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Jan 28, 2015, 10:33 AM
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Quote:
Originally Posted by Brandano
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.
I know Ran has mentioned the possibility of problems when the tail is in the prop wash from the rear motor. Do you know of any videos that would demonstrate this ? And does it matter whether the rear motor tilts or not?
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Jan 28, 2015, 10:35 AM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by JulietKiloMike
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?
As the plane begins to transition into forward flight the down wash from the wing rotors extends backwards and will push down on the horizontal stabalizer. This will push the tail down and the nose will come up preventing forward acceleration. The fully flying horizontal tail matches the incoming air flow allowing good aerodynamic control without this problem. Watch videos of the full scale XC-142 or CL-84 and you will see that the horizontal stabalizer is tilted dramatically in Slow Forward Flight (SFF).

The need for a fully flying horizontal stabalizer can be avoided if you mount the horizontal stabalizer up high, as in a T tail. This is slightly preferable for structural reasons as a fully flying horizontal stabalizer is a little harder to build.
Last edited by Ran D. St. Clair; Jan 28, 2015 at 10:44 AM.
Jan 28, 2015, 10:48 AM
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leadfeather's Avatar

Motor postion


One of the design decisions you can exploit to your advantage is the position of the main motors as mentioned earlier by Brandano.

One effect to exploit is motor forward/aft position. The more forward the motor is mounted, the greater the shift in cg for the entire craft will be when the wing/motors tilt up. this will help provide the load for the tail rotor.

Also the distance the motors are mounted below the wing will determine how far forward the lift from the main fans are relative to the cg in hovering flight. Again the further under-slung the motors are the more weight (during hover) will be transferred to the tail rotor.

When choosing the motor position, there are also other considerations as have been mentioned... torques and moments (structural and aerodynamic) , structural and servo stresses, etc. In aircraft design, it's all about trade off's to accomplish the overall design goals.

I also agree with others in regards to lengthening the aft fuselage. The greater tail volume will dampen pitch and make forward flight and hovering much easier.
Last edited by leadfeather; Jan 29, 2015 at 07:53 PM.
Jan 28, 2015, 11:21 AM
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Ran D. St. Clair's Avatar
You have the entire brain trust working for you now :-)
Jan 28, 2015, 12:18 PM
JulietKiloMike's Avatar
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Quote:
Originally Posted by Ran D. St. Clair
You have the entire brain trust working for you now :-)
Haha yes I do. Thank you so much guys for your awesome input! This plane will be as much mine as it is yours

I've read through your guys' ideas and have decided to re-draft the aircraft with a twin-boom T-tail, a pusher prop tail motor (no ducts--KISS is a good idea!), and lengthened / lowered motor mounts for maximum shift in CG and center of lift from the rotors during transition. I'll sketch it out and post my idea in a sec!
Jan 28, 2015, 01:07 PM
JulietKiloMike's Avatar
Thread OP
Here's a semi-scale sketch of the new design revision. The orange stuff is where we plan to install 1/8" plywood reinforcing (wing spars, servo mounts, internal structure, etc). I had to add landing gear to get enough clearance for the 10" main props after I lowered the nacelles.
Last edited by JulietKiloMike; Jan 28, 2015 at 02:20 PM.
Jan 28, 2015, 02:29 PM
JulietKiloMike's Avatar
Thread OP
Revised again! I lengthened the tail booms to give the elevator better pitch authority.
Jan 28, 2015, 05:57 PM
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leadfeather's Avatar
Looking good!

I would lengthen the booms further still to make the craft easier to control in pitch.

I'd also move the tail fan back further too. This will dampen pitch in hover and give you more pitch authority to overcome the two forces trying to nose the craft up during the transitions. During the transitions, the prop blast from the main fans will try to pull the tail down. Additionally, with the wing at a huge angle relative to the tail during transitions, (large decalage) there will be a strong pitching moment due to the interaction of the wing and the tail, trying to force the nose up. A strong tail fan moment (fan thrust x distance from cg) will help overcome these two pitching moments.

You tail fan support is OK but I would consider instead to run a beam back from the fuselage to clean it up and also to make the yaw servo implementation cleaner as well. Are you planning on tilting the rear fan for yaw control in hover or are you planning on using the ailerons for yaw control in hover?
Jan 28, 2015, 06:05 PM
JulietKiloMike's Avatar
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Quote:
Originally Posted by leadfeather
Looking good!

I would lengthen the booms further still to make the craft easier to control in pitch.

I'd also move the tail fan back further too. This will dampen pitch in hover and give you more pitch authority to overcome the two forces trying to nose the craft up during the transitions. During the transitions, the prop blast from the main fans will try to pull the tail down. Additionally, with the wing at a huge angle relative to the tail during transitions, (large decalage) there will be a strong pitching moment due to the interaction of the wing and the tail, trying to force the nose up. A strong tail fan moment (fan thrust x distance from cg) will help overcome these two pitching moments.

You tail fan support is OK but I would consider instead to run a beam back from the fuselage to clean it up and also to make the yaw servo implementation cleaner as well. Are you planning on tilting the rear fan for yaw control in hover or are you planning on using the ailerons for yaw control in hover?
Thanks LeadFeather! I would make the tail booms a bit longer, but I think I need to do some more CG calculations first to make sure that the aircraft doesn't end up super tail heavy. I'm also trying to cut the side of the aircraft out of one continuous sheet of foam board for better structural rigidity, so I'm limited to a fuselage length of 24" for now. As things start to flesh out I'll see if I can work some more length into the tail.

Right now I'm planning to use the ailerons to control yaw in hover, and haven't yet made provisions for yaw in forward flight. I'm thinking that running two pushrods from two servos to the tail for twin rudders would be a pain...do you think that putting a variable throttle mix on the main engines would work for yaw control in level flight?
Jan 28, 2015, 06:13 PM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by JulietKiloMike
Revised again! I lengthened the tail booms to give the elevator better pitch authority.
Getting better. Now make the tail even longer and the horizontal stabalizer half again larger. The fuselage length to wing span ratio should be about 1:1. The larger horizontal stabalizer will increase your pitch stability margin in fast forward flight and possibly even help make it hover better.

Next step, focus of making it light. Plywood is heavy. Get rid of allmost all of it. Foam board is actually quite strong all by itself. The main wing spar is almost the only real structure you will need. You will need some plywood patches to create hard points for mounting the landing gear but that is about it. Packing tape, like the kind used to seal up boxes is also a structural material. Wrap it around leading edges, trailing edges, and reinforce corners, etc. It even comes in colors if you want to make it pretty.

You don't actually need landing gear, and it is a disadvantage if flying off of grass. It also adds weight you don't need. If you really want to be able to do run on take offs and landings then make the fuselage deeper so the propeller tips will clear the grass. Otherwise, just plan to hover for takeoff and landing.

Find yourself a good pilot or become one yourself (if you aren't already). You need someone who is comfortable with both airplanes and multicopters. If you aren't already a pilot then plan on learning on more conventional and cheaper RC aircraft, not this one. Also plan on a year of weekends to get up to speed.
Jan 28, 2015, 06:17 PM
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Ran D. St. Clair's Avatar
Quote:
Originally Posted by JulietKiloMike
Thanks LeadFeather! do you think that putting a variable throttle mix on the main engines would work for yaw control in level flight?
That will work fine.

Don't worry about the aircraft becoming tail heavy. The battery will be on the floor in the nose and will balance it out.

I recommend 3.3Ah.

Keep it light....Keep it light...Keep it light....Keep it light...Keep it light....Keep it light...
Jan 28, 2015, 07:26 PM
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v22chap's Avatar
This bird is beginning to look really cool ... I might even have to build one of these !!!!
I like the short fat look ... if you can keep it and still get it to work .
Jan 28, 2015, 08:57 PM
JulietKiloMike's Avatar
Thread OP
Quote:
Originally Posted by Ran D. St. Clair
Getting better. Now make the tail even longer and the horizontal stabalizer half again larger. The fuselage length to wing span ratio should be about 1:1. The larger horizontal stabalizer will increase your pitch stability margin in fast forward flight and possibly even help make it hover better.

Next step, focus of making it light. Plywood is heavy. Get rid of allmost all of it. Foam board is actually quite strong all by itself. The main wing spar is almost the only real structure you will need. You will need some plywood patches to create hard points for mounting the landing gear but that is about it. Packing tape, like the kind used to seal up boxes is also a structural material. Wrap it around leading edges, trailing edges, and reinforce corners, etc. It even comes in colors if you want to make it pretty.

You don't actually need landing gear, and it is a disadvantage if flying off of grass. It also adds weight you don't need. If you really want to be able to do run on take offs and landings then make the fuselage deeper so the propeller tips will clear the grass. Otherwise, just plan to hover for takeoff and landing.

Find yourself a good pilot or become one yourself (if you aren't already). You need someone who is comfortable with both airplanes and multicopters. If you aren't already a pilot then plan on learning on more conventional and cheaper RC aircraft, not this one. Also plan on a year of weekends to get up to speed.
Ok, I relent...that's two against one for lengthening the fuselage...I'll stretch it out a bit more

Is there a specific ratio of wing area to horizontal stab area that I should be shooting for?

Ditto on the lightness bit. I've ditched the internal structure from the initial prototype, and will just be doing a few isolated hard points / spars out of plywood or maybe balsa, whichever seems to work best.

I was hoping to install landing gear in order to be able to test the aircraft in phases; we have around 4 months to complete our project, so I think a good proof-of-concept would be a month or so of rolling takeoffs w/ conventional RC gear, then bring in the KK2.0 with OpenAero and get hovering going, then try transitions between the two. My primary flying site is Rancho San Antonio (you have probably flown there a few times), so landing gear is pretty necessary for takeoffs / landings. But, if it proves to be too heavy, maybe I'll try some wire skids or something.

I've had a little over 4 years experience in RC...I don't consider myself an expert pilot by any means, but I am comfortable flying aerobatic quads and fixed-wing. I have a Multiplex Twinstar II that I converted into a CDF tanker a while back, a half-dead cessna, and a few ultra micros that I fly, and am just starting to get into 3D. Hopefully between the airplanes and the multicopters I have some of the experience tidbits necessary to get this thing in the air. But, you do live just a few miles from me...maybe I could pick up some VTOL piloting tips from you sometime!
Jan 28, 2015, 08:59 PM
JulietKiloMike's Avatar
Thread OP
Quote:
Originally Posted by v22chap
This bird is beginning to look really cool ... I might even have to build one of these !!!!
I like the short fat look ... if you can keep it and still get it to work .
Thank you, sir! I appreciate the compliments

I'm hoping to move the tail boom out just a liiiittle bit more, but hopefully the overall look stays similar (I'm planning to keep the tail rotor right where it is). I do like the current look, too
Jan 28, 2015, 09:20 PM
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leadfeather's Avatar
Is there a specific ratio of wing area to horizontal stab area that I should be shooting for?

If you do a Google search for "calculating tail volume" you'll get a bunch of hits. This one is a good site to start with.
Calculating tail volume


You want to error on the side of a large tail volume on a project such as this.


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