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Old Jan 24, 2015, 05:48 AM
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How about adding a couple of fuselage booms to an X8!

Something different!

I decided to modify my X8 to possibly improve the tendency to tip stall and make the beast a bit more stable in the process! so here it is - my cure for (some) of the handling problems with the X8 flying wing.

I reasoned that by extending the elevator to a more conventional moment arm and doing away with the elevons, replacing them with conventional ailerons, would have to make the model a more enjoyable and easier to control machine. Perhaps not quite so touchy in the pitch attitude and with the aid of reasonably sized flaps make the X8 easier to land as well.

It will be equipped with a FY-41AP Lite, Dragonlink RC, 10,000 mah for motor, plus 3300 for OSD, cams, VTX and RX. (total weight around 5.5kg)

It has not been flown yet but almost ready to fly and should be an interesting maiden flight when finally finished and weather is suitable for that first nervous flight.

I have flown many small and large scale model planes and currently fly a Penguin with RVOSD, pan tilt, lights etc and had a lot of enjoyable flights up to a max of nearly 7 klms, so pretty confident I can handle this machine. Will take some video and post here in due course. I'm calling it an X10 ( X8 + 2 booms!)

Any thoughts from you experts about whether the CG should be any way different from an (unboomed) X8 - I would appreciate your thoughts and suggestions..
Cheers,
Supcub
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Last edited by supcub; Jan 24, 2015 at 05:51 AM. Reason: Transferred from "FPV Equipment" to this more appropriate thread
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Old Jan 24, 2015, 05:19 PM
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Old Jan 13, 2015, 09:24 PM
The following Posts were copied from the original thread under the "Equipment" heading.



My apologies to all for any inconvenience caused!

supcub




Heppy Ket
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Interesting indeed. I sure hope you have lots of long grass where you are I'm not sure how you would go at sorting out the CG, it will surely be forward of where it is, do please post some videos.


Dogdude
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Looks cool



Quote:
Originally Posted by Heppy Ket View Post
Interesting indeed. I sure hope you have lots of long grass where you are I'm not sure how you would go at sorting out the CG, it will surely be forward of where it is, do please post some videos.

Supcub
Thanks Heppy Ket, Yes I know what you mean, but will be ultra careful when the time comes to fly!


fOaMmaSh87
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Looks like a stealth plane! Nice.


Daemon
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It may work, but not likely be as efficient as a conventional plane.
A normal conventional plane does all of its lifting with the main wing which
has a cambered airfoil which generates all the upward lift, plus a
negative pitching moment (pulls the nose down).
The CG is usually placed well back from that of a flying wing airfoil, but
still in front of the center of lift which also causes a negative pitching
moment.
The horizontal stab and elevator back at the end of the tail boom/s then
must provide negative lift to create a positive pitching moment countering
the negative moment of of both the cambered airfoil and CG.

A flying wing normally has no elevator at the end of a long boom, so the wing
as a whole must have both camber and reflex so it can generate lift with the forward
portion of the airfoil, and counter its own positive pitching moment with
the reflex. A flying wing airfoil is usually a bit less efficient for this reason,
but usually makes up for it with lower platform drag (no fuse, tail boom or tail).

The problem with adding a tail boom or booms to a flying wing, is that
you've still got the original lower efficiency flying wing airfoil with built
in reflex plus your adding more platform drag with the booms and tail group.
The center of lift of a flying wing airfoil is pretty far forward, so you
can't move the CG back nearly as far as you would with a conventional airfoil,
so the horizontal stab has to run close to zero angle of attack
which doesn't provide a lot of inherent stability (makes it very pitch sensitive).

For your rig, I would start with the CG pretty close to stock recommended (rear
end of front finger holes), and keep the decalage (relative angle of
horizontal stab versus that of main wing) pretty close to zero, with a pretty
low elevator rate. I wouldn't be surprised if you find it challenging
to find the initial working angle of the horizontal stab and elevator.



Supcub
Quote:
Originally Posted by Daemon View Post
It may work, but not likely be as efficient as a conventional plane.
A normal conventional plane does all of its lifting with the main wing which
has a cambered airfoil which generates all the upward lift, plus a
negative pitching moment (pulls the nose down).
The CG is usually placed well back from that of a flying wing airfoil, but
still in front of the center of lift which also causes a negative pitching
moment.
The horizontal stab and elevator back at the end of the tail boom/s then
must provide negative lift to create a positive pitching moment countering
the negative moment of of both the cambered airfoil and CG.

A flying wing normally has no elevator at the end of a long boom, so the wing
as a whole must have both camber and reflex so it can generate lift with the forward
portion of the airfoil, and counter its own positive pitching moment with
the reflex. A flying wing airfoil is usually a bit less efficient for this reason,
but usually makes up for it with lower platform drag (no fuse, tail boom or tail).

The problem with adding a tail boom or booms to a flying wing, is that
you've still got the original lower efficiency flying wing airfoil with built
in reflex plus your adding more platform drag with the booms and tail group.
The center of lift of a flying wing airfoil is pretty far forward, so you
can't move the CG back nearly as far as you would with a conventional airfoil,
so the horizontal stab has to run close to zero angle of attack
which doesn't provide a lot of inherent stability (makes it very pitch sensitive).

For your rig, I would start with the CG pretty close to stock recommended (rear
end of front finger holes), and keep the decalage (relative angle of
horizontal stab versus that of main wing) pretty close to zero, with a pretty
low elevator rate. I wouldn't be surprised if you find it challenging
to find the initial working angle of the horizontal stab and elevator.

Thanks for your excellent advice there Daemon. I can see that you have given this a lot of thought and that is the sort of information I was hoping to get by creating this hybrid wing/ twin boom model and posting here on RC Groups.

I also had similar thoughts about this modification, but the way I figured is if I make the stabilizer a full flying tailplane, with less camber on the upper surface (10%) than a fully symmetrical unit the result would be at least somewhere near stable. I anticipate having to make whatever changes needed to trim the flying stab to suit level flight especially at cruising speed.

I admit to not actually crunching the full numbers on this, but have drawn on more "gut instinct and a lifetime of experience with models" than anything more than basic aerodynamics.
So its just a bit of a combination of hope and good luck if it does fly with the sort of good manners and ease of control we all look for in our models... Time will tell!

All opinions and input much appreciated!

Supcub


fOaMmaSh87
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Have you flown it yet?

supcub

Quote:
Originally Posted by fOaMmaSh87 View Post
Have you flown it yet?

Not Yet Ben, too many other priorities atm.. not enough hours in the day. Also weather here in SA not suitable for the first flight anyway.

I will post a video of the flight when the time comes.




Supcub
Update Jan 22 15:
Ongoing delays because of problems with electronics. Mainly problems with FY-41AP Lite. unable to be easily resolved, so am in process of fitting RVOSD G5 in its place.
First test flight anticipated this coming weekend if autopilot performs as it should and depending on weather of course...

Supcub
The balance point can be seen here, tell me what you think - is this gonna be OK, or should it be somewhere different to this (to start with) ?
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Old Mar 05, 2015, 05:45 AM
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At last I can report to all interested, the twin boomed X10 (formerly X8) has flown.

Without going into details, I have to say (mildly) it was a "handful" to fly!

Despite my modifications to the wing tiplets,the X10 tip stalled twice - it went very close
to being styro confetti, the second tip stall and resulting spin occured at only 50 metres
above ground level, going down below tree level and LOS in the process, but thanks to good
luck and RVOSD in FBW mode the plane narrowly missed the deck. I was able to land the beast
although not very tidily as the throttle seemed to be stuck open about one third or so.

The landing was a little on the wild side and I was lucky to get out of it with only a
busted nose landing gear.
In the excitement of that first flight I had forgotten to deploy the flaps fitted to the
modified wings to slow things up a bit for the landing. They might have allowed the plane
to touch down at a reasonable speed instead of the 71 kph indicated on the nav screen when
played back after the mayhem. (At 73 Im still learning!)

Intense analysis of the flight afterwoods, led me to some conclusions about how to improve
the situation. Better, less flexible mounting of the RVOSD should help FBW stability,
removing the wing tiplets and reshaping the stubs to a more aerodynamic form should help
the poor rudder response. Adding leading edge slats to the outboard ends of the wings should
improve the tip stalling resistance and maybe add some stability at the same time.
All of these and a few other modifications including a new nose gear suspension are nearing
completion, so should be ready to go shortly.

I am looking forward to trying again to se if I can get this beast to fly in some sort of
reasonable manner and will keep you posted with results when the time comes to fly again.
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Old Mar 06, 2015, 02:48 AM
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maybe add some wing fences to prevent so much stall, like on a Mig15 wing. or add some outboard wing washout . Or just fly faster, more thrust! I think these X8 need the speed. For a more stall resistant big flying wing, maybe try the fat airfoil, wide wing like the Hercules, or the smaller Z2. My Z2 was always pretty stable, once flying (b--- to launch without a hyper motor)
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Old Mar 06, 2015, 04:31 AM
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Quote:
Originally Posted by BCSaltchucker View Post
maybe add some wing fences to prevent so much stall, like on a Mig15 wing. or add some outboard wing washout . Or just fly faster, more thrust! I think these X8 need the speed. For a more stall resistant big flying wing, maybe try the fat airfoil, wide wing like the Hercules, or the smaller Z2. My Z2 was always pretty stable, once flying (b--- to launch without a hyper motor)
Funny you should mention wing fences BC, exactly what I also had a mind to do. I have some 1mm light carbon sheeting, might be worth a try.
Thanks for the suggestion!

SC
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