Author Topic: Top-heavy hovering easier  (Read 2939 times)

Offline docmordrid

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Top-heavy hovering easier
« on: 02/12/2012 02:56 pm »
Some interesting, and counter-intuitive, research results in these days of VTVL rocketry -

http://www.physorg.com/news/2012-02-hard-youre-top-heavy.html
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Offline Andrew_W

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Re: Top-heavy hovering easier
« Reply #1 on: 02/12/2012 08:03 pm »
I'd always assumed that this was why the Shuttle ET had the LOX rather than the LH2 tank on top.
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Offline john smith 19

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Re: Top-heavy hovering easier
« Reply #2 on: 02/13/2012 07:41 am »
Some interesting, and counter-intuitive, research results in these days of VTVL rocketry -

http://www.physorg.com/news/2012-02-hard-youre-top-heavy.html

Very counter intuitive. Balencing a pool cue on its narrow end is a *classic* control systems demonstration of the problems of active balance control.

The question is are the aerodynamic restoring forces *big* enough to make it worthwhile doing things this way?
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Offline Proponent

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Re: Top-heavy hovering easier
« Reply #3 on: 02/13/2012 07:46 am »
I'd always assumed that this was why the Shuttle ET had the LOX rather than the LH2 tank on top.

For the Shuttle's ET, I suspect the major issue is loads rather than controllability.  The ET is dragged from the thrust beam, which is near the forward end.  Putting the heavy oxygen tank forward of the relatively light hydrogen tank means that not so much force needs to be transmitted aft of the oxygen tank, making the ET's structure lighter.

Offline john smith 19

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Re: Top-heavy hovering easier
« Reply #4 on: 02/13/2012 09:28 am »
I'd always assumed that this was why the Shuttle ET had the LOX rather than the LH2 tank on top.

For the Shuttle's ET, I suspect the major issue is loads rather than controllability.  The ET is dragged from the thrust beam, which is near the forward end.  Putting the heavy oxygen tank forward of the relatively light hydrogen tank means that not so much force needs to be transmitted aft of the oxygen tank, making the ET's structure lighter.

I think there is also an issue with moment arms and centres of gravity. Putting the LOX tank at the top end sort of balances the SSME weight at the bottom end of the stack, requiring smaller steering angles for both the SSME's and the SRB nozzles.

It's been a long time since I looked at this issue.
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Offline MP99

Re: Top-heavy hovering easier
« Reply #5 on: 02/13/2012 11:29 am »
I'm not sure why the mechanics & aerodynamics of a bug hovering when flapping it's wings is relevant to rocketry.

Separately, though, I thought that keeping the CoM forward was a well understood principal of control in rocketry.

cheers, Martin

Offline simonbp

Re: Top-heavy hovering easier
« Reply #6 on: 02/13/2012 07:44 pm »
I'm not sure why the mechanics & aerodynamics of a bug hovering when flapping it's wings is relevant to rocketry.

Separately, though, I thought that keeping the CoM forward was a well understood principal of control in rocketry.

For the later case, it's a matter of keeping the center of pressure below the center of mass, thus resulting passive dynamic stability.

For hovering, the airspeed is zero and so a different effect is going on. Specifically, it's a matter of how easy it is to actively control a hover, how forgiving the system is to incorrect inputs, etc. Helicopters are notoriously hard to control, but were their rotors at the bottom (e.g. below the center of mass), this research implies that they would be much more forgiving to control.

Offline go4mars

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Re: Top-heavy hovering easier
« Reply #7 on: 02/13/2012 07:50 pm »
I'm not sure why the mechanics & aerodynamics of a bug hovering when flapping it's wings is relevant to rocketry.

Separately, though, I thought that keeping the CoM forward was a well understood principal of control in rocketry.

For the later case, it's a matter of keeping the center of pressure below the center of mass, thus resulting passive dynamic stability.

For hovering, the airspeed is zero and so a different effect is going on. Specifically, it's a matter of how easy it is to actively control a hover, how forgiving the system is to incorrect inputs, etc. Helicopters are notoriously hard to control, but were their rotors at the bottom (e.g. below the center of mass), this research implies that they would be much more forgiving to control.
Makes landing legs and getting in/out a little hard in the case of a "chopper" though...
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Offline RanulfC

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Re: Top-heavy hovering easier
« Reply #8 on: 02/15/2012 02:10 pm »
I'm not sure why the mechanics & aerodynamics of a bug hovering when flapping it's wings is relevant to rocketry.

Separately, though, I thought that keeping the CoM forward was a well understood principal of control in rocketry.

For the later case, it's a matter of keeping the center of pressure below the center of mass, thus resulting passive dynamic stability.

For hovering, the airspeed is zero and so a different effect is going on. Specifically, it's a matter of how easy it is to actively control a hover, how forgiving the system is to incorrect inputs, etc. Helicopters are notoriously hard to control, but were their rotors at the bottom (e.g. below the center of mass), this research implies that they would be much more forgiving to control.
Makes landing legs and getting in/out a little hard in the case of a "chopper" though...
Well it could get "interesting" all right... Though we've solved this issue before:
http://www.aviastar.org/helicopters_eng/gluharev_meg-3.php

http://www.vectorsite.net/avplatfm.html

Several others, the main point being you locate the landing gear below the rotor and WAIT for it to stop before stepping off :)

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Offline kevin-rf

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Re: Top-heavy hovering easier
« Reply #9 on: 02/16/2012 12:29 pm »
Several others, the main point being you locate the landing gear below the rotor and WAIT for it to stop before stepping off :)

But, that takes all the fun out of it!!!
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Offline RanulfC

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Re: Top-heavy hovering easier
« Reply #10 on: 02/16/2012 06:09 pm »
Several others, the main point being you locate the landing gear below the rotor and WAIT for it to stop before stepping off :)

But, that takes all the fun out of it!!!
Whilst "probably" true it helps to keep in mind that the majority of "airfoil" shapes are not conducive to "cutting" but instead tend to "smash" objects they impact which in turn turns the "jaunty" hop off your VTOL platform into a comedy of truely "stoogic" proportions...

Randy
From The Amazing Catstronaut on the Black Arrow LV:
British physics, old chap. It's undignified to belch flames and effluvia all over the pad, what. A true gentlemen's orbital conveyance lifts itself into the air unostentatiously, with the minimum of spectacle and a modicum of grace. Not like our American cousins' launch vehicles, eh?

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