Tail Sizing Help.

[CloudyIFR]

Hello,

For a long time I wanted to convert my Waco Millenium thermal duration contest plane from a V-tail to Conventional tail but never did. When I broke a V-tail attachment recently it was a good opportunity to do so. Plus, I wanted to learn how to cut foam with a hot wire and also try out a new vacuum bagging process in the preparation of the flying wing I’m building.

I test flew her today and the first flight was very interesting, too much elevator control and not enough up elevator trim, so the launch up the hi-start was actually straight and level. I reduced the elevator throw and trimmed some up elevator in and the rest of the flights were great! I then started the flight trimming and dive test and have removed about 1/3rd of the nose weight I added. Still need to do more flight testing but that will have to wait for better weather.

The inflight photo is over Memaloose, OR of the original with the V-tail.
The next two are with the new Cruciform Tails and my new paint scheme underneath the wing.
The last photo is a chart of tail sizing values I chose. Vh/Vv.

I'd appreciate any design comments on my methodology. Seems the horizontal looks small and the vertical looks large, but the numbers make sense. Of course the proof isn't the numbers but in the flying. In flight I noticed no pitch issues and she flew real well during the dive test, making a nice gradual pull out.

I'll need more flight testing and experience to what I should be looking for to determine if the vertical size is correct.

Thanks for any comments.
Curtis
Montana

[HugePanic]

did you respect the stabalizer distance from the COG (or AC) or did you do your calculations only with the stabalizers sizes???

the vert-stab. looks pretty large, but this depends on what you whant to do with it...

[CloudyIFR]

From Wing AC to Stab AC.

I want it to thermal in turns and land well.

Curtis
Montana

[BMatthews]

I suspect you'll find that at some bank angle in the turns that the model will require some "up" aileron to avoid it wanting to wind harder into the turn. If that point occurs around a 60 degree angle then it's not hard to deal with. Turns at a lower bank angle will then be either neutral around the 30 to 45 degree range or slightly self leveling at lower angles. That's a nice range to shoot for. If you find you need to hold "top" aileron at lower than around a 60 degree bank other than the occasional blip to respond to turbulence or airflow in a thermal then your fin area is a little too big. If the model tends to hang a little tail low in the turns and wag side to side during turn transitions it's too small. Also you'll find that this spiral stability is tied to the CG location as well. If you're on the large side area wise moving the CG back will also produce a stronger tendency to want to wind into the turns a little harder.

[CloudyIFR]

Here's a video of the flight.

http://www.youtube.com/watch?v=0esRNxUfRu0

Curtis
Montana

[gyrocptr]

From my old FF experience, I agree with BMatthews. Wing-drop/nose-drop in a tight turn indicated oversize vertical fin.
How do you feel about adding some tip-dihedral at each wingtip? This could provide some automatic self-correction when the plane does that sudden wing-drop-down-slip.

[BMatthews]

It's impossible to tell how it feels to you. It's obvious you're getting some wing drop but it could be a bit of turbulence or it may be a tip stall because you tried to fly too slow or it could be the fin size.

If it's the fin then it'll be very consistent. If you are finding that you need to hold a constant amount of "top" aileron to hold the model in a consistent bank then that is a fin size related issue. If it's more a sporadic need then it's from turbulence in the thermal or from concentrated "slope" lift from the tree line like in the video or other odd airflow issues with the area you fly.

I'd suggest you get in some more stick time unless it's so bad that it's trying to lawn dart itself all the time. Once you get in some calm air flying where you can have the model trimmed to where it's almost flying itself then you'll be in a much better position to really study the situation.

The fin and dihedral is linked as gyrocptr suggested above. Adding even a 1/2 inch of dihedral to the very tips starting from the outer aileron point can make a huge difference.

You see, it's all related to the vertical tail volume coeffient. This is a number that is used to study and compare various airplanes. There's no "right" value other than a range for each aircraft type that has been determined over time and with experience. I saw such a chart for about 2 dozen designs some years back. The aileron models all had a lower range of values while the polyhedral models had a higher range of values. This is because the higher amount of side area presented by the dihedral needs more tail area to balance it out. With very little dihedral the optimum range of tail size decreases.

But as mentioned a while back go too small and the tail will show a lot of side to side wandering and indecision.

Here's a picture of the hack model that I ran some testing on about 15 or more years ago. You'll notice that the vertical area is quite small. But as shown here the model was a total delight to fly. When the vertical was only 1 inch taller it was a real bear to fly with constant top wing pressure needed for all but the most gentle turns. When it was 1.5 inches taller the model would not even fly level for long. I had to constantly correct it just to fly straight. Left to itself it would go into a spiral dive with that extra area. It was like balancing a broom handle in your palm. Now granted there's more area than shown in the picture. The fuselage blends to a flat tail area in the lower portion so the effective vertical area is about another 1/3 again as large as the upper portion. You may be able to see how this is in the picture.

[CloudyIFR]

Thanks guys, I really appreciate what I should be looking for. I only have about a half dozen flights with the new tails and it was pretty calm on the ground but around 20,000' there were a lot of standing lenticulars and the lift down low was very tight, turbulent and spotty. So like ya'll said, I need more stick time to really know. But at least I know what to look for.

I'll report back when work and Montana weather allows more flying.

Thanks
Curtis

[HerkS]

Curtis - If you do decide that the fin is too big, I'd suggest cutting that down, rather than cutting into those beautiful wing panels.

Also, remember that with the Millennium's graceful wing sweep you do get dihedral effect - particularly when you are at the higher angle of attack needed for thermalling.

[CloudyIFR]

Herk,

I agree with the wings, there is no way I'd ever cut into them but I do understand the effect it'd give but I'd cut my fin down as a first and probably last resort. But time will tell, I don't compete any longer but just enjoy the art of soaring.

Curtis

[markdrela]

From the video it appears that the wing drop is just common tip stall, due to an inappropriate planform/airfoil/washout layout. The tail areas don't have anything directly to do with it.

If you want to fix it, I suggest adding a bit of washout twist. You want to concentrate the twist over the inner portion of the outer panel, roughly between 50%-80% semispan, so that the entire 80%-100% tip portion gets a uniform reduced AoA. I'd start with maybe 1 degree of twist, which on a 6" chord is 0.1" at the TE. Increase only if necessary.

One effective way to twist a bagged wing is to hold the twist, while pouring boiling water over it. The epoxy will soften, and then harden again and hold the set when it cools.

[CloudyIFR]

Thanks Mark, I really appreciate your input. At least I know it's not my tails!

First thing I'll do before anything is get a bunch more flights on it then go from there.

Thanks again.
Curtis
Montana

[HerkS]

Curtis - This is an interesting thread. Please keep it going by letting us know more after you've flown the plane and made adjustments.

Curtis and I had communicated about this off the thread. I'm adding it here for the record.

What I saw in the video is that when the plane began to turn, it nosed down and picked up speed in the turn, then the nose would come up --- then it looked as if the flight speed just got too slow and it stalled. My gut feel is that it was nose-heavy.

However, unless you are flying the plane it's difficult to tell if the symptoms come from the natural behavior of the plane or the inputs of the pilot - or some combination. When you're not flying the plane it's really hard to tell.

[CloudyIFR]

Curtis - This is an interesting thread. Please keep it going by letting us know more after you've flown the plane and made adjustments.


Will do but don't hold your breath, two inches of snow on the first day of Spring!

Curtis
Montana

[Texas Buzzard]

From my old FF experience, I agree with BMatthews. Wing-drop/nose-drop in a tight turn indicated oversize vertical fin.
How do you feel about adding some tip-dihedral at each wingtip? This could provide some automatic self-correction when the plane does that sudden wing-drop-down-slip.
.................................................. ...........

Yes, if the fin (vert. stab.) is too large the ship will not allow the dihedral to act (take over) to correct automatically. That results in an ever increasing turn. Not too good for that makes one have to make control corrections.

I am a believer in some polydihedral ( tip dihedral) - a little will do.

If you have made a small handlaunched all balsa glider (16 to 20 inch w.s.) start with a 7% fin. Fly it. Gradually cut away area of the fin while flying it - oh yes do the cutting while the glider is on the ground - hard to cut in the air - ha ha.

When you have cut away a bit too much of the fin you will notice "DUTCH ROLL". A hunting from side to side....looks like the tail is rotating a bit. That means that the fin is too small.

In my humble opinion, the ideal fin size would be a bit larger that the area that allows Dutch Roll. Why?

Too large a fin forces the ship to stay in a turn or even makes the turn more steep.... not good. But the smaller fin will allow the dihedral to make the TURN MORE FLAT. I hope I said this so it's understandable. One can learn something from 16" w.s. handlaunched gliders quickly...and it is cheap too.

[CloudyIFR]

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If you have made a small handlaunched all balsa glider (16 to 20 inch w.s.) start with a 7% fin.

If you mean that the vertical tail area should be 7% of the main wing area;

Then I have:
Main wing area - 977sq inches
Thus my tail area should be: 0.07*977=68.39 sq inches vertical area

The current vertical area is: 72.88 or 7.4%

Still need to flight test before anything else.

Thanks
Curtis

PS Cutting dihedral into the wings is not an option as there is already aerodynamic dihedral due to sweep. Anyone know how to calculate the effect of sweep and dihedral?

[JetPlaneFlyer]

Anyone know how to calculate the effect of sweep and dihedral?
Yes :)

From my own experience with small scale swept wing jet models I' concluded that (measuring wing sweep at the 25% chord line) 5 to 6 degrees of sweep was needed to give the effect of 1 degree of dihedral.

I always appreciated that it was not quite this simple as the effect of sweep varies with angle of attack, sweep being much more effective (in terms of dihedral effect) at high AoA... but for my little free flight jets 5 -6 Deg sweep = 1 Deg of Dihedral worked well enough.

Some one better at math than me came up with a formula to calculate the effect scientifically... this is it:

Wing sweep equivalent to 1 Deg of Dihedral = ½ Arcsine(a/Cl)

Where:
a = slope of lift curve
Cl = Coefficient of lift

Here are the numbers for a 0.08 lift slope:

Lift coefficient: 0.1___0.2____0.3___0.4___0.5___0.6___0.7___0.8
Sweep______: 26.6__11.8___7.7___5.8___4.6___3.8___3.3___2.9

You can see that at high Cl sweep has a very strong dihedral effect, only 2.9 Deg of sweep being equal to 1 Deg of Dihedral at Cl=0.8, but at low Cl the dihedral effect of effect of sweep in much less pronounced. My rule of thumb 5-6 Deg matches well with the 0.4 - 0.5 Cl that my little jets fly at.

[HerkS]

I think that JPF has the sweep-dihedral question just about right for our purposes. You can slice it finer and finer, but in terms of practical usefulness, I think he has it pretty well in the middle of the ball park.

With regard to the size of the vertical tail:
Sailplanes with high aspect ratio (long - relatively heavy wings) have another issue that relates to fin size. The long wings have a lot of inertia about the yaw axis. So, there is a need for strong aerodynamic damping in yaw to control the effect of the mass of the wings. A built-up model that has an aspect ratio of ten has much - much less need of a big fin than one with a fiberglass and foam wing with an aspect ratio of 15. I believe that is one of the reasons that full scale competition sailplanes have such (apparently) large vertical tails. There is a square function in the equations, so the effect of long wings is magnified. You don't want the yaw-wobbles either.

Bottom line is that you can't make comparisons between the types very directly. Putting the plane in the air and working it through its maneuvers will tell the tale. Then as Texas Buzzard has suggested, make changes if they are needed.

[Texas Buzzard]

From an old-time model aerodynamicist Raul Hoffman:

3 degrees of sweep equals 1 degree of dihedral.

That would be 1 degree dihedral under each wing tip.

(Yes I know this is old, but it works.)

So a 30 degree sweep ( of both wings) = 10 degrees of dihedral.

[JetPlaneFlyer]

From an old-time model aerodynamicist Raul Hoffman:

3 degrees of sweep equals 1 degree of dihedral.


Buzzard,
If you read my post above you will see it's not quite that simple. If you are flying at quite high Cl (i.e. high AoA) then, yes, 3 deg of sweep can very well equal 1 deg of dihedral but if you are zipping along at high speed at a Cl of about 0.2 then you may need 12 Deg of sweep to give the same 1 deg dihedral effect.

You need to work out a compromise based on the intended use of your model.

[HerkS]

Curtis is interested in computing the effective sweep angle for a multi panel wing. That is where the wing panel has perhaps two or three sections, each with a different area and a different sweep angle. Does anyone have a method for computing the effective sweep angle for this kind of wing - so that Curtis can use the JPF supplied algorithm for dihedral effect due to sweep????

[CloudyIFR]

Herk,

See post #9.

http://www.rcgroups.com/forums/showthread.php?t=788953

I've about got it worked out in the spreadsheet, if you think 6.27 degrees of sweep for the Millenium seem about right?

Curtis

[gyrocptr]

Here's a link to a technique for determining an "equivalent dihedral angle" for a multi-panel wing.

http://www.rc-soar.com/tech/spiral_eda.htm

Perhaps you can take the sweep-angle of the quarter-chord line for each swept-wing-panel; convert it to corresponding "dihedral" angle (using the 1:6 ratio); and then use the technique to calculate an "equivalent dihedral angle" for the swept panels (?).

[HerkS]

Curtis - if by centroid of a wing panel he means the MAC for that panel. Then I have to say that it sounds reasonable to me. Giving the outer panels of the wing more influence on effective sweep angle sounds right (when you're doing this to figure dihedral effect due to sweep).

I do think that the Millennium number sounds about right.

[CloudyIFR]

Curtis - if by centroid of a wing panel he means the MAC for that panel. Then I have to say that it sounds reasonable to me. Giving the outer panels of the wing more influence on effective sweep angle sounds right (when you're doing this to figure dihedral effect due to sweep).

I do think that the Millennium number sounds about right.

Herk,
I used the MAC as that seems to work out as the centroid as far as I can figure. So now I have the Cl added and the sweep, I'm not sure if I'll add any info for the dihedral effect to the spreadsheet or not but just keep it as a note for myself. Hmmm? It's been an interesting exercise.

Curtis

PS More snow tonight so now flying in the next few days. They are threatening us with 60 degree weather next week!

[CloudyIFR]

Yesterday Montana weather and my job cooperated and I was able to fly the Millenium again with the new tails. The weather was just perfect! The dropping tip was still there so I removed 40% of the aileron to rudder compensation and now she doesn't drop a tip till she's really slow in a turn. I've also removed more nose weight and she's flying and landing like a dream!

I'm really pleased with my conversion, even though the vertical still looks too big! But at least I can recognize my model from somone else's!

Thanks for all the tips, I really appreciate them.

Curtis
Montana