RBS, precession & LZ
This thread has begun at the suggestion of Lu (see vote thread).
I have seen many of Lu’s posts, and I believe that his search for understanding needs to start back with first principles, so I will have a go at retreating blade stall whilst (hopefully) demonstrating to Lu that he needs to go back to basics first…………………
To Lu: you need to grasp first aerodynamic POF before you can hope to grasp the explanations offered by so many in various threads. You particularly need to understand how FLAPPING TO EQUALITY, achieves EQUALITY OF LIFT. These concepts were ably explained to you in the last thread by Arm out the window, with almost no acknowledgement from you, but once you understand these basic POF concepts, most of the explanations offered to you will become more easily understood. After all, your stated aim is to understand.
Why am I going down this path? Some examples….
You have persisted with the notion that the rotor disc has different lift in different areas: You said: >> If it were pure aerodynamics the helicopter would roll to the right because of the differential of lift across the disc<<. This is NOT TRUE. The DISSYMETRY OF LIFT (not differential) is equalised by the blades FLAPPING TO EQUALITY, thus we have the steady state situation of lift being equal across the entire disc from entirely aerodynamic processes.
You said: >>When a helicopter is in the flight regime that will result in retreating blade stall the blade does not stall like a wing on an airplane. Most people think that when the blade stalls it drops out of the tip path and strikes the tail boom this is not true. What happens, is the left side of the rotor system although it has higher pitch it is generating less lift than the right side of the disc. This differential of lift will cause the perturbing force to be on the right side and like the rotor on a gyro the disc will tip up over the nose and down over the tail<<.
NOT TRUE
A rotor blade stall is entirely consistent with a wing on an aeroplane that enters a stall. BECAUSE A ROTOR BLADE IS A WING (maybe that’s why they call them “rotary wing” aircraft). The blade does NOT become >>suddenly unstable<< as you have asserted. It does NOT have a higher pitch angle. It does not suddenly fall out of the plane of rotation. It does not even lose all lift. The stall is caused by the blade exceeding its critical angle of attack (AoA), as opposed to pitch angle, due to the airspeed/AoA relationship, just like a fixed wing. As the blade approaches the critical AoA, it is still generating significant lift, but the drag is beginning to increase dramatically. Further increase in the AoA produces the buffet where airflow begins to break its laminar flow over the aerofoil and lift dramatically decreases whilst drag dramatically increases.
At this point in the process, a DISSYMETRY OF LIFT situation is created because of the rapid loss of lift from the retreating blade side and the blade is unable to flap to equality from this situation. Accordingly FOR PURELY AERODYNAMIC reasons, the aircraft will roll toward its retreating blade side. The stalled blade begins to reduce its upward movement (remember the retreating blade is normally climbing), but due to rotation, the airflow changes and the blade quickly becomes unstalled, and resumes its climb toward the highest point of the disc over the tailboom. Because the stall interrupts the blade’s climb, it can no longer reach its pre stall height over the tail boom, hence the tip path plane is lower at the rear when in retreating blade stall. Accordingly, the aircraft will pitch nose up. How gyroscopics applies to this is beyond me. So what the pilot feels and sees is:
A number of blades per rev vibration (ie a two per in a B212, a four per in a S76) rapidly building in severity, a pitch up and roll moment toward the retreating blade side.
By lowering the collective, the pilot can bring the blade out of the stall and recover the aircraft. BUT here is the crunch: when it recovers this way, there is a restoration of flapping to equality and lift is again equalized across the disc and therefore the disc DOES NOT SUDDENLY ROLL BACK TOWARD THE ADVANCING SIDE. So why des the nose try to gently pitch down if there are no GYROSCOPIC roll moments applied? Because the blade again climbs to a higher high point over the tail (the upward movement around the retreating side is no longer disrupted by the stall) and the aircraft will pitch VERY gently nose down (if you are brave enough to let it). IE an aerodynamic situation.
There is more:
You said: >> I won’t challenge your statement that you allude to as pure crap as that would take too much time but I will address one point. I made the comment about the Bell blades not having equal lift when the blades passed over the longitudinal centerline. I caught a lot of flack on that so; I contacted the that showed what causes the two per rev. At approximately 5-7 degrees ahead of the lateral centerline the advancing blade will cause an upward movement. Just like the explanations of Nick and Frank. He explained that this upward movement was the cause of the two per rev. Now as I addressed the movement of the transmission on the 214 it moved not only up, it moved down by the same amount. that if the advancing blade made the transmission move up then what makes it move down. is the blade loses lift at some point and the transmission falls. When the next blade comes around the trannie moves up and this happens at twice the rotor speed. Based on this, I would like to hear some of your opinions.<<
My opinion? You caught a lot of flack because what you are saying is NOT TRUE.
The Facts? See Nick Lappos’ excellent description of the causes of the vibration. If you look back over your statement, you will see the key to understanding what people are trying to tell you……
You said: >>[The]senior aerodynamicist at Bell….sent me some engineering diagrams. I didn’t fully understand the diagrams. My mind tells me. The only thing I can think of.<<
When you don’t understand, you do not ask, you create a theory in your own mind and reject explanations that do not accord with your theory. I believe that if you understood equality of lift as achieved by flapping to equality, you will be equipped with the basic tools you need to help you understand the rest of the discussions presented to you. Good luck in your search for understanding Lu.
For others.....have I grasped retreating blade stall? It has been a long time since I have reached into these memory banks.
AAARRRGGGGHHH my helmet is on fire……………..
[ 04 August 2001: Message edited by: helmet fire ]