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Heather Kay

1/72 Fairey Rotodyne: Heather relives her childhood!

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In addition to appreciation of your modelling skills your clear, correct and engrossing writing has kept me riveted (in a good way) to every page.

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1 hour ago, Chewbacca said:

Without wishing to hijack this fascinating thread and excellent build, I thought this was worthy of a bit of exploration and fortunately my son is currently studying aeronautical engineering at one of the UK's top engineering universities so I thought I would summon his help.  As a former Lynx Observer, I have a pretty good idea about the aerodynamics affecting rotor blades including the blade tips approaching supersonic speed, the effects of retreating blade stall etc, but I had no idea how to calculate the amount of lift from a given rotor disc with blades of a specific aerofoil section at a given angle of attack, or how to calculate the amount of energy you needed to accelerate them and then maintain a constant speed.  And after an hour on the phone last night in which he explained a whole raft of stuff involving differential equations, I'm still not entirely sure, but in a nutshell, it is this. 

 

The speed of sound at sea level in a standard ICAO atmosphere, is give or take, 1,100 feet per second (fps).  An aircraft travelling forward at 100 kts is around 165 fps, at the Rotadyne's cruise speed of 168 kts it is 280 fps.  You then have to add the forward rotational speed of the advancing blade.  Given that the effects of compressibility start acting around M0.85 (actually they start lower than that but they become much more noticeable at that M number), aircraft designers aim to keep the maximum speed of the blade tip below 935 fps (1,100 x 0.85).  That means that at cruise speed the maximum speed of the rotors can be 935 - 280 = 655 fps.  That in turn with a 45 ft blade equates to a rotor RPM of about 140.  The next bit of maths I didn't understand but essentially 140 RPM with that rotor blade aerofoil section cannot generate sufficient lift to get the aircraft off the ground.  To achieve a vertical take off, a minimum rotor speed of around 200 RPM would be necessary.  This equates to 940 fps.

 

Since the rotor disc itself is unpowered, it acts like an autogyro.  In other words, the disc only generates lift in forward flight.  And so to get that 200 RPM, you have to somehow drive the blades.

 

After this he had to make some assumptions about rotor blade mass (early rotor blades were extremely heavy; Rotadyne blades had a cast steel main spar) and the likely energy available from the compressed air bleed.  Based on a main spar of 25mm x 50 mm, 3 mm thick, the blade would weigh ~ 60 kg.  It's important when advancing the rotor speed to do it as quickly as possible to avoid the phenomena known as droop when the blades drop down below their normal flight path and can hit the airframe.  The YouTube video of a Rotadyne engaging rotors seems to show it going from stopped to full speed in about 15 seconds although I suspect its actually slightly longer and the video has been edited.  Either way the torque needed to accelerate the tip of a 60 kg blade from zero to 200 RPM in let's say, 20 seconds, is around 690ftlb.  The next bit got very hazy as he lost me in the maths, but essentially the compressed air fed alone would be insufficient to accelerate the blades up to 200 RPM and therefore needed the added thrust afforded by the fuel injectors - a sort of early afterburner if you like.

 

Of course once the Rotadyne starts to transition to forward flight, it benefits from transational lift and the energy needed to drive the rotor disc reduces.  At this point the fuel supply to the jets can be cut off and the compressed air supply withdrawn.  At that point the rotor disc generates lift but only in forward flight.  It's actually a very simple and neat system and although the pipework would be an engineering challenge, its a lot simpler than adding a drive from the engines to a main rotor gearbox (and then a tail rotor gearbox to counter the torque) or a separate lift engine (again with a tail rotor),

 

Hope that helps and once again apologies if this has hijacked.  Back to the build!

 

 

 

 

What a wonderful explanation of the intricacies of this aircraft. I had to read it two or three times to grasp the mathematics, but I think I got there in the end........I put it down to age!

Thank you to both you and your son for taking the time to explain the process so clearly.

I've actually printed it off to keep for future reference.

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Heather, nice job on the Rotodyne, it actually turned out most impressively - shame about the transfers but given their age you salvaged them well and repaired what you couldn't save excellently B)

 

It's been fun, thanks :D

 

Cheers,

 

Stew

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Well done Heather and thanks for a brilliant build thread :goodjob:

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Thanks for all the lovely replies. Watch out for the gallery pics later. :winkgrin:

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Fantastic build and as everyone else has said, kudos for the ‘recovery’ work. I also agree that there should be a place for builds like this in magazines. Just because a kit is old, it doesn’t mean that spectacular results cannot be achieved as you’ve more than amply demonstrated.

 

That pen thingy looks interesting. Could you use it for painting canopy frames.....?

 

Trevor

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12 minutes ago, Max Headroom said:

That pen thingy looks interesting. Could you use it for painting canopy frames.....?

 

Thanks Trevor. The pens could potentially be used for canopy frames, but this particular brand lacks density. I think @CedB has used some paint pens. Perhaps he might pipe up and advise on the ones he uses. 

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Thanks Heather :)

I have some Liquidex pens that I use (for their bright colours) on i/ps and the like and I like them, even if I have to blob some out and use a small brush.

I think Trevor might be referring to the bow pen - I had a go with one a while ago and found it difficult to get acrylic paint to flow properly, but that's probably just me not trying hard enough. Must have another try, one day...

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Can't believe how quickly you've put this together!

 

Great job! 👍

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15 hours ago, Heather Kay said:

 

. Soldering is bread and butter work for me in the "day job". I know it's something some of the BM community struggle with it. I'm not about to set myself up as any sort of expert. 

 

15 hours ago, Heather Kay said:

 

Hi Heather

This caught my eye, your explanations have been wonderfully clear, so if you could a soldering tutorial I'm sure would be a greatly appreciated.

Even a link to one on YouTube that you thought wasn't cobblers would be good.

 

Really enjoyed this build, as others said, the "saves" have been inspirational.

 

Cheers

T

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50 minutes ago, Troy Smith said:

so if you could a soldering tutorial I'm sure would be a greatly appreciated.

As I said, I’m not about set myself up as an expert. I can still make a right Horlicks of things quite easily! That said, I will have a think about a short tutorial. I’ll also see if I can find some decent online stuff as well. 

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Heather, that build was a treat to watch as you progressed through thick and thin. Thank you for showing what can be done with an old kit. I too have a liking for the older models due to their 'simplicity' and a harking back to days of yore. The Rotodyne has just gone up towards the top of the pile now, and it too may make an appearance in this GB, but it would be nothing like yours for quality of finish. I mentioned to a friend at our model club meeting earlier this week that when it comes to seams, filler and me, there is only one loser, and that is not the seams or filler! In all my years of modelling, I have never got the hang of eradicating seams!

 

I am looking forward to seeing this one in the Gallery.

 

All the best,

 

Ray

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She looks gorgeous in the gallery Heather, lovely job :)

 

7 hours ago, Troy Smith said:

Even a link to one on YouTube that you thought wasn't cobblers would be good.

I follow Paul Budzik on YouTube - some great techniques. Here's his guide to soldering:

 

 

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Thanks Ced!

 

I think that video sums things up nicely. I don't think there's anything I can add to that. 👍

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Absolutely fantastic! It's amazing what a difference those window runners make. One question: have you considered adding any droop to the rotor blades?

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19 hours ago, cngaero said:

What a wonderful explanation of the intricacies of this aircraft. I had to read it two or three times to grasp the mathematics, but I think I got there in the end........I put it down to age!

Thank you to both you and your son for taking the time to explain the process so clearly.

I've actually printed it off to keep for future reference.

Apparently he enjoyed it.  To quote: "it was a refreshing break from gas turbine thermodynamics revision"!

 

10 hours ago, CedB said:

I think Trevor might be referring to the bow pen - I had a go with one a while ago and found it difficult to get acrylic paint to flow properly, but that's probably just me not trying hard enough. Must have another try, one day...

I was stuck on a train to London today for a meeting unable to get internet connection on my work laptop and so having read about bow pens on here I thought I would Google them on my personal phone.  To my utter amazement I discovered that they are those funny screw devices that I inherited from my father in his engineer's drawing set.  I never had any idea what they were!  They say that everone learns something every day, that was it for me!  Thanks to Heather for introducing them in this thread.

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Well done Heather, you have made a fantastic job of this aged-kit. You did the right thing in removing the rivet detail and the all of the little enhancements you have made have improved the look. Great recovery work on the decals and making good the stripes. You may already be aware but for future reference you could find these Decal Stripes handy, they come in several colours and I find them useful for tidying up cheatlines and other straight edges.

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22 hours ago, Chewbacca said:

Without wishing to hijack this fascinating thread and excellent build, I thought this was worthy of a bit of exploration and fortunately my son is currently studying aeronautical engineering at one of the UK's top engineering universities so I thought I would summon his help.  As a former Lynx Observer, I have a pretty good idea about the aerodynamics affecting rotor blades including the blade tips approaching supersonic speed, the effects of retreating blade stall etc, but I had no idea how to calculate the amount of lift from a given rotor disc with blades of a specific aerofoil section at a given angle of attack, or how to calculate the amount of energy you needed to accelerate them and then maintain a constant speed.  And after an hour on the phone last night in which he explained a whole raft of stuff involving differential equations, I'm still not entirely sure, but in a nutshell, it is this. 

 

The speed of sound at sea level in a standard ICAO atmosphere, is give or take, 1,100 feet per second (fps).  An aircraft travelling forward at 100 kts is around 165 fps, at the Rotadyne's cruise speed of 168 kts it is 280 fps.  You then have to add the forward rotational speed of the advancing blade.  Given that the effects of compressibility start acting around M0.85 (actually they start lower than that but they become much more noticeable at that M number), aircraft designers aim to keep the maximum speed of the blade tip below 935 fps (1,100 x 0.85).  That means that at cruise speed the maximum speed of the rotors can be 935 - 280 = 655 fps.  That in turn with a 45 ft blade equates to a rotor RPM of about 140.  The next bit of maths I didn't understand but essentially 140 RPM with that rotor blade aerofoil section cannot generate sufficient lift to get the aircraft off the ground.  To achieve a vertical take off, a minimum rotor speed of around 200 RPM would be necessary.  This equates to 940 fps.

 

Since the rotor disc itself is unpowered, it acts like an autogyro.  In other words, the disc only generates lift in forward flight.  And so to get that 200 RPM, you have to somehow drive the blades.

 

After this he had to make some assumptions about rotor blade mass (early rotor blades were extremely heavy; Rotadyne blades had a cast steel main spar) and the likely energy available from the compressed air bleed.  Based on a main spar of 25mm x 50 mm, 3 mm thick, the blade would weigh ~ 60 kg.  It's important when advancing the rotor speed to do it as quickly as possible to avoid the phenomena known as droop when the blades drop down below their normal flight path and can hit the airframe.  The YouTube video of a Rotadyne engaging rotors seems to show it going from stopped to full speed in about 15 seconds although I suspect its actually slightly longer and the video has been edited.  Either way the torque needed to accelerate the tip of a 60 kg blade from zero to 200 RPM in let's say, 20 seconds, is around 690ftlb.  The next bit got very hazy as he lost me in the maths, but essentially the compressed air fed alone would be insufficient to accelerate the blades up to 200 RPM and therefore needed the added thrust afforded by the fuel injectors - a sort of early afterburner if you like.

 

Of course once the Rotadyne starts to transition to forward flight, it benefits from transational lift and the energy needed to drive the rotor disc reduces.  At this point the fuel supply to the jets can be cut off and the compressed air supply withdrawn.  At that point the rotor disc generates lift but only in forward flight.  It's actually a very simple and neat system and although the pipework would be an engineering challenge, its a lot simpler than adding a drive from the engines to a main rotor gearbox (and then a tail rotor gearbox to counter the torque) or a separate lift engine (again with a tail rotor),

 

Hope that helps and once again apologies if this has hijacked.  Back to the build!

 

 

 

 

60 kg. Believe me they are heavier than that.

 

Rich

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Excellent build,you might be interested in Aeroplane Monthly this month as it has a large article all about the Rotodyne.

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3 minutes ago, fatalbert said:

Excellent build,you might be interested in Aeroplane Monthly this month as it has a large article all about the Rotodyne.

Saw that. Its a very informative article.

 

Terry

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13 hours ago, fatalbert said:

Excellent build,you might be interested in Aeroplane Monthly this month as it has a large article all about the Rotodyne.

Something to look out for in a couple if months, especially after Heather's wonderful build.

Steve.

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