Dad-O-Lot said:ah, but what if the helicopter was on a turntable that matched the rotary speed of the blades but in the opposite direction; would it take off? (probably not before the pilot threw up from dizziness)Ag with kids said:
The best thing about helicopters is that they don't need conveyor belts in order to take off.
MOOT!Complaint Investigator said:Dad-O-Lot said:ah, but what if the helicopter was on a turntable that matched the rotary speed of the blades but in the opposite direction; would it take off? (probably not before the pilot threw up from dizziness)Ag with kids said:
The best thing about helicopters is that they don't need conveyor belts in order to take off.
Yes.
And the friend owns a damn Ferrari, too.Iowaggie said:
I think one problem with being a helicopter pilot is you would have that friend that keeps mooching rides just to solve his own problems.
Complaint Investigator said:Dad-O-Lot said:ah, but what if the helicopter was on a turntable that matched the rotary speed of the blades but in the opposite direction; would it take off? (probably not before the pilot threw up from dizziness)Ag with kids said:
The best thing about helicopters is that they don't need conveyor belts in order to take off.
Yes.
i did nerd out on this so i'll give it a try (disclaimer, all of my helicopter knowledge is from Wikipedia and all my physics knowledge is from A&M)Pirate04 said:Complaint Investigator said:Dad-O-Lot said:ah, but what if the helicopter was on a turntable that matched the rotary speed of the blades but in the opposite direction; would it take off? (probably not before the pilot threw up from dizziness)Ag with kids said:
The best thing about helicopters is that they don't need conveyor belts in order to take off.
Yes.
I wanted to nerd out on this but had a couple drinks and decided better not.
Pirate04 said:
Okay maybe I'm missing something.
What you are saying would be true if the turntable were spinning in the same direction of the main rotor blade however, the scenario is opposite of main rotor. Which, at least in my limited knowledge, would lead me to believe the velocity of the wind would double, causing greater lift at less power and less need for pedal input since seemingly the tail rotor it offset with the increased torque velocity. In essence, making the main rotor more efficient.
But then again, I have had a couple drinks and maybe be completely misunderstanding the scenario.
You are correct that the main rotor rotates at XYZ RPM relative the the fuselage. So, if the fuselage rotates in the opposite direction of the main rotor at the same XYZ RPM then the main rotor WOULD be at 0 velocity relative to the ground.BlackGoldAg2011 said:i did nerd out on this so i'll give it a try (disclaimer, all of my helicopter knowledge is from Wikipedia and all my physics knowledge is from A&M)Pirate04 said:Complaint Investigator said:Dad-O-Lot said:ah, but what if the helicopter was on a turntable that matched the rotary speed of the blades but in the opposite direction; would it take off? (probably not before the pilot threw up from dizziness)Ag with kids said:
The best thing about helicopters is that they don't need conveyor belts in order to take off.
Yes.
I wanted to nerd out on this but had a couple drinks and decided better not.
I'm going to go with no for a few reasons and will use a black hawk for the following assumptions
The first, assuming the axis of rotation in your turntable is centered under the main rotor, that puts the center of the tail rotor 388" out from the axis of rotation. The radius of the main rotor sits right at 322" so the tail of the aircraft is going to be moving faster than the tips of your rotors, and I don't imagine the tail of the helicopter was designed to withstand that kind of load. Assuming my googling is correct and the rpm of a black hawk rotor is 258 rpm, that puts the tips of the main rotor moving at 725 ft/s linear (they are designed to move through the air at this speed) and the tail of the craft moving at 880 ft/s or 20% faster. so my hypothesis is that this scenario will break the craft apart before it can ever take off.
second, and the more hypothetical answer, is no due to basic physics of how a helicopter gets lift. The rotor turns at a fixed rpm relative to the fuselage (in this case 258 rpm). the fuselage remains rotationally fixed in relation to the ground and air around it held so by the tail rotor and sometimes friction with the ground. with the fuselage held fixed, the rotor then is turning at 258 rpm relative to the ground and surrounding air as well, giving it the motion through the air it needs to create acceleration through the air. if we start turning the fuselage relative to the ground though, the rotor's rotation stays at 258 rpm relative to the fuselage so for every rpm i rotate the fuselage "backwards" i lose an rpm in the rotor relative to the ground. for instance if we rotate the helicopter 58 rpm in the negative direction, the rotor is now only moving at 200 rpm relative to the ground in the positive direction. we take this trend to the limit where the fuselage is turning at -258 rpm, the rotor relative to the fuselage will be turning +258 rpm, but that will put it at exactly 0 relative to the ground. so unless i am missing something in the mechanics of the rotor engine that would allow it to compensate for this negative rotation and essentially double the turnover rate of the blades to 516 rpm, the air craft will not be able to take off. in fact, in this perfect scenario, from standing on the ground off the turntable the blades would appear motionless and you could reach out and grab them. but only for a second because then you'd get smacked by the tail going 880 ft/s
Ag with kids said:
However, it's not that velocity relative to the ground that's important. In this scenario, the main rotor blades would have 0 velocity relative to THE AIR...
True...BlackGoldAg2011 said:Ag with kids said:
However, it's not that velocity relative to the ground that's important. In this scenario, the main rotor blades would have 0 velocity relative to THE AIR...
Yea. I realized that but decided to stick with comparing it to the ground since it's easier to visualize as a stationary reference. And for all intents and purposes the air is also stationary to the ground in ideal takeoff conditions.
GAC06 said:
Helicopters fly because the Earth rejects them due to sheer ugliness. The turntable is moot.
first off, yes, the scenario presented is entirely unrealistic, mostly because of the fact that this rotation would likely rip the tail off.Complaint Investigator said:
You're assuming we can't overcome the rotation with the anti-torque pedals (and our tail rotor generates about 400lbs of lift) and a whole slew of other assumptions (that we are fixed and unable to turn on top of the turntable etc.) We will crash and not fly far, but I'll bet a scenario could be created to where we could using the pedals to counteract the spin. We spent an entire period discussing this crap on a wx day in flight school. Helped with aerodynamics knowledge.
Otherwise you are correct. Rotors are not generating any lift and that vector is 0 based on general assumptions
thanks for the clarification. i thought about getting into the relative speed to the air but then my brain went into figuring out what the local air currents would be like, generated from the rotating body of the air craft, but then decided to simplify since my day job probably would appreciate me spending my time doing and FEA model on the scenario of spinning a helicopter really fast...Ag with kids said:True...BlackGoldAg2011 said:Ag with kids said:
However, it's not that velocity relative to the ground that's important. In this scenario, the main rotor blades would have 0 velocity relative to THE AIR...
Yea. I realized that but decided to stick with comparing it to the ground since it's easier to visualize as a stationary reference. And for all intents and purposes the air is also stationary to the ground in ideal takeoff conditions.
I was kind of just adding to the explanation and clarifying it.
BlackGoldAg2011 said:first off, yes, the scenario presented is entirely unrealistic, mostly because of the fact that this rotation would likely rip the tail off.Complaint Investigator said:
You're assuming we can't overcome the rotation with the anti-torque pedals (and our tail rotor generates about 400lbs of lift) and a whole slew of other assumptions (that we are fixed and unable to turn on top of the turntable etc.) We will crash and not fly far, but I'll bet a scenario could be created to where we could using the pedals to counteract the spin. We spent an entire period discussing this crap on a wx day in flight school. Helped with aerodynamics knowledge.
Otherwise you are correct. Rotors are not generating any lift and that vector is 0 based on general assumptions
for trying to counter the spin, if somehow the tail was able to stay attached and you try to start the tail rotor when, (i assume it's spinning backwards freely because of the wind across it from the rotation) it is going to have to transition from spinning backwards, to forwards. just at the moment where the rotor is "stationary" it will be seeing somewhere in the vicinity of 24,000 to 36,000 lbs of drag force from the air. if our theoretical turntable has the force to keep spinning against this force, the 400 lbs from the tail rotor aren't going to make a difference. even if we drop the drag coefficient of the tail rotor to that of a stalled airplane wing (0.15) it still is creating of drag force of 2,460 lbs. and that is just form the tail rotor blades themselves, its before even considering the drag from the actual fuselage and tail section. all that to say it takes a hell of a lot of force to spin a helicopter that fast. far more than the tail rotor will generate.
i assume the aircraft is fixed to the turntable because there is no way it would stay on the table just from friction with the ground spinning at that rate. I will take your word on that in any real scenario we could create that doesn't destroy the bird, a skilled pilot could likely find their way out of it. But this hypothetical scenario either, a) creates the hypothetical 0 lift and "stationary" main rotor or b) in any "real" world scenario tears the bird apart because to the best i can tell from digging, the fuselage simply isn't designed to rotate that fast.
