Aerodynamics Discuss the concepts of aerodynamics here

V-bar for a twin boom

Old 06-08-2010, 05:07 AM
  #1  
hillbillynamedpossum
Rode the short bus
Thread Starter
 
hillbillynamedpossum's Avatar
 
Join Date: Aug 2009
Location: West Plains MO USA
Posts: 721
Default V-bar for a twin boom

Hey guys,

I built a twin boom.... it flew pretty good, but it had some stability issues. So Im building a new one.


So whats a good v- bar ratio for a full flying elevator? Also can the elevator be on the same plane as the wing.....does it need to be higher, lower, or what?

I feel like ideally it needs to be plumb out of the propwash and in the clean air....but that aint gonna happen.....the props too big....


HELP

Possum

Wing Root Chord (A): 9

Wing Tip Chord (B): 7

Wing Sweep Distance (S): 2

Wing Half Span (Y):24


Stabiliser Root Chord (AA):6

Stabiliser Tip Chord (BB):6

Stabiliser Sweep Distance (SS):0

Stabiliser Half Span (YY):5.875

Distance between both LE's (D): 20

Type of Stabiliser: std
Std. T-tail

Enter Static Margin, then 10%
%

Mean Aerodynamic Chord MAC =

Sweep Distance at MAC (C) =

From Root Chord to MAC (d) =

From Wing Root LE to AC =

From Wing Root LE to NP =

From Wing Root LE to CG =

Wing Area = 384

Stabiliser Area = 72

Wing Aspect Ratio = 6

Tail Volume Ratio, Vbar =.43

Last edited by hillbillynamedpossum; 06-08-2010 at 05:18 AM. Reason: how thw heck can I cut and paste that grapg from adamore?
hillbillynamedpossum is offline  
Old 06-08-2010, 06:10 AM
  #2  
Larry3215
Look out for that tree!!!
 
Larry3215's Avatar
 
Join Date: Mar 2006
Location: Gig Harbor, Wa USA
Posts: 7,061
Default

Do you have a picture?

What exactly were the stability issues?
Larry3215 is offline  
Old 06-08-2010, 11:54 PM
  #3  
hillbillynamedpossum
Rode the short bus
Thread Starter
 
hillbillynamedpossum's Avatar
 
Join Date: Aug 2009
Location: West Plains MO USA
Posts: 721
Default

Its "hunting" up and down......very very twitchy to elevator input. I think its called porposing or pungiod ossilation or some crap like that. Any way, Im gonna try making the booms longer, and the elevator bigger, with less throw, and more mecanical advantage to the servo......about 1:2 or 3:5. Mark 1 was 1:1 ......it had a huge rudder throw with the EPA at 100%


Possum
hillbillynamedpossum is offline  
Old 06-09-2010, 01:59 AM
  #4  
Water Boy
☺ ↓ My co-pilot ☺
 
Water Boy's Avatar
 
Join Date: Sep 2006
Location: Greenview IL
Posts: 907
Default

Sure it was not a CG issue? I know if you have the cg off it can make it twitchy.
Water Boy is offline  
Old 06-09-2010, 02:39 AM
  #5  
Larry3215
Look out for that tree!!!
 
Larry3215's Avatar
 
Join Date: Mar 2006
Location: Gig Harbor, Wa USA
Posts: 7,061
Default

That could be several things.

From the numbers listed above, I think the elevator is large enough but it sounds like the plane is maybe a bit short coupled. Is the span 48" and the distance from leading edge of wing to leading edge of stabilizer 20"?

Thats stubby enough to be sensitive in pitch but not really all that bad. Many biplanes are that short.

Id look at linkages and servos first before I started to re-build.

Remove the prop first!

Turn the plane rx ON and then grab the trailing edge of the elevator and try to move it UP/DOWN by hand.

Is there any movement at all? Any slop? Any play?

If so - fix it.

Next, turn the rx off and remove the linkage from the elevator. Move it up/down by hand. Does it move easily, freely and siiiiiiiiilky smoooooooth?

If not - fix it.

Are the booms and the elevator structure itself strong enough to resist any flexing, bending?

If not - fix it.

Using the Tx, move the elevator up/down several times then stop. Note the exact location of the trailing edge of the elevator when the tx stick is centered. Now move the tx stick UP to full deflection then release the stick.

Does the elevator return to the exact same place at center?

Now move the tx so the elevator goes to full down, then release the stick.

Does the elevator return to the first exact same center point?

If not, then you have a centering problem that could easily cause your porpoising.

You have some binding in the linkages or hinges or a bad servo or too small a servo or too cheap a servo or some of all that.

Fix it

Wouldnt hurt to be sure the CG is in the 25% to 30% cord range while your at it.
Larry3215 is offline  
Old 06-09-2010, 04:20 AM
  #6  
hillbillynamedpossum
Rode the short bus
Thread Starter
 
hillbillynamedpossum's Avatar
 
Join Date: Aug 2009
Location: West Plains MO USA
Posts: 721
Default

Larry,

The numbers posted apply to the plane that Im building. The last plane was about 20% smaller.

My question was really.....Is the elevator big enough?


I think I tracked most of the oscillations down to stretch in the pull-pull lines. If this one does it, Ill use some E-string or just break down and pay $10 for some spider wire. Im using a SG90 servo from Jeff for the rudder and it seems to be holding up pretty good. If MK 2 still acts up Ill go with a futaba metal gear servo....Ive got a few laying around, I just cant hardly bring myself to use a $20 servo on a 20 cent plane. Besides that Ive never crashed on account of Jeffs $4 servos. Ive really only torn up about 3 of em in all my mis-adventures. (I doggone near hit a deer last night, but It run under some trees before I could) The 3 gram servos are crap though.

There is almost no flex at all in the booms....only a little side to side.....zero up and down. Theyre reinforced with aluminum.


I had a thought.....If the prop wash is actually my culprit....why not use two full flying elevators out side of the booms glued to one dowell running thru the whole mess? I might have to experiment.

I agree that Im getting a little close coupled.....I cant help it. I cant break out any thing longer than 24" without using the big brakes at work and they tend to stress crack the aluminum.

Heres some pics of MK 1... http://www.wattflyer.com/forums/showthread.php?t=55950 The one (MK 2) Im building now is bigger, has no rudders(sorry Larry), will only use pull pull on the elevator, and Im gonna use a LOT more aileron and less fixed wing.

The ailerons on MK 2 will be about 45% of the wing. Im gonna try 3 gram servos, one for each aileron, just cause I got some extra that I aint broke yet. In the future Ill probabally go with SG90's from Jeff. I got the Y harness built and the servos glued down. I really need a 6 channel FM computer radio....but alas, I dont got one.

I have the dowell rod (pivot) at 25% of the chord of the rudder....Seems like I read a discussion about that somewhere.


Well Im tired and stinky....Its time for a shower and bed....gonna be warm again tommorrow and them doggone air conditioners wont fix them selves


Possum
hillbillynamedpossum is offline  
Old 06-09-2010, 04:31 AM
  #7  
Larry3215
Look out for that tree!!!
 
Larry3215's Avatar
 
Join Date: Mar 2006
Location: Gig Harbor, Wa USA
Posts: 7,061
Default

Having the elevator in the prop wash isnt the problem. Other than pushers, 99% of all planes have the elevator in the prop wash and they dont have that proplem

Its your pull/pull set up.

Ive run pull pull setups like that before and they dont work well at all. You have way to many turns and bends in it. No way thats ever going be a 'tight' set up. Im quite certain it doesnt center worth beans either. Better string will help but not enough.

Thats the whole reason for the oscillation.

Your going to have to mount one or two servos out on the booms and go with straight runs.

I also think 3 gram servos are way too small for anything other than a very light indoor plane.
Larry3215 is offline  
Old 06-10-2010, 01:51 AM
  #8  
hillbillynamedpossum
Rode the short bus
Thread Starter
 
hillbillynamedpossum's Avatar
 
Join Date: Aug 2009
Location: West Plains MO USA
Posts: 721
Default

Thanks Larry,

I had a notion that bending the string was causing some issues. Im pretty far into the MK 2 build....far enough to find out I need to move the motor forward as far as I can. I really made a mess of things, but I think it'll work out.

Against both of our better judgement (I agree....I HATE 3 gr servos) Im gonna use 3 gr servos for the ailerons. I only have one SG90 left and its a spare for the bipe. Im gonna give them about a 4:1 mechanical advantage and hope for the best. Im scrapping the single servo pull-pull on the ailerons and going with one servo for either side.


Possum
hillbillynamedpossum is offline  
Old 06-10-2010, 03:41 AM
  #9  
hillbillynamedpossum
Rode the short bus
Thread Starter
 
hillbillynamedpossum's Avatar
 
Join Date: Aug 2009
Location: West Plains MO USA
Posts: 721
Default

Heres a few pics of MK2. Itll weigh about 17 oz max all up.


Possum
Attached Thumbnails Click image for larger version

Name:	06092010_007.jpg
Views:	125
Size:	156.9 KB
ID:	129943   Click image for larger version

Name:	06092010_008.jpg
Views:	130
Size:	158.8 KB
ID:	129944   Click image for larger version

Name:	06092010_009.jpg
Views:	142
Size:	155.6 KB
ID:	129945  
hillbillynamedpossum is offline  
Old 06-10-2010, 03:58 AM
  #10  
Larry3215
Look out for that tree!!!
 
Larry3215's Avatar
 
Join Date: Mar 2006
Location: Gig Harbor, Wa USA
Posts: 7,061
Default

Those itty bitty servos are going to have fun with those HUGE ailerons

It wuold help if you could counter balance that elevator but that would also make your CG problem worse.

Here is a good site that will help you figure out the servo torque you need for any given control surface size.

I think you will find that you need a LOOOOOOOT more servo than you have

http://www.mnbigbirds.com/Servo%20To...0Caculator.htm

Dont worry, you're not alone in that. Most of the planes we fly use servos that dont have enough torque to really do the job properly
Larry3215 is offline  
Old 06-11-2010, 03:28 AM
  #11  
hillbillynamedpossum
Rode the short bus
Thread Starter
 
hillbillynamedpossum's Avatar
 
Join Date: Aug 2009
Location: West Plains MO USA
Posts: 721
Default

I dont really disagree about the aileron servos...I know they are too small, but thats what I have on hand.

I ended up using piano wire on the pull pull for the elevator.......there is no slop any where. Holding the plane by the nose and looking down the center pod I saw part of the problem. The whole elevator flexes and flutters at WOT. Oh well, I can live with that for now.

I took it out for the maiden tonight and it flew pretty good. I tried to get a few hi-speed passes heading for the camera, but my wife didnt really go for that.

I did a quick pitch speed calc and my pitch speed is somewhere around 40 mph.... Power up 400 sport 1050 kv on a 3s1p and a 9050 gws prop if you dont care to check my math. It doggone sure FELT faster than that. At WOT the fancy oscillation was back. But I did finally discover the cause.....wing flex. I actually saw the thing flapping the wings on a hi speed dive pull out.......well what the heck can I expect for a flat foamy, huh?

All in all, I learned a few things, and like all my planes, this has really just been an experiment.....Id call it a success.


Possum
hillbillynamedpossum is offline  
Old 06-11-2010, 03:47 AM
  #12  
Larry3215
Look out for that tree!!!
 
Larry3215's Avatar
 
Join Date: Mar 2006
Location: Gig Harbor, Wa USA
Posts: 7,061
Default

Cant ask for more than that as long as your having fun

Congrats!
Larry3215 is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
Crash9
Aerial Photography
82
12-12-2010 08:12 PM
hillbillynamedpossum
Scratch and Kit Built Aircraft
0
05-30-2010 03:47 PM
hillbillynamedpossum
Scratch and Kit Built Aircraft
2
05-28-2010 10:47 PM
hillbillynamedpossum
Aerodynamics
2
05-25-2010 04:10 AM
GreenAce92
Scratch and Kit Built Aircraft
14
02-08-2008 02:54 AM

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Quick Reply: V-bar for a twin boom


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.

Page generated in 0.09460 seconds with 17 queries