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Old Sep 15, 2012, 04:41 PM
I don't like your altitude
Stupot46's Avatar
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Originally Posted by Shedofdread View Post
Stuart,
I can't comment on the overall design (sorry, not been paying enough attention) but I would also caution against that really small tip chord. At chords that low, behaviour isn't predictable. To be safe, treat 120(ish)mm as a practical minimum. The last 10% of the semi span can go less but if you stick to 120ish, 10% in from the tip, there'll be less probs.
Richard
Thanks Richard.I think this whole discussion is moving towards a bigger span and maybe slightly lower aspect ratio.
This will,as Herks points out leave more room in the wing; and also the centre section without much increase in the weight of the drive.
Regards Stuart.
Ps I think another maybe 30mm x250mm piece of your fancy F1 foam will be ideal as a drive mount.
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Old Sep 16, 2012, 04:27 AM
I don't like your altitude
Stupot46's Avatar
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An idea for the drive.The channels supported from the pillow block bearings in the pic,direct drive to the screwed rod by a geared motor.Not to scale,if the engineering faculty approve the concept I'll source the bits and pieces to build it.
Regards Stuart
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Old Sep 16, 2012, 08:01 AM
Herk
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Virginia USA
Joined Jun 2007
1,653 Posts
Good morning Stuart -- morning here anyway.

I was thinking about your screwjack drive yesterday. Looking at your current design, I conceived the idea that the same mechanism could also shift the drive engine, and perhaps the battery with it, as a way of shifting cofg should that become desirable.

If the drive mechanism was mounted on a sliding tray, an extension of the screwjack could move it aft as the main follower travels forward to sweep the wings. It would take a shaft extension with the opposite thread cut - or it could work with a similar thread through a gear drive that reverses the rotation direction.

With the gear drive option, the gear ratio could adjust the range of travel to obtain the desired effect.

As I've said before - I doubt this will be necessary - but we won't know until more is determined regarding the actual physical characteristics of the whole craft.

Also --- as I was looking at the drawing I made of the wing planform, I was thinking about the wingtips. Something that would look good and work well through the sweep range would be nice. What I drew on the paper was a half circle - round tip. Something like that, or a modest deformation of it, seems to fit the purpose.
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Old Sep 16, 2012, 09:31 AM
I don't like your altitude
Stupot46's Avatar
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Good afternoon(2pm)Herks.
Trying to combine a weight shift has been on my mind since early on,lh thread one of the ideas.
Another method I considered was driving the wing sweep using a timing belt,the top moving the wings,the bottom a weight shift in the opposite direction.I discarded this idea(sketch in post 232) because of belt tension and keeping the nut rigid.
Latest idea,sparked by Marks suggestion of a pod is shown in the quick sketch.It would need some clever math to work out the lever lengths etc,but by putting the pivot in a slot along the length of the lever it would be adjustable.
DOH!!!! just realised the lever could be moved by the same nut as the wings(sound of hand slapping forehead)and could be a rod moving through swivelling eyes.


On the wing design question,can I ask your thoughts on Richards suggestion of a 150mm tip chord and subsequent enlargement of the wing.I presume the semicircular tip is for aerodynamic reasons,to try to present the same profile to the airflow at different sweep angles(hand up at the back of the class)
Regards Stuart
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Old Sep 16, 2012, 01:22 PM
Deniable plausibility
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Derbyshire, UK
Joined Aug 2008
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Errrrr.... I said 120(ish)mm.

Have you looked at how the airfoil changes with sweep angle? One presumes that as the wing sweeps back, the [effective] airfoil becomes less thick and cambered as a % of chord length. The increased spanwise flow (with increased sweep) may further exacerbate that effect.

Richard
(keeping his geometry fixed )
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Old Sep 16, 2012, 01:53 PM
I don't like your altitude
Stupot46's Avatar
Joined Sep 2011
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Quote:
Originally Posted by Shedofdread View Post
Errrrr.... I said 120(ish)mm.

Have you looked at how the airfoil changes with sweep angle? One presumes that as the wing sweeps back, the [effective] airfoil becomes less thick and cambered as a % of chord length. The increased spanwise flow (with increased sweep) may further exacerbate that effect.

Richard
(keeping his geometry fixed )
Not paying attention againApologies
Stuart
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Old Sep 16, 2012, 06:44 PM
I don't like your altitude
Stupot46's Avatar
Joined Sep 2011
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Quote:
Originally Posted by Shedofdread View Post
Errrrr.... I said 120(ish)mm.

Have you looked at how the airfoil changes with sweep angle? One presumes that as the wing sweeps back, the [effective] airfoil becomes less thick and cambered as a % of chord length. The increased spanwise flow (with increased sweep) may further exacerbate that effect.

Richard
(keeping his geometry fixed )
Richard,When I first came up with this idea I did a lot of reading and research on rcg and elsewhere,looking for examples of similar ideas.Whilst not much of relevance emerged, I did rediscover the HoXIII.If the notes on this are to be believed,the wing was constructed from a pair of HoIII wings(24.3*) mounted at 60*
At that point,and until quite recently I wasn't giving much thought to the aerodynamics of the idea.While I am aware that it can be dangerous to take examples from full size and apply them to models,it was the example of the XIII that made me think that the variable sweep may be possible.
My hope is that the present discussion will come up with a combination of planform,airfoil and control surfaces which will result in a flyable wing.
Best regards,Stuart
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Old Sep 17, 2012, 06:01 AM
Deniable plausibility
Shedofdread's Avatar
Derbyshire, UK
Joined Aug 2008
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Stuart,
Don't mind me - I'm just trying to catch up. Been 'out of the loop' for a while...
Regards,
Richard
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Old Sep 17, 2012, 06:22 AM
I don't like your altitude
Stupot46's Avatar
Joined Sep 2011
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Hi Richard,I've been doing a bit of reading on the span wise flow thing,some very interesting stuff.I've lost it now,but I came across a discussion on rcg from 2008 involving Herks,Mark Drela and Norm(who as usual provided some useful links)amongst others.
My understanding of the problem has increased,but I'm afraid I have no answers,other than to suggest that limiting the degree of rearward sweep may help?
I've come up with a different planform which I'll put up later.
Regards Stuart
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Old Sep 17, 2012, 08:10 AM
Deniable plausibility
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Derbyshire, UK
Joined Aug 2008
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Stuart, I'm aware of the compexity issues that this will raise so feel free to ignore this but have you considered boundary layer fences that are geared with the wing sweep to stay in line of flight? Rather like the way the pylons under a Tornado's wing are but on the top surface? Make the fences large enough and 'hey presto' that's yaw stability sorted...?

Richard

PS wait till 2019 when the GR4 goes out of service and just grab the bits off one of those...
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Old Sep 17, 2012, 08:54 AM
I don't like your altitude
Stupot46's Avatar
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Quote:
Originally Posted by Shedofdread View Post
Stuart, I'm aware of the compexity issues that this will raise so feel free to ignore this but have you considered boundary layer fences that are geared with the wing sweep to stay in line of flight? Rather like the way the pylons under a Tornado's wing are but on the top surface? Make the fences large enough and 'hey presto' that's yaw stability sorted...?

Richard

PS wait till 2019 when the GR4 goes out of service and just grab the bits off one of those...
Short answer is yes.Fairly simple mechanically(how many)but could they be kept flush to the surface at different angles?I suppose that depends to a great extent on the airfoil/s used.Complicated
Regards Stuart.
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Old Sep 17, 2012, 09:22 AM
Herk
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Virginia USA
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Hi Stuart - just lurking here.

I've been reminded of a comment I heard years ago in Saint Louis - while attending a meeting at the plant where they were building the F-15 and F-18 aircraft. One of the managers told me that when they were building the F-15 prototypes, their philosophy was to get it in the air as soon as possible and "tame" it later.

I'd say keep it as simple as possible and get it flying. Think of the fun you will then have as you try to refine it.
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Old Sep 17, 2012, 11:30 AM
Deniable plausibility
Shedofdread's Avatar
Derbyshire, UK
Joined Aug 2008
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Originally Posted by HerkS View Post
Hi Stuart - just lurking here.

I've been reminded of a comment I heard years ago in Saint Louis - while attending a meeting at the plant where they were building the F-15 and F-18 aircraft. One of the managers told me that when they were building the F-15 prototypes, their philosophy was to get it in the air as soon as possible and "tame" it later.

I'd say keep it as simple as possible and get it flying. Think of the fun you will then have as you try to refine it.
Wise words. Ones that I REALLY need to remember...
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Old Sep 17, 2012, 12:17 PM
I don't like your altitude
Stupot46's Avatar
Joined Sep 2011
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Ok Herks,you've tipped the balance Enough discussion!
I've settled on a root chord of 240mm,disc diameter 75%of this@180mm.
Disc thickness at 15%-36mm.
The subtleties of wing design can resume when I've got the mechanical bits sorted

Richard,this means two sheets from Mr Buttons footwell 390mm x180mm.
I'm afraid there'll be a lot of questions heading your way as to the best way to proceed,but first I'll do another drawing of the drive section to illustrate what I have in mind.
Regards to both Stuart
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Old Sep 17, 2012, 01:13 PM
Herk
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Virginia USA
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Originally Posted by Stupot46 View Post
The subtleties of wing design can resume when I've got the mechanical bits sorted
Regards to both Stuart
Stuart - I think this is a good start. It'll be very helpful to know the dimensions and approximate flying weight of the center structure and mechanism. That way you can get a good idea of how much wing you will need to fly it. That also makes it possible to begin to work out any balance issues. Your starting chord and disk dimensions sound a bit more workable to me as well.
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