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Old Sep 12, 2012, 02:31 PM
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It's all a compromise ---
If were not for the VTO setup -it would all be different
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Old Sep 12, 2012, 02:57 PM
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The preliminary designs and the early prototypes of the F4 Phantom had a straight wing and stab. The dog tooth and dihedral on the outer portion of the wing were a fix for some quirks in its flight behaviour, and the tail anhedral was introduced to counteract the increased dihedral from the wing. It ended up being a brilliant and long lived plane, but started its life like a runt.
[edit] correction, my bad. The spiral stability issues that dictated the adoption of the polyhedral wing were detected in the wind tunnel, so the prototypes were born with the oddly shaped polyhedral wing. Incidentally, I think it works better than plain dihedral to keep the wingtips safe in a landing, and it might be a bonus in carrier operations.
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Old Sep 12, 2012, 03:25 PM
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Originally Posted by Brandano View Post
Well, the harrier has a strongly swept and high set wing. the anhedral in that case is to obtain neutral stability in a roll.
One thing I remember from my few hours in the Harrier is that there are flight regimes where pushing the left pedal makes you roll left and others where pushing the left pedal makes you roll right. That will get your attention.

Of course it doesn't quite get your attention the way that being 30 feet above the runway with no airspeed gets your attention.
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Old Sep 12, 2012, 03:34 PM
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I think that most planes are designed for cruise, or to have the best behaviour possible for the pilot in the portion of the flight where he can pay the least attention to the airplane quirks. Of course, practice makes sure you can handle things, but you don't want the pilot to puzzle whether pressing the left pedal is the right thing to do while he is busy evading a threat. In the case of the Harrier I think you are supposed to spend as little time as possible hovering, even though that is what makes it an unique plane.
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Old Sep 12, 2012, 05:05 PM
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Originally Posted by Brandano View Post
I think that most planes are designed for cruise, or to have the best behaviour possible for the pilot in the portion of the flight where he can pay the least attention to the airplane quirks......
That is very much the case with most man carrying airplanes which are intended for covering significant distances most of the time. Airliners are very good examples of such. They are designed to have the most energy efficient state when cruising at a narrow range of speeds at the usual cruise altitude conditions. The rest is simply making it easy to get up to altitude and to come down without too much of an issue.

Sparky Paul may be able to shed some light on this but during one of the many "energy crises" some authourity demanded that passenger jets slow down to "save fuel". Not only did this produce the opposite effect but the food trolleys had to be pushed uphill towards the nose and held back on the return "run" to the galley. By cruising at a speed which required a higher angle of attack the fuselage was no longer aligned with the direction of the airflow which caused more drag and used more fuel. And the wing had to run at a higher angle of attack which put it into an area of the lift drag for those airfoils where they generated more drag at slower speeds than at the proper cruise speed. All in all a major fail on all counts.

The Predator UAV is no exception to this. With the long sailplane like wings it is very much optimised for a fairly narrow range of efficient speeds.

Jet fighters, on the other hand, are all about the maneuvering. Only with the arrival of better engines and the ability to compute an airframe which would maneuver well AND be somewhat drag efficient did we see the first of the "super cruise" jets which would cruise supersonic without afterburning.
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Old Sep 15, 2012, 05:36 PM
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Gentlemen,

Here's a YouTube video
Prototype Inverse VTail RC Electric Plane Test Flight, Scratch build. (2 min 53 sec)

It's my flying buddy's "what would happen if...?" design. Note that it has a flat wing of no special planform.

Comments are welcome.

Jim R.
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Old Sep 15, 2012, 06:54 PM
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Nice job - -If you want to try a little more stability -just add a couple of 2" wide tips angled up a few degrees
These are very effective - so tape em on at first with some micropore or blenderm tape
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Old Sep 17, 2012, 08:59 AM
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Here's my design

So, here is my design -->
Name: inv_v-tail.jpg
Views: 60
Size: 58.1 KB
Description: Is the tail area too large?
The problem is that according to XFLR5 it's very sensitive to control surface movements. I can only use 2 degrees of up elevator otherwise the plane stalls.
The ruddervator hinge is at 70% of the chord.
Name: cm_alpha.jpg
Views: 34
Size: 70.1 KB
Description: The upper curve is with 2 degrees of elevator up deflection.
I used the following tail volume coefficients:
Vh=0.45
Vv=0.04
And the corresponding areas are:
Ah=634 cm^2 (98 sq in)
Av=780 cm^2 (120 sq in)
The total area of the V-tail is Ah+Av and the dihedral angle is -48 deg.
The tail moment arm is 1000 mm (39 in).
The wing area is 6500 cm^2 (1008 sq in) and the EDA is 10,5 deg.
Static margin is 10%.

This is the second plane design I'm working on using XFLR5. The first design was a flying wing. And XFLR5 showed that it's elevator was very sensitive. I didn't believe it because the elevons were already so small that I did not dare decrease their area. So I built the plane, went flying aqnd guess what - it was really sensitive. Now I'm more inclined to believe what XFLR5 is telling me.

What do you think - is the v-tail too big or am I misusing XFLR5 somehow?
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Old Sep 18, 2012, 01:53 AM
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It's hard to tell from that sketch but it does seem like it's got a generous amount of tail area.
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Old Sep 18, 2012, 02:19 AM
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The total tail area is 1414 cm^2 (219 sq in). It's seems quite a lot to me too, but that's the area I got with the chosen tail volume coefficients.

I compared the design with Mark Drela's Bubble Dancer and found that the horizontal tail area is comparable, although the fin area is smaller on the BD.
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Old Sep 18, 2012, 12:48 PM
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Comparing one design to another in that way only works if the wing area and tail moment are the same. It's all about the percentages for area comparison.

In any event the scaling of your drawing is obviously WAY out of kilter given the areas you mention in the table of sizes and such. With a wing area of 1008 sq inches your effective horizontal tail area of 120sq inches is only 12%. That is on the critical side of things although the relatively long tail moment of 39 inches obviously makes up for a lot on that count.

Out of curiousity where did the calculations tell you to put the CG to achieve that 10% SM?

I also see from the data in the 3D drawing that it's showing a 0 wing loading. You may want to insert a resonable weight for the wing to work with the XFLR5. It may be getting confused by that.

As for your 2 degrees of elevator you need to consider what you're looking at when it tells you that more than 2 degrees will produce a stall. What it is saying is that in level gliding you will have a 2 degree TRIM range before it slows down enough to stall from whatever speed you start from. If you start from the minimum sink speed as your "normal" condition then of course it'll only require a degree or two to slow down and reach a stall speed. But if you start from the Vms, which is about as slow as you'll want to fly in any event, and see how much down trim angle it takes to reach the best L/D and from there a sporty but non diving high speed penetration mode for fighting back upwind then you'll likely find that the trim range from a high speed penetration flying setting to the minimum sink speed is more like a 4 to 5 degree range. As you lower the stability margin down to a more efficient 5 to 6% you'll find that this range of angle narrows somewhat.

But during actual flight maneuvers such as pulling up in turns and other pitch maneuvering you'll need a lot more than this amount of trim range to work with.

You're also fighting with the disparate needs of lots of control surface angle and area to achieve a good yaw response to work with the polyhedral wings vs the small amount of angle and area needed for pitch. This is simply the nature of the game with V tails and polyhedral. It simply means that you need to use good servos and build tight slop free control linkages that are not temperature sensitive.

In my experience this means that you will avoid Nyrod type plastic tube in tube pushrods like the plague since they expand and contract a lot with temperature changes. And cable in tube systems tend to have too much internal slop for truly demanding setups. It's primitive but after trying so many options I keep coming back to wood and wire. Pushrods made with wood n' wire seem to hold their trim well over a wide range of temperature changes. And designed and executed well they provide a good solid linkage.

I must admit though that I have not tried using carbon tubes or rods for pushrods at this point. I've stuck with wood simply because wood was just easier to work with and does the job very nicely.
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Old Sep 18, 2012, 01:12 PM
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Quote:
Originally Posted by martig View Post
So, here is my design -->
Attachment 5162253
The problem is that according to XFLR5 it's very sensitive to control surface movements. I can only use 2 degrees of up elevator otherwise the plane stalls.
What do you think - is the v-tail too big or am I misusing XFLR5 somehow?
I've given up on XFLR5 entirely. The pretty graphs make it look like there is far more accuracy in the results than there is, and it produces a lot of errors. AVL is harder to use, but produces better results for stability and control. Dan's CG calculator produces AVL output for your design, which helps:

http://www.rcgroups.com/forums/showthread.php?t=1106300

You can even use the AVL output to create your airplane in CRRCSim, and fly it before you build it!

Pitch sensitivity, even for a flying wing, is entirely dependent on the static margin. Move the CG forward and/or reduce the control throws if you want it less sensitive in pitch. A lot of RC airplanes are quite flyable with zero static margin. where it takes no elevator input to trim to any airspeed. Many aerobatic airplanes and discus launch gliders are flown like that. They basically stay in whatever pitch attitude you put them.

If you use Dr. Drela's recommendations for tail volumes, you will have a glider that handles well (this looks like a glider, is it?) - attached.

Kevin
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Old Sep 18, 2012, 02:10 PM
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Guess I'll have to learn to use AVL then. I looked into creating CRRCSim models. Seems that there is quite a bit of manual labor involved. Could be worth it though.

I happened to read your thread about XFLR5 inaccuracy. You may be on to something.
I've seen some comparison with wind tunnel data. The comparison looked good enough to me. It was an older version of XFLR5 though.

My calculations are based on the same article you attached.

It will be an aerial photography platform. So it will be heavier and with a larger fuse than a glider.
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Old Sep 18, 2012, 02:42 PM
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Originally Posted by BMatthews View Post
Comparing one design to another in that way only works if the wing area and tail moment are the same. It's all about the percentages for area comparison.
The wing area and tail moment are very similar. BD 's wing area is 1000 sq in and tail moment arm is 36 inches.

Quote:
Originally Posted by BMatthews View Post
In any event the scaling of your drawing is obviously WAY out of kilter given the areas you mention in the table of sizes and such. With a wing area of 1008 sq inches your effective horizontal tail area of 120sq inches is only 12%. That is on the critical side of things although the relatively long tail moment of 39 inches obviously makes up for a lot on that count.
So, it seems to you that the tail is too small?

Quote:
Originally Posted by BMatthews View Post
Out of curiousity where did the calculations tell you to put the CG to achieve that 10% SM?
133 mm (5.24 in) from the leading edge.

Quote:
Originally Posted by BMatthews View Post
I also see from the data in the 3D drawing that it's showing a 0 wing loading. You may want to insert a resonable weight for the wing to work with the XFLR5. It may be getting confused by that.
The drawing is just for illustration. I have inserted the weights of the various parts of the plane. You can see the total mass there. For some reason it's showing zero wing loading. I think it's always been that way. Maybe I'm doing something wrong here.

Quote:
Originally Posted by BMatthews View Post
As for your 2 degrees of elevator you need to consider what you're looking at when it tells you that more than 2 degrees will produce a stall. What it is saying is that in level gliding you will have a 2 degree TRIM range before it slows down enough to stall from whatever speed you start from. If you start from the minimum sink speed as your "normal" condition then of course it'll only require a degree or two to slow down and reach a stall speed. But if you start from the Vms, which is about as slow as you'll want to fly in any event, and see how much down trim angle it takes to reach the best L/D and from there a sporty but non diving high speed penetration mode for fighting back upwind then you'll likely find that the trim range from a high speed penetration flying setting to the minimum sink speed is more like a 4 to 5 degree range. As you lower the stability margin down to a more efficient 5 to 6% you'll find that this range of angle narrows somewhat.

But during actual flight maneuvers such as pulling up in turns and other pitch maneuvering you'll need a lot more than this amount of trim range to work with.
I did the analysis in the constant lift mode. The lower curve corresponds to the trim AoA.

Quote:
Originally Posted by BMatthews View Post
You're also fighting with the disparate needs of lots of control surface angle and area to achieve a good yaw response to work with the polyhedral wings vs the small amount of angle and area needed for pitch. This is simply the nature of the game with V tails and polyhedral. It simply means that you need to use good servos and build tight slop free control linkages that are not temperature sensitive.

In my experience this means that you will avoid Nyrod type plastic tube in tube pushrods like the plague since they expand and contract a lot with temperature changes. And cable in tube systems tend to have too much internal slop for truly demanding setups. It's primitive but after trying so many options I keep coming back to wood and wire. Pushrods made with wood n' wire seem to hold their trim well over a wide range of temperature changes. And designed and executed well they provide a good solid linkage.

I must admit though that I have not tried using carbon tubes or rods for pushrods at this point. I've stuck with wood simply because wood was just easier to work with and does the job very nicely.
I was thinking of incorporating the servos into the tail surfaces. I know it's a bad idea to add weight to the tail, but I think it will make my life a little easier than using long pushrods.
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Old Sep 19, 2012, 11:54 AM
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Canada, ON, Toronto
Joined Jun 2012
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I can confirm that inverted v-tail works well without dihedral and ailerons. I really like the way it flies... smooth, stable, and with nice banking turns. It's not particularly aerobatic, but I have managed to do some decent loops... and some fairly sloppy rolls, but all my rolls tend to be a little sloppy.

I've tried it on both a twin-boom pusher and a small delta. Here's an overly long video (with too loud music) of my little delta. It was underpowered, but still managed to be a fairly nice flyer.

Simple Mini Delta Wing (RC plane, KFm2 airfoil) (5 min 17 sec)


Attached is a pic of the bigger version that I plan to maiden soon.
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