Vortex Ring Autorotation
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If you descend with the Heavy and Light helo at the same speed, pitching up both the same manner you get exactly the same initial downwash speed on both helos (refered to the helicopters)
The only difference is the deceleration rate, at early stages the heavy helo simply falls through the VR, with the higher integral downwash compared to the light to the environment.
Also the deceleration rate plays a role. Wich is for shame not linear, now try to get an nonlinear speedchange in an windtunnel.
Disc load itself is only a mathematical simplification for power calculations since the blades on varying modern day helicopters are nearly equal.
A smaller disc is actually providing a VR, since the outer vortex has to be smaller, the proper reasons for higher disc loads are:
Smaller diameter provides landing nearer to obstacles
Materials, for shure(since we have seen that tapered blades also provide VR)
Blade systems (with more blades the diameter decreases)
Greetings,
The only difference is the deceleration rate, at early stages the heavy helo simply falls through the VR, with the higher integral downwash compared to the light to the environment.
Also the deceleration rate plays a role. Wich is for shame not linear, now try to get an nonlinear speedchange in an windtunnel.
Disc load itself is only a mathematical simplification for power calculations since the blades on varying modern day helicopters are nearly equal.
A smaller disc is actually providing a VR, since the outer vortex has to be smaller, the proper reasons for higher disc loads are:
Smaller diameter provides landing nearer to obstacles
Materials, for shure(since we have seen that tapered blades also provide VR)
Blade systems (with more blades the diameter decreases)
Greetings,
Sokol - I'm still not sure what your point is regarding VRS.
As for your assertions about rotors, might I suggest you read Prouty to better understand how helicopters have developed over the years.
Can I ask, are you in fact a helicopter pilot?
As for your assertions about rotors, might I suggest you read Prouty to better understand how helicopters have developed over the years.
Can I ask, are you in fact a helicopter pilot?
Oh, please Crab. You certainly read his profile. Ray Prouty wasn't a pilot either (AFIAK). We all know there are many stick jockeys out there that still believe in gyroscopic precession in a rotor. Being a helicopter pilot is not really a proof of understanding helicopter aerodynamics.
Sokol, I agree with you, that the disk loading is just a mathematical simplification, but the downwash speed on a helicopter with a lower disk loading is really lower than one with a higher disk loading.
An approximation for this in a stable flight condition is:
square root(M(helicopter)/(2x(air density)x(disk area))
And that has nothing to do if you are going up or down or around the church spire.
The higher the mass of the helicopter or the lower the air density or the disk area, the higher the downwash speed.
Nonlinear speed changes in wind tunnel. Tricky. That's why they researched VRS on sleds.
Sokol, I agree with you, that the disk loading is just a mathematical simplification, but the downwash speed on a helicopter with a lower disk loading is really lower than one with a higher disk loading.
An approximation for this in a stable flight condition is:
square root(M(helicopter)/(2x(air density)x(disk area))
And that has nothing to do if you are going up or down or around the church spire.
The higher the mass of the helicopter or the lower the air density or the disk area, the higher the downwash speed.
Nonlinear speed changes in wind tunnel. Tricky. That's why they researched VRS on sleds.
Rotorbee - no, I don't make a habit of reading people's profiles but now I have, I understand Sokol's viewpoint - ie theoretical rather than practical.
You are right that not all pilots understand P of F but plenty of engineers think they know more about cockpits and flying than most pilots
You are right that not all pilots understand P of F but plenty of engineers think they know more about cockpits and flying than most pilots
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Oh, please Crab. You certainly read his profile.
(e.g. understanding that and why an lighter helo gets into VR faster came until this discussion)
but the downwash speed on a helicopter with a lower disk loading is really lower than one with a higher disk loading.
Blade load would be a more appropriate term. But I do agree that the formula gets really complex though.
The higher the mass of the helicopter or the lower the air density or the disk area, the higher the downwash speed.
Nonlinear speed changes in wind tunnel. Tricky. That's why they researched VRS on sleds
You are right that not all pilots understand P of F but plenty of engineers think they know more about cockpits and flying than most pilots
Try IT. Everyone is expert in that world.
Greetings,
However cleverly you design the blades - a heavier helicopter always has to displace more air than a lighter one in a stable, free-air hover - hence the higher downwash speed
Nonlinear speed changes in wind tunnel. Tricky. That's why they researched VRS on sleds
Embarassing that I didīnt thought of it.
Embarassing that I didīnt thought of it.
At the same downwash speed, the heavier ship will fall faster into or through VRS. Is it that what you mean? In that case, that would be obvious. Otherwise I still don't get your thing with the constant downwash speed either.
By the way, the formula would not be that much different. There is a similar formula that uses the rotor diameter. Blade load or disc load is more or less the same thing. It only gets ugly, when you want to find out the regions where the blade stalls at what Vd and it gets even worse if you have to take blade twist into account.
Who knows, if this brings us any further.
Known that video for a long time. Been in IT for a long time. Had to deal with smart-arses in IT for a long time. It's not a communication problem, believe me.
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Crab,
While a heavier aircraft does displace more air in a hover than a lighter one, this does not imply a higher downwash speed. The rotor diameter can be increased, hence resulting in more air being displaced for the same downwash.
Imagine two rotorcraft of the same weight, but one has a rotor diameter of 1 mile, while the other has a diameter of 1 inch. The downwash for the 1 mile rotor would barely be perceptible, while the 1 inch rotor will essentially be a rocket.
While a heavier aircraft does displace more air in a hover than a lighter one, this does not imply a higher downwash speed. The rotor diameter can be increased, hence resulting in more air being displaced for the same downwash.
Imagine two rotorcraft of the same weight, but one has a rotor diameter of 1 mile, while the other has a diameter of 1 inch. The downwash for the 1 mile rotor would barely be perceptible, while the 1 inch rotor will essentially be a rocket.
SimFlightTest - yes I know that and if you read back a few posts you will see that the discussion was qualified by the 2 helicopters having the same size of disc.
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See Sokol, that's why you should read Prouty ... and some NACA papers ...
However: Thanks for literature recommendation.
I don't really see a problem with wind tunnels.
At the same downwash speed, the heavier ship will fall faster into or through VRS. Is it that what you mean?
In that case, that would be obvious.
Blade load or disc load is more or less the same thing.
Who knows, if this brings us any further.
Greetings,
Interesting. If I do build an helo with 2 Blades and an helo with 5 Blades of the same shape as the 2 bladed one, that changes nothing.
Not for a man that only refers to the environment 3 times during layout. And thought (honestly) first about an magneto-fluid like behaviour.
The problem occurs simply in scale....
The thing is only the point and time you are referring to. The downwash referred to the environment is higher on the heavier helo after an infinitessimal short time period after inducing the same controls on both helos.
The VRS is, while quite good understood (but not completely), not a very important subject in helicopter aerodynamics. I believe there wasn't any fundamental research on this for decades. Despite some self-proclaimed pilot gods opinion, it does not kill that many people. CFIT is way more important, but that has nothing to with aerodynamics.
If you are interested:
Shawn Coyle: Little Book of Autorotations
Shawn Coyle: Cyclic & Collective
These are as close to the bible for pilots as it gets.
Interesting. If I do build an helo with 2 Blades and an helo with 5 Blades of the same shape as the 2 bladed one, that changes nothing.
Sokol - yes scale is an issue with wind tunnels but quite a well understood one - have you studied Reynolds numbers?
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Sokol - yes scale is an issue with wind tunnels but quite a well understood one - have you studied Reynolds numbers?
There is BTW, an interesting piece in the Prouty, how to design a helicopter and how to decide on the number of blades.
Would you care to explain? I don't get it. Both.
Magnetofluid: a long story. Oil in your pan is for example a magnetofluid. If your oil gets hot, little bosses occur. This are convection rolls. These can be vectored by an induced magnetic field.
Private message for more information.
Why? VRS occurs in every size of rotor. The drone web-community is full of tips and tricks for avoiding VRS and so are the RC sites
OK, that was unfair.
The point I want to make here is, that helicopter design involves to the day still trial and error.
let's hear it for rotor solidity ratio........ how many helicopters with a small, two-bladed system do you see?
Greetings,
I am waiting for that PM. I know magneto fluids, I did not know that hot oil is one. How that has anything to do with a rotor will be interesting.
Helicopter aerodynamics is very interesting. We can beat each other over the heads for hours and then find out that nobody was right ... or all ... or some. May take a few years sometimes.
... go on ...
Helicopter aerodynamics is very interesting. We can beat each other over the heads for hours and then find out that nobody was right ... or all ... or some. May take a few years sometimes.
you MAY get an exact VRS behavior, but the triggering part is a little different.
Well, Reynolds numbers are for laminar incoming flow. Which you canīt expect fully on an rotorhead
You made a point about scale in wind tunnel tests which is why I mentioned Reynolds numbers - I didn't say it was applicable to VRS and flow around a real rotor system.
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... go on ...
In the VRS itself the behavior should be nearly the same as I would call it an "nearly enclosed system". But I have read a bit more prouty to enforce this thesis though.
Helicopter aerodynamics is very interesting. We can beat each other over the heads for hours and then find out that nobody was right ... or all ... or some. May take a few years sometimes.
Greetings,
the only thing an engineer can ashame an physician for
