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Author Topic: "V" STABILIZER, REQUEST FOR DESIGN HELP  (Read 1319 times)
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Thurman
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« on: December 23, 2018, 11:03:04 AM »

P-30 ADDICT needs help,

I have recently been interested in using a "universal joint" to achieve the generally down and right thrust of the propeller (another subject).  As a test platform for this idea, I am building a new P-30.  The "visual" design (not a rolled tube or rectangular box work) is such that I would like to use a "V" tail.  Having never used this type of stabilizer, I have no clue about where to start.  I would rather not learn all of what I need to know through trial and error or my interpretation of the generalized suggestions I have seen (for equivalent horiz, to V tail).  Don't trust my math, don't know the subtleties between power/glider, stability margins, etc.

I have read some powerful stuff on this part of the forum.  I cannot follow the math all the time, but I know the suggestions will be a rock solid starting point for me.

DESIGN BASICS: A rather standard P-30 (30" span & O.A. length, 50 grams all up weight). The general proportions, starting with a wing of about 130 sq. in., I have had reasonable success with stabs of 25%. and an aspect ratio around 4.3.  I use the PGI trim approach, with the thrust line through the vert./horiz. CG (65-75% mean chord), wing incidence set at a positive 2.5 degrees to this thrust line.  Flat bottom Stab incidence is set  for fast glide recovery.  Powered trim is determined by test flying, keeping the wing/stab setting constant, until full power results in a more or less vertical climb and positive cruise. (Well, that is the idea - right?)

Plain Jane pop-up wing D.T., tracker, exterior rubber winding, carbon capped rib const., etc.

My guess, is that I need to know A.) Area of the stabilizer; B.) Average chord and/or shape (aspect ratio); C.) V angle in degrees.  Any other suggestions for airfoil shape, settings for left glide turn, etc. would be greatly appreciated.  Photographs of implemented recommendations will be shown here.

Best regards, Thurman


 
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flydean1
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« Reply #1 on: December 23, 2018, 01:30:14 PM »

Where is George Perryman when we need him.  Oh!  He is passed on...and he didn't use math either.  This is only an opinion backed by observation which is many times faulty.

As a starting point, I would go with same stab area, but as projected as in seen from above or below.  30 degree (120 deg included angle).  If effective fin area is insufficient, like George, I would add a sub-fin at least for turn trim.

Agree whole heartedly on PGI force setup.

We're all looking forward to it.
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Thurman
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« Reply #2 on: December 23, 2018, 02:39:25 PM »

Thanks Dean,

Good ideas.  I forgot to add in description:

DESIGN BASICS: A rather standard P-30 (30" span & O.A. length, 50 grams all up weight). The general proportions, starting with a wing of about 130 sq. in., I have had reasonable success with stabs of 25%. and an aspect ratio around 4.3. 

The rear  mounted rudder of 9,o sq. in. is 7% of the wing - on top or bottom of the  fuselage as a fin.  I find no difference.  My smaller winged (103.5 a  Rising Light had a rudder of only 7,4 sq. in.

I use the PGI trim approach, with the thrust line through the vert./horiz. CG (65-75% mean chord), wing incidence set at a positive 2.5 degrees to this thrust line.  Flat bottom Stab incidence is set  for fast glide recovery.  Powered trim is determined by test flying, keeping the wing/stab setting constant, until full power results in a more or less vertical climb and positive cruise. (Well, that is the idea - right?)
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Yak 52
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« Reply #3 on: December 23, 2018, 03:19:47 PM »

Hi Thurman,

I would suggest you have a play with Curtis Suter's 'SailplaneCalc' excel file. It's designed as a tool for sizing the tail for sailplanes and for converting from conventional tails to V-tails. It's obviously not intended for P30 but does exactly the same job on any model, taking the dimensions and doing the maths for you.

There is a quick converter file which will work if your tails are mounted the same distance from the wing. If the fin is significantly ahead of or behind the tailplane then I'd suggest you put in the dimensions of your model into the full version of 'SailplaneCalc Cruciform Tail' to find the Vertical and Horizontal Tail Volumes (Vh and Vv) of you current P30. Then play around with the areas and angle of in 'SailplaneCalc V-Tail' until you get the same numbers with a V version.

Rubber duration models have very large horizontal tail volume (Vh) and modest vertical tail volume (Vv). this means that the V-tail will probably come out a bit flatter than the 120 degree included angle as the tail needs to do much more work in the pitch axis and less in the yaw axis.

The projected area method leads to rather less area than you actually need. The finished V-tail should have the same total area as the original tailplane plus the fin area. If you think about it the V version is now working to give the same pitching force as the tailplane was but now also has to provide the neccessary yawing forces at the same time. So it needs a bit more area than just the projection.

The excel files can be found here:
http://tailwindgliders.com/Files.html

Using your numbers in the converter I get
Current Tailplane area: 32.5in2
Fin area: 9in2
New V-Tail area: 20.75in2 each side
V tail dihedral angle: 27.8 degrees (included angle 124.5 degrees)


Jon
« Last Edit: December 23, 2018, 03:32:51 PM by Yak 52 » Logged
slopemeno
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« Reply #4 on: December 30, 2018, 11:26:41 AM »

Worth a read: http://www.charlesriverrc.org/articles/design/vtailmsganthology.htm
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Thurman
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« Reply #5 on: December 31, 2018, 02:46:17 PM »

WOW!,

A strong  note of thanks to Dean, Jon and Slopemeno.

Jon, I am going with your numbers derived from your recommended 'SailplaneCalc'.  I wasn't kidding about my lack of computer knowledge.  Hey, I tried several times to open the software...not knowing that I needed Excel !

Slopemeno's recommendation of the Charles River site was nearly life-changing and most enjoyable.  I understand now why there is such enthusiasm and participation in the glider events at the NATS.  Astonishing models/technology/aeroscience.

Makes me wonder how these heavy-hitters would approach the design of a P-30 model.  That high point of Western Civilization.

Thanks to all,

Thurman

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Tmat
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« Reply #6 on: December 31, 2018, 04:14:21 PM »

Thurman,

The Charles River site gives you some good guidelines on how to size the Vee tail. I'm curious as to why you would want to use a Vee tail on a P-30? Just to be different?
I think it might get tricky to trim a duration rubber model (like a P-30) with a Vee tail. Typically, side thrust is balanced with rudder offset and possibly stab tilt to trim the turn in the climb and the glide. And stab decalage along with down thrust and CG is used to adjust the climb attitude and glide for minimal sink. But with a Vee tail, a change to rudder will also affect decalage and so on. To get a rudder effect you might have to skew the entire tail assembly. I think it will be much trickier to trim than with a separate vertical and horizontal stabilizer. I know of no current winning rubber duration models with Vee tails. And I think the difficulty in separating the yaw and pitch effects for climb and glide is why.
But by all means give it a try and let us know how you make out!

Tmat
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flydean1
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« Reply #7 on: December 31, 2018, 04:55:12 PM »

Rudder turn issues could be dealt with using a sub-fin and rudder, like George Perryman.  Then the V-tail could be used in the conventional pitch trim sense.
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FLYACE1946
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« Reply #8 on: December 31, 2018, 07:07:54 PM »

Did someone mention George Perryman? He designed the Sig Mini Maxer and after his recommendation to increase the dihedral from 1 3/4 per the plan and make it 2 1/4 inches. I took his advise and built it that way. The sig mini maxer has a smaller wingspan than P-30 size but the way the improved Mini Maxer performed well it was lost oos way back just after getting flown and chased along with a flock of birds until I went after the wrong bird. You know George studied the bird wing shapes for a reason.
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OZPAF
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« Reply #9 on: December 31, 2018, 08:30:57 PM »

You have been given some good advice here but thought of a possible problem.

I noticed that you are trimming using the PGI approach and this may affect your V tail approach as part of the optimisation of the climb may require changing the fin area. If you are confident of your area being close than this is not an issue however if you would like to have some room for change then perhaps you could move say 20 -30% of the fin  to an under slung fin for alteration/adjustment.

This would alter the V Tail calculation that Jon has thoughtfully provided with a reduced total area and flatter included angle

As a quick example I moved  2sq" into a sub fin changing the VTail total surface area to 39.7 sq".

The included angle then becomes 130 deg.

Thus a VTail of 130deg included with a underslung fin of approx. 2sq".

This would also help in isolating rudder/elevator trim adjustments as mentioned by Flydean and Tmat.

Happy New Year and good luck

John

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lincoln
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« Reply #10 on: February 03, 2019, 06:06:29 PM »

You said you don't like math, but if you are confident you know the proportions of a conventional tail that would work, the following is a very quick way to come up with a comparable v-tail:
http://www.charlesriverrc.org/articles/design/markdrela_vtailsizing.htm
DJ Aerotech has a slightly different way. I'm not sure which is which, but one of the two is based on control authority and one on stability.  I would imagine the latter would give slightly better results on a free flight model.

If you don't have a calculator handy that has stuff like arctangent on it, there's one on Windows 10. Or at least there's one on my copy. The little up arrow on the left changes what the upper buttons mean, and you'll see that one of them says tan-1. You can also find scientific calculator apps on line. The one I use is for an HP-41. That uses RPN, though. RPN is superior but takes a little getting used to.
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Thurman
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« Reply #11 on: February 05, 2019, 12:32:00 AM »

Thanks Lincoln!

Just in time.  I am about to cut balsa.  Oh, I like! math, just not very proficient.  My computer calculator has all the functions needed so I will see if I can learn something.

I am also tempted to add a small subfin like Perryman used to uncouple the turn setting.  In the past I have used an (all) sub rudder with a conventional stabilizer.  I can see no difference in performance - some tall grass scrubbing when glide testing.  No problem on normal DT since a pop up wing, which I prefer, makes for a wild and crazy descent which makes every "landing" contact at a different place.

MANY Thanks again to all commenters as each has been most helpful.  I did notice that the Perryman Speckled something-or-other was a One Design P-30 Event at the Nats last year.  I have yet to find the results, but I did find the plan and it must have been a fun time.

I am still planning to post pictures/drawings of the final configuration.

Best regards to all.

Thurman
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