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Jan 27, 2015, 06:10 PM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by PlumbBob View Post
I happened to notice an rcgroups post the other day where the author suggested using a small pipe cutter.
I would have to try it before recommending it. For reasonably small CF Rod up to about 3/32" I use curved shears like this Shimano brake housing cutter:

http://www.ebikestop.com/shimano_tl_...FYqIfgodC5UALg

I still have to sand the ends to remove the frayed edges, which I do in running water in the sink.
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Jan 27, 2015, 08:44 PM
JulietKiloMike's Avatar
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Quote:
Originally Posted by Ran D. St. Clair View Post
Your prop sizes and motors are reasonable. That makes an 8:1 ratio in terms of the distance from the CG. If you locate the tail rotor as necessary to maintain the correct ratio, and put the horizontal stabalizer in front of the tail rotor, you will end up with a very short coupled design and likely poor pitch stability in forward flight. You would be better off to go with twin booms and put the horizontal stabalizer behind the tail rotor. If your fuselage is wide, like 5 1/2 inches, then you can extend twin vertical stabalizers from the sides of the fuselage in the manner of the WUV.

https://www.rcgroups.com/forums/show....php?t=2307837

I am not really recommending it, as there are better solutions, but it would solve your CG problem with minimal change to the overall design.

Cutting carbon fiber is no big deal. Wrap the tube, rod, or whatever with scotch tape and cut it with a fine tooth saw. A hack saw works in a pinch. Suck up the dust immediately with a vaccume cleaner or do your cutting and sanding under running water in the kitchen sink. You can wear rubber gloves to avoid itchy skin, but I never bother. CF cuts easily and is not really dangerous like some other materials.
Okeydoke, I'll try it! I have a CF rod sitting somewhere unused...I'll probably mess around with plywood for the initial prototype and start working with CF once / if things get ironed out.

Also, thank you PlumbBob for the pipe cutter suggestion. I might try that too!
Jan 27, 2015, 08:46 PM
JulietKiloMike's Avatar
Thread OP
Quote:
Originally Posted by Brandano View Post
if the motor pods are slung under the wing the rotor disk will move forward during the conversion, and give you a little more leeway in placing the CG. This comes at the cost of greater torque on the tilt servo, that is usually already under a very stressful load. A better option to move the GC back would be to increase the horizontal stabilizer size. How do you plan to control yaw? Ailerons in the rotor slipstream, tilting tail rotor or both?
Our current wing tilt servo is a Hitec 645MG Ultra Torque Servo (7.7kg / 55g / 0.24sec)
http://www.horizonhobby.com/645mg-ul...ervo-hrc32645s

I'm hoping that it'll be enough to tilt our wing in-flight, but that stage is still a long way off...
Jan 27, 2015, 09:24 PM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by JulietKiloMike View Post
Our current wing tilt servo is a Hitec 645MG Ultra Torque Servo (7.7kg / 55g / 0.24sec)
http://www.horizonhobby.com/645mg-ul...ervo-hrc32645s

I'm hoping that it'll be enough to tilt our wing in-flight, but that stage is still a long way off...
I checked the specifications on the servo and it is a reasonable servo to use. Make sure the linkage uses its full range of travel and you should be OK.
Jan 27, 2015, 09:48 PM
JulietKiloMike's Avatar
Thread OP
So I played around a little with the CAD mock-up looking for a way to move the tail rotor forward while keeping the T-tail further back. Do you guys think that something like this would work?

Notching the vertical stab would let me fit a tail rotor forward of the main tail plane...the only problem is that the air needs a way to get through the fuselage. Maybe a duct of some sort? Or I could ditch the whole rear fuselage chunk and make it a boom-type system. What do you think?
Jan 27, 2015, 09:49 PM
JulietKiloMike's Avatar
Thread OP
Quote:
Originally Posted by Ran D. St. Clair View Post
I checked the specifications on the servo and it is a reasonable servo to use. Make sure the linkage uses its full range of travel and you should be OK.
Good to hear! I got the highest-torque servo I could find in a reasonable form factor.
Jan 27, 2015, 10:28 PM
Winging it >
leadfeather's Avatar
The Canadair CL-84 from back in the 1960's is a very similar arrangement to what you are proposing. Based on the video below it appears to be a great VTOL craft. I think their arrangement/placement of the tail fan is a good choice for this type of design.

Of particular interest in the video is the amount of tail plane deflection during transitions to keep the fuselage level. This may be important for your craft as well.

Best of luck!

Canadair CL-84 (12 min 44 sec)
Jan 28, 2015, 12:17 AM
JulietKiloMike's Avatar
Thread OP
Quote:
Originally Posted by leadfeather View Post
The Canadair CL-84 from back in the 1960's is a very similar arrangement to what you are proposing. Based on the video below it appears to be a great VTOL craft. I think their arrangement/placement of the tail fan is a good choice for this type of design.

Of particular interest in the video is the amount of tail plane deflection during transitions to keep the fuselage level. This may be important for your craft as well.

Best of luck!

https://www.youtube.com/watch?v=pGH2Lg_pg-k
Thanks for the cool video link, leadfeather! I hadn't seen that particular airplane before...interesting! Do you know why the tailplane deflects so much during wing transitions?

It's interesting to see the tail rotor so far back, much like the XC-142. Do you know why they chose to put it so far behind the CG? Rand was advising that a CG distance ratio of around 8:1 would be preferable to maintain pitch stability, but the long tail boom on the CL-84 seems to suggest a much larger ratio. It's curious to see more than one tiltwing aircraft with the same far-aft tail rotor configuration, given its inherent instability.
Jan 28, 2015, 12:31 AM
Registered User
Ran D. St. Clair's Avatar
The CL-84 is a fine aircraft but not a good basis for your design in terms of pitch stability. It has counter rotating, and most importantly variable pitch propellers on the tail. When the wing was vertical it was mostly balanced on the forward propellers with the tail rotor acting as control and trim.

With a fixed pitch propeller for a tail rotor you have a very different situation The tail rotor can only lift, it can never push down. In order to provide adequate pitch control and stability it needs to be reasonably loaded.

Another possible solution would be to spin the tail rotor backwards. This has recently become a popular method to allow quads to fly inverted, so it can be done, but I am not familiar with generally available ESC's that are capable. The ESC's and flight controllers that do it on the commercially available quads are highly integrated and very custom to their specific application. It also requires a special propeller with a symmetrical airfoil. You could probably modify a regular prop if you had to, or you might be able to use one of the ones from the aerobatic quads.

As for the tail plane, it either needs to be fully flying, with a wide range of angle as on the CL-84 and the XC-142, or it needs to be mounted up high, or the tail rotor needs to be powerful enough to hold the tail up despite the downwash on the horizontal stabilizer during transition. I mention the latter solution because that is the one used by the commercially available model of the CL-84 that you can buy.

That model deals with the fixed pitch tail rotor issue by using a complex wing tilt mechanism that shifts the wing forward as it tilts up. This puts more weight on the tail rotor for adequate pitch stability.

That model also tilts the tail rotor from side to side to provide good yaw control. The ailerons alone provide yaw control on the XC-142 model, and it is adequate but weak. Fortunately good yaw authority is less critical for a model than for the full scale aircraft.
Jan 28, 2015, 12:56 AM
JulietKiloMike's Avatar
Thread OP
Quote:
Originally Posted by Ran D. St. Clair View Post
The CL-84 is a fine aircraft but not a good basis for your design in terms of pitch stability. It has counter rotating, and most importantly variable pitch propellers on the tail. When the wing was vertical it was mostly balanced on the forward propellers with the tail rotor acting as control and trim.

With a fixed pitch propeller for a tail rotor you have a very different situation The tail rotor can only lift, it can never push down. In order to provide adequate pitch control and stability it needs to be reasonably loaded.

Another possible solution would be to spin the tail rotor backwards. This has recently become a popular method to allow quads to fly inverted, so it can be done, but I am not familiar with generally available ESC's that are capable. The ESC's and flight controllers that do it on the commercially available quads are highly integrated and very custom to their specific application. It also requires a special propeller with a symmetrical airfoil. You could probably modify a regular prop if you had to, or you might be able to use one of the ones from the aerobatic quads.

As for the tail plane, it either needs to be fully flying, with a wide range of angle as on the CL-84 and the XC-142, or it needs to be mounted up high, or the tail rotor needs to be powerful enough to hold the tail up despite the downwash on the horizontal stabilizer during transition. I mention the latter solution because that is the one used by the commercially available model of the CL-84 that you can buy.

That model deals with the fixed pitch tail rotor issue by using a complex wing tilt mechanism that shifts the wing forward as it tilts up. This puts more weight on the tail rotor for adequate pitch stability.

That model also tilts the tail rotor from side to side to provide good yaw control. The ailerons alone provide yaw control on the XC-142 model, and it is adequate but weak. Fortunately good yaw authority is less critical for a model than for the full scale aircraft.
Ah, that makes sense. I forgot that the tail rotors on the XC-142 and CL-84 use variable pitch props--bringing a fixed-wing prop to a halt and spinning it backwards seems like a complicated ordeal that would preferably be avoided through CG placement.

I don't currently have plans to hinge the tail rotor, so I might end up mounting it further forward on the fuselage with some sort of duct assembly to allow airflow through the fuselage from top to bottom.

I don't mean to tire you with my endless stream of noobish questions, but why must the tailplane be fully flying? Is the tail plane angled with a positive AOA during transition to keep the nose of the aircraft down, or the tail of the aircraft up?
Jan 28, 2015, 01:05 AM
JulietKiloMike's Avatar
Thread OP

Look at my beautiful artwork! (not)


I sketched up an idea (not to scale) of what the aircraft might look like with a duct assembly and the tail rotor further forward. I'm thinking about switching out the GWS 5043's I have now for one of these and making the tail rotor a pusher configuration, which would get rid of the weird / possibly problematic notch in the vertical stab. I probably also want to move the engine nacelles forward more so that the CG shift is more pronounced during transition. Do you guys see any glaring issues that I'm missing?
Jan 28, 2015, 04:40 AM
Registered User
If you make the tail large enough the plane will be closer to a tandem wing, and the CG can be much further back than 25% of the wing's MAC. It's the plane's MAC as a whole that determines pitch stability. I'd try to keep the motor thrust axis fairly close to the tilt axis, to reduce servo loads, but lengthening the nacelles won't really help in moving the rotors forward. Lowering them would, but in a full sized plane it would also reduce the ground clearance for conventional landings, if they make part of the plane's mission. The reason for a T tail is to keep it at least partially out of the main rotor wake when the plane is transitioning. There's other approaches to the issue, like the all moving tail on the CL-84 and XC142, or the very large elevator of the V22.
Jan 28, 2015, 08:40 AM
Winging it >
leadfeather's Avatar
Quote:
Originally Posted by JulietKiloMike View Post
I sketched up an idea (not to scale) of what the aircraft might look like with a duct assembly and the tail rotor further forward. I'm thinking about switching out the GWS 5043's I have now for one of these and making the tail rotor a pusher configuration, which would get rid of the weird / possibly problematic notch in the vertical stab. I probably also want to move the engine nacelles forward more so that the CG shift is more pronounced during transition. Do you guys see any glaring issues that I'm missing?
The T tail as you have drawn has a huge notch in the vertical stabilizer; not much structure left. Instead you might want to consider twin fins. Twin fins will be much stronger and will not need to be notched.
Jan 28, 2015, 08:52 AM
Winging it >
leadfeather's Avatar
Below is an RC Cl-84 performing very well. Transitions to forward flight look really good. Transitions to hovering flight have a little bump in the first video; it appears to me that the pilot should have slowed the forward speed more before initiating the transition to hover to avoid this. In the second video, the (different) pilot slowed down properly before tilting the wing up nd the transition to hover was much smoother.

RC Canadair CL-84 Dynavert by Shenzhen Unique Model Tech (7 min 54 sec)


Unique Model CL 84 Dynavert advertising video (vertical takeoff and landing) (3 min 28 sec)
Last edited by leadfeather; Jan 28, 2015 at 09:05 AM.
Jan 28, 2015, 10:31 AM
Registered User
Ran D. St. Clair's Avatar
Quote:
Originally Posted by JulietKiloMike View Post
I sketched up an idea (not to scale) of what the aircraft might look like with a duct assembly and the tail rotor further forward. I'm thinking about switching out the GWS 5043's I have now for one of these and making the tail rotor a pusher configuration, which would get rid of the weird / possibly problematic notch in the vertical stab. I probably also want to move the engine nacelles forward more so that the CG shift is more pronounced during transition. Do you guys see any glaring issues that I'm missing?
Your idea has merit. Just move the tail back farther until it is no longer near the Prop. That will also improve forward flight stability. Props in slots are generally a bad idea as they are noisy and lose some efficiency. Make sure your tail boom is narrow enough so it doesn't block a significant portion of the prop disk. Alternatively mount the motor and prop below the tail boom. It doesn't matter if the tail blocks the inlet to the prop disk as long as it is 1/2 diameter away or so as the inlet air velocities are much lower. Alternatively, use twin booms on either side of the tail rotor. Alternatively cut a hole in the rear of the fuselage and make a round duct. Alternatively, extend twin vertical stabalizers back behind the stub fuselage and put the tail rotor in between. There are at least a half dozen ways to do this and it mostly comes down to what is simplest to build and is also structurally sound. Save yourself a lot of grief and follow the KISS principle (Keep It Simple...) Stay away from ducts. They are not KISS.

Finally, I do not recommend the prop you have chosen as the pitch is too high. Hovering aircraft need low pitch props. A typical ratio is that the pitch is half the diameter or a bit more, like a 9-5 for example. It does not matter whether you use a normal rotation or reverse rotation prop for this purpose. Electric motors can turn either way, and the prop can be mounted as a pusher or a puller. You will need to tilt the tail rotor slightly to counter the torque of the tail rotor. That is usually just a matter of a small shim or washer under the motor mounts.
Last edited by Ran D. St. Clair; Jan 28, 2015 at 10:53 AM.


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