore and aft blades come back into balance as the helicopter reaches a steady rate of roll.

 

The problem with the 180-degree autorotation to the left is the pilot deals with some additional issues. When the pilot lowers the collective to enter the autorotation the helicopter responds with a slight nose-down pitching moment. At the same time, the pitch-roll coupling due to the left roll causes an additional nose-down moment. Had this been a right roll, the pitch-roll coupling would have caused a nose-up moment tending to neutralize any nose-down moment caused by the down collective.

 

The other issue is the need for right pedal to counter the reduction in torque after the collective reduction. It is some times confusing at this point, because the pilot is turning left and the required pedal is right. The right turn follows more naturally, right turn with right pedal.

 

It’s at this point things can get messy. If the pilot is late with right pedal or adds left pedal to aid the left turn, they will be grossly out of trim. The helicopter at that point will on longer be aligned with the relative wind. The pilot while trying to turn left is skidding away from the turn to the right. The relative wind now strikes the right side of the fuselage, causing a right rolling motion along with the previous issues.

 

The pilot is now trying to correct a combination of problems as the helicopter enters the 180-degree autorotation to the left with an out-of-trim yaw, right roll, and nose-down pitching moment. The result of all this is an increase in the rate of descent. As the pilot sees the helicopter will be short of the intended landing point, the turn is tightened causing rapid increase in rotor RPM. The pilot’s workload will continue to increase, as the cycle of chasing rotor RPM with collective begins.

 

The entry into the 180-degree autorotation to the right is easier for most students to coordinate and more stable. Left turns require positive trim control and slight aft cyclic during the collective reduction to oppose any nose-down pitching.

Edited November 11, 2012 by iChris

[Spirit of '69]

i'm thinking for simplicity's sake, giving as short of an explanation as possible. for all autos, enter as if its a straight-in. (lower coll. aft cyclic right pedal). all autos will require right pedal, to stay in trim. right turn autos require a little more. on a left 180 auto, the nose will want to pitch down, just correct with slight aft cyclic, to maintain airspeed, scan to confirm. in a right 180, expect pitch up of nose, so forward cyclic, to maintain airspeed, scan. i can see twisting the student pilots' brain in knots with too complicated of an explanation. should say also this is in a 300c. i just want to get this right, so any suggestions are welcome.