Gyroscopic load change while doing 360's

C

Cool Money

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I leave this for the prop manufacturers to answer but this was explained to me years ago by an old AC builder and it made a lot of sense. This is something I think should be included when you purchase a new prop is a diagram of how gyroscopic loads change from the center of the prop and travel down the blade while doing 360's not to mention end loads on the blade when water is being introduced over the side of the boat. I've seen a lot of props explode and cranks broken and most owners didn't know why other than Blame the prop guy. I'm not a professional at props just an old airboater, but when I see problems I try to put some thought to why it happened. Hope I didn't bore y'all to death with this but I couldn't find a good demonstration of what happens when the load travels from the center of the prop down the blade.

Spencer
 
Do a search on "360s. Surprising forces are exerted"
(Sorry, can't copy the link on my phone)

This is a post written by David Wine a few years ago. You may also want to look through his posts, there are a LOT of white papers and valuable info on everything from props to whole boat configurations.
 
An airplane or airboat propeller is intended and designed for forward and or reverse direction of travel. Both blades within reason are loaded equally in this case. When a prop travels in a direction 90 degrees to forward or reverse it becomes a helicopter bade at least to some percentage. Now we have a leading and a retreating blade. THIS unbalances the load to the point that can be destructive. Ask any helicopter pilot or designer about leading and retreating blades. It is an whole different world than an airplane propeller. Asymmetrical blade loading is a whole science unto itself. Huntinbigun here is a helicopter man, maybe he will be able to chime in as an authority on this subject. Listen closely to what he has to say, it is his profession. :thumbleft:

W4ERC DE KE4YGS We could do a simplex 2M chat.....73 Scotty
 
Think of a spinning propeller as a gyroscope. Remember when you were a kid and you'd hold a bicycle wheel by the axle and then your buddy would give it a spin? You'd try to turn it left or right and instead it would want to pop up or dip down. That's called gyroscopic procession.
When you're trying to control a spinning gyroscope it's necessary to put the control input for the change in 90 deg. before you want it to occur. All of the flight controls on a helicopter are set up just that way .... a component called a 'swash plate' takes the control input from the pilot and puts it in 90 deg. ahead of any change needed of the plane of the main rotor blades. Igor Sikorsky was the genius who first made rotary wing flight possible when he engineered a rotor system that addressed just that.

If you think about it most propeller driven aircraft make nice gentle turns, which reduces the strain of a course change on the engine mountings and the airframe in general. I sometimes cringe when I see somebody doin 360's on an airboat ....... the forces against the engine stand are huge when you do that, and if that stand gives way and a spinning prop touches the cage? It won't be pretty !! :shock:
 
Right on Ken and the asymmetrical loading torque is WHY the need for the little rotor in the rear to keep the thug facing forward. Without that the craft just wants to spin opposite of the main rotor. At least that my guess anyway.

Gyros do not have a powered rotor or swash plate but they still suffer from vibrations from the asymmetrical loads of a leading and retreating blade in the main rotor. In fact you can feel it in the control stick. They have a forward thrusting engine and prop so the yaw is taken are of by a rudder in its slipstream.

The thing about Asymmetrical loading is this. The forward moving bade has the blade speed PLUS the crafts forward speed for a load and the retreating bade has the crafts forward speed MINUS the rotor speed loading. This can be a tremendous amount of stress and can in many cases break the blades, hub or crank if they were not designed for exactly this scenario. I remember in the early personal Gyro days it was not unheard of for craft to shed the rotors and or the engine prop in a catastrophic chain of events. Not even mentioning blade flex and rebound which also has killed more than a few novices. These are the same conditions that are set up in an airboat doing 360s. Asymmetrical blade loading.

I may have some of this not exactly right but I am at least fairly close with the idea.
 
Good info and Bear and Olf Art are dead on, lots goes on to setting limitations of props in regards to speed of blades, loading and extra. The main problem we have with failures is happens when you need it the most such as when Helicopter is hovering and the tail rotor is putting max load on keeping aircraft straight, that's when blades fail, links break etc. So good advice to watch your 360's it may not fail when you do it but later when your trying to climb a dike and prop lets go that duck just came home 8) does not do the engine much good ether as oil is being forced away from the pickup, that why on aerobat engine has special oil and fuel system to compensate for flying upside down or spins etc so not just what a 360 does to the prop....................
 
I would imagine the forces at play are wholly dependent upon rpm of the blades, the length of them and the rate of degree change.

For instance, if you a turn at x deg per second heading a larger prop will be affected more than a smaller one given the same rpms.

The people drive like I do it may be in the back of your mind but when you have an oh sheet moment and some smuck hauls asz around a blind corner and you go get out the way your rate of turn and rpms are likely to me about maxed out. I guess the main other time boat really takes a beating is when the hogs are spotted and try to get away.... All bets are of then it's game on.

Any way cool subject it's kind of fascinating. I just know unless you drive like my grandpa you probably have some pretty regular events that cause you to turn sharp and hit the throttle. At the end of the day that's how you make an airboat turn.....


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Wayne, I hadn't even considered the problem with the oil pickup. That's a big one too. 8)

Guts, rotary wing flight is almost a 'black science' .... the damn things probably shouldn't be able to fly at all, but somehow they manage to! :lol:
There is a whole 'nother discussion on advancing and retreating blades, and retreating blade stall. Simply put, when one of the rotor blades on a helicopter crosses the nose of the ship and heads for the tail, that blade is retreating because it's headed in a direction opposite the direction of flight, so in effect it's traveling at a speed much slower than the opposite blade which has already crossed the tail and is now headed forward, in the direction of flight. If You keep in mind that they're called rotary wings, then the blades actually have to adjust their pitch on every revolution to keep the lift balanced on both sides of the rotor head.
 
That is why a helicopter has a swash plate. The rotor blades constantly change pitch in rotation in forward motion but stay equal in pitch while in a hover, but gyro copter blades never change . That's why helicopter pilots say you fly a gyro copter in autorotation all the time for they fly due to air passing under the rotor blades where a helicopter fly's due to the amount of pitch you put in the blades with the collective.
 
Exactly right. I've never been in retreating blade stall, but I've been close. Trust me .... it's a very spooky place. Everything starts to get real 'mushy'. Fortunately I had enough altitude to take some collective out of her and get that retreating rotor blade flying again.
 
One big problem with a helicopter is that you can let it get into its own rotor wash and when that happens you will go down like a rock if you don't realize it and immediately recover.
 
Lwells said:
One big problem with a helicopter is that you can let it get into its own rotor wash and when that happens you will go down like a rock if you don't realize it and immediately recover.
Click to expand...
Hover has often been described as trying to sit on top of a big beach ball. You're right. If you're coming ashore for example and you want to sit down on a big dock? It's heads up because the airflow under you changes big time. Terra firma is a piece of cake compared to times when you have to take rotor wash into account around structures that will upset it. It's been nearly 50 years since I flew for the Army .... I think I still could if I had to, but there have been so many changes what with fly-by-wire flight controls and rigid rotor technology, let alone the GPS navigation, that it would probably be like learning to fly all over again. My wings have been clipped now, but I do love me some aircraft. :)
 
Olf Art said:
Wayne, I hadn't even considered the problem with the oil pickup. That's a big one too. 8)

Guts, rotary wing flight is almost a 'black science' .... the damn things probably shouldn't be able to fly at all, but somehow they manage to! :lol:
There is a whole 'nother discussion on advancing and retreating blades, and retreating blade stall. Simply put, when one of the rotor blades on a helicopter crosses the nose of the ship and heads for the tail, that blade is retreating because it's headed in a direction opposite the direction of flight, so in effect it's traveling at a speed much slower than the opposite blade which has already crossed the tail and is now headed forward, in the direction of flight. If You keep in mind that they're called rotary wings, then the blades actually have to adjust their pitch on every revolution to keep the lift balanced on both sides of the rotor head.
Click to expand...


So I guess you aren't going to be the one that can explain to me how a helicopter can perform a loop. Cause I sure can't figure it out!

http://www.bing.com/videos/search?q=helicopter+loop&view=detail&mid=DD3AE4420F9EF492A76FDD3AE4420F9EF492A76F&FORM=VIRE4
 
Olf Art said:
Wayne, I hadn't even considered the problem with the oil pickup. That's a big one too. 8)

Guts, rotary wing flight is almost a 'black science' .... the damn things probably shouldn't be able to fly at all, but somehow they manage to! :lol:
There is a whole 'nother discussion on advancing and retreating blades, and retreating blade stall. Simply put, when one of the rotor blades on a helicopter crosses the nose of the ship and heads for the tail, that blade is retreating because it's headed in a direction opposite the direction of flight, so in effect it's traveling at a speed much slower than the opposite blade which has already crossed the tail and is now headed forward, in the direction of flight. If You keep in mind that they're called rotary wings, then the blades actually have to adjust their pitch on every revolution to keep the lift balanced on both sides of the rotor head.
Click to expand...
Makes me think Disney rides when younger


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Uhairball said:
So I guess you aren't going to be the one that can explain to me how a helicopter can perform a loop. Cause I sure can't figure it out!

http://www.bing.com/videos/search?q=helicopter+loop&view=detail&mid=DD3AE4420F9EF492A76FDD3AE4420F9EF492A76F&FORM=VIRE4
Click to expand...
That's called a 'parabolic arc', and something that people who design roller coasters use all the time. It works because of speed and centrifugal force. You go into it with a lot of speed and pulling hard for altitude, and then as the loop starts to tighten up you would drop the pitch out of the rotors until the aircraft literally pitches over the top. When it does, then you start getting collective (pitch- lift) back and fly away from it. It's not a perfect circle, but it's a loop.

The first one I'm aware of actually happened quite a few years ago ('68) when I working at Sikorsky up in Stratford, CN. Sikorsky was trying to develop one of the first ridged rotor systems for helicopters, and the test mule they'd chosen was a CH-53 that most everybody knew as a Jolly Green Giant. Anyway, one afternoon the word through the hanger was that one of their test pilots had actually looped one out over Long Island Sound. L.I.S. wasn't far away and most of the test flights were made out there so that if anything went wrong there wouldn't be any danger to the local population.

There was also a great little neighborhood bar in town where you could get a bowl of the best clam chowder you'd ever eaten, a slice of home baked bread, and a cold draft beer for $2.00. :)
 
I like to know how everything works so I was thinking about this. I wondered if the pitch changed on rotors with forward speed. After a moment I figured it has to because hovering required pitch tone the same but going forward fast required pitch to change not only port to starboard but also a greater delta with faster speeds. My next though was this means there is a limitation for helos both from a pitch angle that can be achieved for lift, which I assume isn't significant because a wing can only function within certain pitch any way... But the forward speed of the craft would be limited because as the forward speed of the helo and the rearward rotation of the blades could theoretically match leaving half the rotor with little to no lift while the other forward spinning side was at maximum lift. Seems the faster the rotor spins the more forward speed the chopper could achieve just from that perspective. Only way I see around that is counter rotating rotors that way each side always has maximum available lift at least from one rotor on each side and the more blades the better.


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And yes, this is t about doing 360s any more, it's helo theory.... [THUMBS UP SIGN]


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I have read that they have managed to get some test rotorcraft up into the 300 MPH range but like you pointed out, there appears to be a forward speed limitation to the rotors.
 
Yup, at something like 300 mph. ( :shock: ) there would be a 600 mph differential between the advancing and retreating blade on every revolution, but add to that the differential for forward airspeed.

When you need more lift from a wing you increase the angle of attack (pitch) of that wing. Rotor blades are no different .... they're rotary wings, When any wing reaches a pitch component where the air no longer wants to flow over it smoothly and make lift however, the air over that wing begins to 'burble' .... the wing starts to stall. No lift on the retreating half of the rotor. When that happens you're only option is to drop the pitch to the rotor head and get that port side flying again so that you can scrub off some speed. If you don't have the altitude to do that you're soon gonna look like you're trying to plow a field.
 
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