Got a helicopter aerodynamics question which is bugging me. I think I have got the "Billy and Johhny" solution but I would like to check my reasoning. The aerodynamic question is this:
Why does the disc tilt forwards when I reduce the collective (and vice versa)?
I think it is because the NR momentarily drops leading to an relative increase in flapping up on the retreating side (ie disc tilts forward). Although I don't think it has an impact on this phenomenon, my aircraft is a turbine with US rotation and anticipators. Any ideas?
Changing the collective will change the pitch in the blades, therefore will change the amount of flapping to equality in the entire disc region, so if you lower the lever you flap forward (nose down) and raise the collective, you flapback.........
GM If your aircraft is a Lynx it will not do this as the flying control runs have a Collective to Fore and Aft Cyclic interlink that will feed an increasing nose down cyclic input with increased collective selection. Up collective > Nose down cyclic Down collective > Nose up cyclic IIRC it was to compensate for the fixed horizontal stabliser pitching influence on the aircraft to changes in vertical airflow at slow speed,I maybe wrong it's a long time ago since I played with a Lynx.. W
This disc tilt will only happen with an airflow over the disc (forward speed or wind). If you, say, raise the lever the pitch angle will increase equally around the disc. However, because the advancing blade has a higher airflow then the retreating one it will experience a greater increase in lift than the retreating blade (remember 1/2 rho V squared S). As always happens, the effect is noticed 90 degrees on, so the disc flaps back. Lowering the lever has the opposite effect and the disc flaps down.
I maybe wrong but.....I thought the reason is more simple than that. I thought it was because as you lower the lever there is less down force on the stabilisor causing the nose to pitch down. As the nose pitches down the disc also pitches down (relatively !). The aircraft will descend, speed increases disc will then flapback.
In forward flight (airspeed across disk), there is dissymmetry of lift, which is being countered by decreasing the pitch on the advancing blade and increasing the pitch on the retreating blade - forward cyclic essentially controls the blade's desire to flap up on the one side and down on the other.
If the collective is raised to maximum allowable power, there is more lift, therefore more dissymmetry. With collective lowered to minimum, there is less lift, so less dissymmetry. If you leave a high-lift flapping correction (forward cyclic) when reducing power, you will cause "flapping" the other way, so the nose will want to pitch down.
With the helicopter in forward flight, the cyclic is used to hold the disc in such a position that the desired airspeed is maintained. This position is an equilibrium position into which the disc has settled, after the effect of flapback has been allowed for with forward cyclic in setting the desired speed. If the lever is then raised, there is an overall increase in lift across the disc, and the disc will rise to a higher point in the direction of travel i.e. the nose will rise. If airspeed is to be maintained, then further forward cyclic will be required to overcome the new level of flapback. Conversely, if the lever is lowered, there is an overall reduction in lift, and the disc will move to a lower position in the direction of travel i.e the nose will drop. The cyclic is still at its original position, and is now too far forward for this reduced level of flapback, and must be moved rearward to maintain the original airspeed.
Helidriver, the tail stabiliser effect only occurs once the aircraft starts to climb or descend and the airflow over it changes but the disc flaps back or forward as soon as the lever is raised or lowered.
Flap Forward, as aerodynamic force, does not actually exist. The disc actually pitches (flaps) forward because of a decrease in the value of Flap Back (which is an aerodynamic force).
As HB describes your cyclic is positioned (forward) to counter for a certain value of Flap Back. As you lower the collective you reduce the value of Flap Back....the cyclic is still positioned forward to maintain equilibrium (constant airspeed) and it appears has if the disc is flapping forward. Its not flapping though, the forward cyclic position is causing it to pitch (flap) forward.
I do admit that it is somewhat semantic......as obviously there is an aerodynamic force at play whether its increasing or decreasing.
Thank you everyone. Nice to know I was on vaguely the right track. I "resdicovered" this during a rapid descent recently and the rest of the crew cried when I tried to work out why it occured. Of course, I was probably taught the whys and the wherefores at groundschool but no-one's perfect.
Oldbeefer - Don't suppose you have a title for the section of Prouty's that it's in? I have the combined works but I can't seem to find the right bit. (Excellent book btw - Helibooks.com).
Blow back of disk The blowing back of the disk mainly depends on two parameters: 1. forward speed 2. coning If you reduce collective, you reduce the total force on the disk and the coning reduces. There is also a more detailed (flapping-geometry-precession etc.etc...) explanation of that phenomenon, but it may be as convenient to remember this more simple and still correct 'umbrella' reasoning. Note that if for some weard reason the rotor would cone negatively, then it would blow forward just as an umbrella would do...