New Products Flash Sale
Thread Tools
Old Jul 09, 2015, 12:32 PM
beecat is offline
Find More Posts by beecat
Getting Ahead of Himself
beecat's Avatar
United States, AR, Little Rock
Joined Mar 2015
73 Posts
Discussion
Elevons with poor pitch control

I'm a novice builder, and I seem to have a recurring problem. That problem is that my delta-style planes with elevons don't have much pitch authority.

I'm mostly concerned with my most recent build. It's not the best design, as I think the airfoil is too steep on the leading edge, but it flies decently. I also had to add a whole lot of weight to the nose to get it balanced (almost 12oz). I think my control surfaces are sufficient, as it has extremely fast roll rates (see attached photo).

I can't get it to loop, though.

Any ideas?
beecat is offline Find More Posts by beecat
Reply With Quote
Sign up now
to remove ads between posts
Old Jul 09, 2015, 12:52 PM
balsa or carbon is offline
Find More Posts by balsa or carbon
Hot glue held together by foam
United States, WA, Vancouver
Joined Aug 2010
4,089 Posts
I don't know what you can do for the delta in the photo - but if you were to make a new airframe , try making the elevons larger . And moving the motor forward ( either on the nose as a tractor , or in a slot at about 30% - 40% ) so you won't have to add dead weight to get a good CG balance . It also looks like your motor is small ( under-powered? ) relative to the airframe size .

Here is my design for a delta ( Simplest Simple Delta ) , will roll and loop all day long :
http://www.rcgroups.com/forums/showthread.php?t=2145562

This video is of an SSD built recently by Marblekit here on RCG :

Red Dragon "On Steroids" (2 min 19 sec)
balsa or carbon is offline Find More Posts by balsa or carbon
Reply With Quote
Old Jul 09, 2015, 01:05 PM
kcolraw is offline
Find More Posts by kcolraw
Kcolraw
United States, NC, Asheville
Joined Feb 2015
412 Posts
Your elevon's are to small. Bigger elevon will change the cg as well since your adding to the wing cord. My 30 inch wing I built has elevon that taper from 1.5 inches at the tip to 1 inch at center line. Even the preview shot in that video has bigger (proportional to the wing area ) elevons.
kcolraw is offline Find More Posts by kcolraw
Reply With Quote
Old Jul 09, 2015, 02:36 PM
eflightray is offline
Find More Posts by eflightray
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
15,199 Posts
Quote:
Originally Posted by beecat View Post
I'm a novice builder, and I seem to have a recurring problem. That problem is that my delta-style planes with elevons don't have much pitch authority.

I'm mostly concerned with my most recent build. It's not the best design, as I think the airfoil is too steep on the leading edge, but it flies decently. I also had to add a whole lot of weight to the nose to get it balanced (almost 12oz). I think my control surfaces are sufficient, as it has extremely fast roll rates (see attached photo).

I can't get it to loop, though.

Any ideas?
Is it based on a plan ?, if so which.
12 oz in the nose seems ridicules, isn't the batter as far forward as possible ?
How do you know the CG is correct ?, what you are describing, (low pitch authority and not looping), could be a very nose heavy model.

Try this CG Calculator check where it should be --

http://fwcg.3dzone.dk/
eflightray is offline Find More Posts by eflightray
Reply With Quote
Old Jul 09, 2015, 03:00 PM
rocketsled666 is offline
Find More Posts by rocketsled666
If it flies, I can crash it.
rocketsled666's Avatar
Joined May 2012
805 Posts
Do you need a lot of up trim to fly level? If you invert, do you then need a lot of down elevator to fly level? If so, your CG is too far forward.
rocketsled666 is offline Find More Posts by rocketsled666
Reply With Quote
Old Jul 09, 2015, 03:28 PM
beecat is offline
Find More Posts by beecat
Getting Ahead of Himself
beecat's Avatar
United States, AR, Little Rock
Joined Mar 2015
73 Posts
Thanks for all the info!

It's my own design (I design for fun, not function ). I calculated CG using that site, which is what led me to the crazy amount of nose weight. Being pusher (motor/esc near rear, battery as far forward as I could manage but still behind CG), and a blunt-nose delta(the body itself is heavily skewed toward rear weight) and being built without enough forethought about CG led to the absurd amount of nose weight.

I actually thought about giving up on it at one point with how heavy it was getting, but my motor calculations said it would fly so I stuck with it.

It seems well powered, both by eCalc and by flight observation. It launched exceptionally well considering it's size and lack of a decent way to throw it. It did need a good amount of up trim to fly level, so I dropped a little weight and it stayed the same. I didn't try inverted because it's a little on the large side and didn't feel all that stable (best I can describe is that it felt like balancing on a log). That made my relatively small flying area feel VERY cramped.

Can anyone suggest a good rule of thumb for elevon-area to wing-area ratio?
beecat is offline Find More Posts by beecat
Reply With Quote
Old Jul 09, 2015, 03:40 PM
kcolraw is offline
Find More Posts by kcolraw
Kcolraw
United States, NC, Asheville
Joined Feb 2015
412 Posts
I use a minimum of 1.5 inches unless a tapered design , worst case scenario you cut down on control surface travel.
kcolraw is offline Find More Posts by kcolraw
Reply With Quote
Old Jul 09, 2015, 04:38 PM
Whiskers is online now
Find More Posts by Whiskers
Build straight - Fly twisty
Whiskers's Avatar
Australia, QLD, Little Mountain
Joined Feb 2010
4,767 Posts
Glue on tapered extensions to the elverons so that their cord is wider in the middle and not changed at the tips.
Whiskers is online now Find More Posts by Whiskers
Reply With Quote
Old Jul 09, 2015, 04:42 PM
beecat is offline
Find More Posts by beecat
Getting Ahead of Himself
beecat's Avatar
United States, AR, Little Rock
Joined Mar 2015
73 Posts
Quote:
Originally Posted by Whiskers View Post
Glue on tapered extensions to the elverons so that their cord is wider in the middle and not changed at the tips.
Is there any particular advantage to tapered elevons other than looking nice? I always assumed it was more or less aesthetic vs just making larger straight ones of the same area.
beecat is offline Find More Posts by beecat
Reply With Quote
Old Jul 09, 2015, 05:04 PM
springer is offline
Find More Posts by springer
treefinder
springer's Avatar
SE MI
Joined Oct 2004
10,726 Posts
Actually,I was gonna suggest tapering the other way, more chord on the tips. Both my OneSheetWing and the Quartersheetwing use that type of elevon. Aileron effectiveness is great and the extension gives good elevator control function. Looks different too, everybody extends chord in center.... 12 oz is pretty crazy unless your plane is a 60inch span (maybe even then!) You say your battery is forward, but still behind the needed CG, why can't you put it right in the nose? Make the weight be useful? This may get us into the discussion about whether to extend motor or battery leads, and I have come down on the side of motor, extension, based on several threads here on RCG. You can get extenders from Hobby King, just remember to match the connecters on your setup
springer is offline Find More Posts by springer
Reply With Quote
Old Jul 09, 2015, 05:12 PM
beecat is offline
Find More Posts by beecat
Getting Ahead of Himself
beecat's Avatar
United States, AR, Little Rock
Joined Mar 2015
73 Posts
It's as far forward as I can get it given wire lengths. I may make some extensions. Mostly I was concerned about damaging the ESC. The scare title of that one post in Electric Planes about ESC lead length sticks with me.

As for the weight, it's definitely more than is reasonable. The span is in the high 40s, if I recall correctly (I'm away from home now). For reference, that prop is 8 inches.
beecat is offline Find More Posts by beecat
Reply With Quote
Old Jul 09, 2015, 05:29 PM
avigator is offline
Find More Posts by avigator
Registered User
United States, CO, Boulder
Joined Feb 2015
14 Posts
I don't know for sure, but I would guess that tapering the elevons toward the tip (larger at the root) brings the center of the elevon force inward, so by adding larger, tapered elevons you would be increasing pitch authority without increasing roll authority as much. If the airplane were to have lots of trailing edge sweep, then tapering the other way (larger at the tips) might be better because then most of the surface would be farther away from the CG, giving more pitch authority.

I've never built a delta, but when I build flying wings, I taper the elevons so that the elevon chord is always a fraction of the wing chord, so the elevons might be 25% of the wing chord. If the root chord is 16 inches and the tip chord is 8 inches, then the elevons would taper from 4 inches at the root to 2 inches at the tip. For a delta you would probably want much smaller surfaces than 25% chord, but tapering in that manner has worked well for me, giving good pitch authority without excessive roll authority.

To add to what balsa or carbon said about moving the motor forward, another advantage of moving the motor would be propwash over the elevons if you extend them inboard to the centerline. It isn't really a substitute for making the elevons bigger since at low power settings control will still be sluggish, but it can increase maneuverbility for some phases of flight, especially post-stall maneuvers like super tight loops or "dragging in" the airplane for a nice slow landing.
avigator is offline Find More Posts by avigator
Reply With Quote
Old Jul 09, 2015, 05:36 PM
tonystro is offline
Find More Posts by tonystro
TonyS
United States, AR
Joined May 2010
634 Posts
I believe reducing the overall weight should be your first step in improving control response. Currently you are trying to get that extra 12 ounces moving, or stopped, which probably takes a lot of elevator force.

I strongly support extending the esc to motor leads and moving the battery as forward as possible or as necessary to achieve desired CG.

The Polaris Sea Plane Parkflyer is a delta wing with rear mounted pylon power, and all sizes in the family (http://www.rcgroups.com/forums/showthread.php?t=922465) use extended esc to motor leads successfully. Many have hundreds of flights on them with no problem. I have three different size Polaris, with extensions from 14in to 22in. With dozens of flights have not had esc nor motor problems, except when I have dunked them.

FYI; Polaris XL's delta wing is approximately 24 by 38 inches. Elevons are approximately 2 3/4" wide inboard, 1 1/2" outboard, and 12 inches long with each elevon centered between fuselage and wing tip.

TonyS
tonystro is offline Find More Posts by tonystro
Last edited by tonystro; Jul 09, 2015 at 05:52 PM. Reason: to add wing & elevon dimensions.
Reply With Quote
Old Jul 09, 2015, 06:10 PM
basicguy is online now
Find More Posts by basicguy
Ayn Rand Sez.............
basicguy's Avatar
United States, MO, La Plata
Joined Dec 2014
580 Posts
I have built a half dozen deltas. What I have learned is that the airfoil is not that important on a delta. I build them thick enough for strength. My F117 clone has a 25" wing span and a 32" length, 16 oz flying weight. Almost a pure arrowhead shape. It has 1.25" elevons about 10" long each. My 22" long F117 has weighs 5.5 oz AUW and does nicely with 1".

How much deflection does yours have? The most expedient way to make it move sharper is to give it more angle.

It does take a fair amount of deflection to maneuver compared to a conventional plane.
basicguy is online now Find More Posts by basicguy
Reply With Quote
Old Jul 09, 2015, 06:10 PM
Whiskers is online now
Find More Posts by Whiskers
Build straight - Fly twisty
Whiskers's Avatar
Australia, QLD, Little Mountain
Joined Feb 2010
4,767 Posts
Quote:
Originally Posted by beecat View Post
Is there any particular advantage to tapered elevons other than looking nice? I always assumed it was more or less aesthetic vs just making larger straight ones of the same area.
Yes. The proposed setup will give more area in the middle which will mostly be effective as 'elevator.'
You are happy with the roll rate so the outer eans of the elevons do not require enhancement.
Whiskers is online now Find More Posts by Whiskers
Reply With Quote


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Help! Poor loiter performance and pitching on yaw. JonnyJ Multirotor Electronics 0 Sep 23, 2014 01:29 PM
Discussion “Drooperons” - leading edge droop, mixed with pitch and roll control inputs ZachAttach Modeling Science 28 Apr 08, 2014 12:01 AM
Discussion Flying Wing with Elevon Control? scharlach Scratchbuilt Indoor and Micro Models 4 Oct 26, 2012 08:39 AM
Discussion F-22 (BZ-22) Raptor with Elevon Control cheap daddy Electric Ducted Fan Jet Talk 2 Dec 20, 2010 02:50 AM
Discussion spectrum DX6i poor elevon mix range mrk Radios 1 Aug 15, 2008 07:22 AM