why does the nose pitch forward when you lower collective?
Join Date: May 2005
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i've had so many possibilities of what may/may not/should/could/might do for this it's getting a little confusing 
i've drawn every option as guys have mentioned it and all have some form of merit...suppose it's what you feel best trying to explain and if someone else comes up with another concept that you prefer you'd probably go to that one... i'd just like to know EXACTLY what causes it
, just incase the examiner asks me...just so i can be ready for the slippery old goat 
34'
i've drawn every option as guys have mentioned it and all have some form of merit...suppose it's what you feel best trying to explain and if someone else comes up with another concept that you prefer you'd probably go to that one... i'd just like to know EXACTLY what causes it
, just incase the examiner asks me...just so i can be ready for the slippery old goat 34'
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Best answer I have is,
In forward flight the advancing blade is producing more lift than the retreating.So when you lower the lever the percentage of lift lost is more on the advancing side.Due to this being felt 90 degree's later the nose pitshes forward.
Hope that helps
In forward flight the advancing blade is producing more lift than the retreating.So when you lower the lever the percentage of lift lost is more on the advancing side.Due to this being felt 90 degree's later the nose pitshes forward.
Hope that helps
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Ah - this was the thread I was thinking of.
http://www.pprune.org/forums/showthread.php?t=214775
It talks about the disk tilting forward, rather than the aircraft. Hmmm.
http://www.pprune.org/forums/showthread.php?t=214775
It talks about the disk tilting forward, rather than the aircraft. Hmmm.
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The way I had it explained to me was it happens because of Flap back.
It is easier to see with a picture but if you can imagine a two bladed machine in forward flight. This machine is producing lift, both blades are producing the same lift but have different AoA's and airspeeds.
Simple so far right!
Now put some values on lift for simplicity I will use small numbers
We currently have a lift value of 20 on each side.
Advancing side has a higher airspeed and lower angle of attack
Airspeed = 10 AoA = 2
10 x 2 = 20
Retreating side has lower speed and higher AoA
Airspeed = 5 AoA = 4
5 x 4 = 20
Now we lower collective by 1 degree
Now advancing side is
10 x 1 = 10
Retreating side
5 x 3 = 15
You have more/less lift on the retreating/advancing side this precesses 90deg to make nose pitch down.
So while flying we automatically correct for flap back with cyclic, but this setting only works at a given speed/collective setting. if you change either of them you have to reset the cyclic.
I am sure there are probably many factors to this question but this is my main understanding of how this works.
Crispy
It is easier to see with a picture but if you can imagine a two bladed machine in forward flight. This machine is producing lift, both blades are producing the same lift but have different AoA's and airspeeds.
Simple so far right!
Now put some values on lift for simplicity I will use small numbers
We currently have a lift value of 20 on each side.
Advancing side has a higher airspeed and lower angle of attack
Airspeed = 10 AoA = 2
10 x 2 = 20
Retreating side has lower speed and higher AoA
Airspeed = 5 AoA = 4
5 x 4 = 20
Now we lower collective by 1 degree
Now advancing side is
10 x 1 = 10
Retreating side
5 x 3 = 15
You have more/less lift on the retreating/advancing side this precesses 90deg to make nose pitch down.
So while flying we automatically correct for flap back with cyclic, but this setting only works at a given speed/collective setting. if you change either of them you have to reset the cyclic.
I am sure there are probably many factors to this question but this is my main understanding of how this works.
Crispy
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Thanks crispy
I'll get the whiteboard into action and see if i can get the brain around it
This examiner had better ask me this question or i'm just gonna give him the info anyway
I'll get the whiteboard into action and see if i can get the brain around it
This examiner had better ask me this question or i'm just gonna give him the info anyway
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in nil wind the nose will not drop.
most of the other answers must not be from pilots. if they are then it is very embarrassing.
this thread demonstrates the main arguement against low time pilots becoming instructors, a complete lack of knowledge of basic aerodynamics.
so much for the jar system.
most of the other answers must not be from pilots. if they are then it is very embarrassing.
this thread demonstrates the main arguement against low time pilots becoming instructors, a complete lack of knowledge of basic aerodynamics.
so much for the jar system.
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we all have to learn the information somewhere... if you have many hours of experience that could be of help to us starting out then that would be great...we don't make the rules...i'm just trying to learn as much as i can...
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34'
If the examiner does not give the helicopter's configuration, you can quite honestly state that 'pitch reduction due to torque reduction, is because of the pitch-torque coupling'.
After he has given you a mark of zero, you can then say that you were talking about the Intermeshing configuration and ask for your full mark.
___________________________
www.unicopter.com/0842.html#Torque_Pitch
If the examiner does not give the helicopter's configuration, you can quite honestly state that 'pitch reduction due to torque reduction, is because of the pitch-torque coupling'.
After he has given you a mark of zero, you can then say that you were talking about the Intermeshing configuration and ask for your full mark.
___________________________
www.unicopter.com/0842.html#Torque_Pitch
I was taught that it was primarily the effect of reduced downwash over the horizontal stabiliser, but that like verything in a helicopter there are multiple forces at play. Those included the moment arm on the CofG (see Shawn Coyle's post) and the possibility of an uneven redistribution of lift across the disk (not flapback) as per fingwing207.
I did find that flying Balck Hawks reinforced the lesson that it is primarily due to downwash reduction over the horizontal stabiliser. The Stabilator (big wing at the back) is driven by a computer that changes it's angle dependant upon a number of inputs, one of which is collective bias. This senses the rate and magnitude of collective movement and angles the stab to keep the nose level. When you lower the collective in the Black Hawk, the nose stays level, but the stab drives up to achieve it.
This explanation would suit all arguements, however the one that made lean toward the downwash theory was what happened when we lowered collective whilst fitted with the external support wings (ESSS). Those wings are situated forward of the rotor mast, and the stab computer doesn't know they are on. When we lower the collective, we get a pitch up. That would make sense with the downwash reduction over the ESSS not being countered by the stab computer, but it also backs up the CofG moment arm arguement. It tends to discount the effect of any redistribution of lift across the disk because that theory would dictate a nose down regardless of the ESSS.
I found the same thing on Hueys without a computerised (but moving) stabiliser. I did a maintenance test flight with an improperly rigged stabiliser, and during the max power check I contacted the forward cyclic stop trying to keep the rising nose under control. The only way out was to reduce collective and that restored forward cyclic authority. This suits the downwash arguement well. Lastly, when fitted with gunship "wings", the aircraft behaves like the Black Hawk with ESSS, ie it pitches nose up rather than down.
I did find that flying Balck Hawks reinforced the lesson that it is primarily due to downwash reduction over the horizontal stabiliser. The Stabilator (big wing at the back) is driven by a computer that changes it's angle dependant upon a number of inputs, one of which is collective bias. This senses the rate and magnitude of collective movement and angles the stab to keep the nose level. When you lower the collective in the Black Hawk, the nose stays level, but the stab drives up to achieve it.
This explanation would suit all arguements, however the one that made lean toward the downwash theory was what happened when we lowered collective whilst fitted with the external support wings (ESSS). Those wings are situated forward of the rotor mast, and the stab computer doesn't know they are on. When we lower the collective, we get a pitch up. That would make sense with the downwash reduction over the ESSS not being countered by the stab computer, but it also backs up the CofG moment arm arguement. It tends to discount the effect of any redistribution of lift across the disk because that theory would dictate a nose down regardless of the ESSS.
I found the same thing on Hueys without a computerised (but moving) stabiliser. I did a maintenance test flight with an improperly rigged stabiliser, and during the max power check I contacted the forward cyclic stop trying to keep the rising nose under control. The only way out was to reduce collective and that restored forward cyclic authority. This suits the downwash arguement well. Lastly, when fitted with gunship "wings", the aircraft behaves like the Black Hawk with ESSS, ie it pitches nose up rather than down.
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without getting to technical
i would have thought it was as simple as a loss of lift on the advancing blade and the postion of the cyclic in foward flight when the collective is lowed
i would have thought it was as simple as a loss of lift on the advancing blade and the postion of the cyclic in foward flight when the collective is lowed
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Originally Posted by stacey_s
Ask a 'good Engineer' and he'll tell you why!!
Munch Munch
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Lot of funny and complicated answers on a straight forward question.. 
Lowering collective reduces AoA, and in forward flight this reduces flapping and make the rotordisc tilt forward, this will cause the liftvector to tilt forward and anybody who been awake on the lesson where the helicopters stability has been the subject, will understand why the fuselage tilts forward.. Stabilizers and precession and those factors are not the main contributor in this matter.
Fly safe
Lowering collective reduces AoA, and in forward flight this reduces flapping and make the rotordisc tilt forward, this will cause the liftvector to tilt forward and anybody who been awake on the lesson where the helicopters stability has been the subject, will understand why the fuselage tilts forward.. Stabilizers and precession and those factors are not the main contributor in this matter.
Fly safe
Dis Mystery has it right.
The instantaneous reaction to lowering collective is for the lift on the advancing side to reduce MORE than the lift on the advancing blade. [The amount of blade angle reduction on adv and ret sides is reduced by the same but the ratio of lift is different]. Move this round 90 (ish) degrees due to flapping to equality and the front of the blade dips - helo noses down.
At the same time, on SOME helos, this goes one step further with a change in C of G dynamics.
Once the helo starts to descend.....then the tail stabiliser comes into effect -exacerbating this dip with the nose.
The instantaneous reaction to lowering collective is for the lift on the advancing side to reduce MORE than the lift on the advancing blade. [The amount of blade angle reduction on adv and ret sides is reduced by the same but the ratio of lift is different]. Move this round 90 (ish) degrees due to flapping to equality and the front of the blade dips - helo noses down.
At the same time, on SOME helos, this goes one step further with a change in C of G dynamics.
Once the helo starts to descend.....then the tail stabiliser comes into effect -exacerbating this dip with the nose.