New Products Flash Sale
Thread Tools
Old Jul 27, 2016, 12:36 AM
bennetbr is online now
Find More Posts by bennetbr
Registered User
United States, WA, Puyallup
Joined Nov 2012
108 Posts
Discussion
EDA and Spiral Stability Question

I'm working on a project to build a carbon pod and boom fuselage for my Riser 100. For the moment I'm using the stock wing until I get around to building a new wing with ailerons and flaps.

My question is I've sized the fin and tail length using Sailplane Calc to have a acceptable amount of spiral stability (4.27), but the EDA is still low for a RES polyhedral model (6.64). Is this going to fly correctly or have some weird handling characteristics? I've modeled the plane using XFLR5 and everything seems to be ok, but I don't really know how to use the program to examine the flight characteristics while under controlled flight. I've only used it to examine the behavior of the model in reaction to a disturbance and how it returns to stable flight.

I'm not really interested in chopping my wing up to add more dihedral since I'm building a new wing later. Plus I plan to still use the wing with the stock fuselage.

Any thoughts?

Thanks

Brian
bennetbr is online now Find More Posts by bennetbr
Reply With Quote
Sign up now
to remove ads between posts
Old Jul 27, 2016, 06:20 AM
ZAGNUT is offline
Find More Posts by ZAGNUT
supreme being of leisure
ZAGNUT's Avatar
Tel Aviv, Israel
Joined Jul 2004
3,748 Posts
built per the plan the EDA should be around 9.5
ZAGNUT is offline Find More Posts by ZAGNUT
Reply With Quote
Old Jul 27, 2016, 10:03 AM
bennetbr is online now
Find More Posts by bennetbr
Registered User
United States, WA, Puyallup
Joined Nov 2012
108 Posts
It should be around 9.5, but even in the stock form the EDA is around 7. Since I'm reusing the wing and flying as RES I need it to work as is. When I build a new wing it will have ailerons and a flatter wing so the EDA shouldn't be a issue. My concern with the EDA is the turning performance as a RES.
bennetbr is online now Find More Posts by bennetbr
Reply With Quote
Old Yesterday, 08:02 AM
slowmatch is offline
Find More Posts by slowmatch
Registered User
slowmatch's Avatar
Joined Dec 2015
224 Posts
Brian,

You've obviously looked at the VvB parameter also? This is a measure of roll control. According to your figures you have a healthy VvB (provided the rudder is a reasonable size) but you're achieving this by means of a large vertical stab rather than large EDA.

As a result, the control with rudder will be adequate but I suspect you will have a model with quite high pilot work load, needing close attention to fly well. The combination of low EDA, large Vv and low B is going to make for a model that goes exactly where it's pointed, and so you will need to be on the ball to keep it pointed where it should! Doing it this way is also going to require large rudder inputs and more yaw for the same roll response and so more drag.

The low EDA will be to the detriment of tight thermal turns. A rudder/elevator model relies on some yaw angle to hold bank angle. For a given turn radius and bank angle, this yaw angle is proportional to EDA - greater EDA requiring less yaw to sustain the banked turn. So a low EDA model in a tight turn will have a lot of drag from the yawed fuselage and so sink rate will be poor for working small lift.

So flyable, yes probably. But low/small thermal performance may be poor and probably a bit of a handful to fly. At least that's what the numbers are saying to me

Let us know how it goes!

Jon
slowmatch is offline Find More Posts by slowmatch
Last edited by slowmatch; Yesterday at 09:48 AM.
Reply With Quote
Old Yesterday, 09:35 AM
bennetbr is online now
Find More Posts by bennetbr
Registered User
United States, WA, Puyallup
Joined Nov 2012
108 Posts
Thanks for the reply Slowmatch. The interesting thing is I entered the stock Riser 100 into the spreadsheet and the EDA was the same since it's a function of wingspan and dihedral angle. But the spiral stability (3.27) and VvB (0.06) were lower. Which makes since because the tail length and fin area are less. The stock Riser 100 flies OK, but I've noticed it has a tendency to roll out if turns and the rudder authority is slow.

I'm going to build a new wing anyways. So I will try it with the stock wing and see how it flies. If its really bad it will motivate me to build a new wing sooner.
bennetbr is online now Find More Posts by bennetbr
Reply With Quote
Old Yesterday, 10:22 AM
BMatthews is online now
Find More Posts by BMatthews
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
14,375 Posts
Quote:
...The stock Riser 100 flies OK, but I've noticed it has a tendency to roll out if turns and the rudder authority is slow.
I've had this with a couple of my own gliders. With one I used the wings and tail with a new electric power fuselage. On this fuselage I kept the tail volume roughly the same but was aiming for a better rudder response and yaw damping by increasing the tail moment by a couple of inches (2 meter design) while making the fin and rudder slightly smaller to maintain the same vertical volume coefficient.

The results were more than I'd hoped for. The new electric power fuselage flies with much better rudder response. It's almost "aileron like" in the response speed. And the tendency to level and leave a thermal if I let the speed get a touch low is gone compared to what it was like with the pure glider version.

So if you have this sort of issue with the stock Riser you could do worse than to make the new fuselage a touch longer in the same way.

Some of the other numbers and acronyms like EDA and VvB you're tossing around I've not heard of before. So I won't even try to talk in those terms. Just the statement in your last post which mirrored my own experience led me to toss in this suggestion.
BMatthews is online now Find More Posts by BMatthews
Reply With Quote
Old Yesterday, 10:54 AM
Gliderfly is online now
Find More Posts by Gliderfly
Registered User
United States, NC, Franklinville
Joined Jun 2016
12 Posts
See below.
Gliderfly is online now Find More Posts by Gliderfly
Last edited by Gliderfly; Yesterday at 10:57 AM. Reason: Double post
Reply With Quote
Old Yesterday, 10:55 AM
Gliderfly is online now
Find More Posts by Gliderfly
Registered User
United States, NC, Franklinville
Joined Jun 2016
12 Posts
I highly recommend adding some dihedral when the time comes to build the new wing. It made a huge improvement in the flight characteristics with the stock fuse on my E-Riser 100.
Gliderfly is online now Find More Posts by Gliderfly
Reply With Quote
Old Yesterday, 11:04 AM
slowmatch is offline
Find More Posts by slowmatch
Registered User
slowmatch's Avatar
Joined Dec 2015
224 Posts
Quote:
Originally Posted by BMatthews View Post
Some of the other numbers and acronyms like EDA and VvB you're tossing around I've not heard of before. So I won't even try to talk in those terms.
EDA - Equivalent Dihedral Angle

Vv - Vertical Tail Volume

B - Blaine Rawdon's Spiral Stability parameter

VvB - (Vv multiplied by B) a measure of roll response to rudder control (ignores actual rudder area and deflection though)

All these parameters are used in the 'tail sizing checks' in Sailplane Calc.
slowmatch is offline Find More Posts by slowmatch
Reply With Quote
Old Yesterday, 11:27 AM
slowmatch is offline
Find More Posts by slowmatch
Registered User
slowmatch's Avatar
Joined Dec 2015
224 Posts
Brian,

Bear in mind that the Riser gets some increased spiral stability from the chunky fuselage. The B parameter ignores fuselage effects. It's probably minimal, but something to bear in mind when changing to a lower drag pod and boom.

Jon
slowmatch is offline Find More Posts by slowmatch
Reply With Quote
Old Yesterday, 11:53 AM
bennetbr is online now
Find More Posts by bennetbr
Registered User
United States, WA, Puyallup
Joined Nov 2012
108 Posts
I should have included a picture of the new fuselage. I'm kinda doing a incremental upgrade of the airframe. New longer fuselage and tail, then build a new wing with Drela airfoils, flaps and ailerons. For the moment I'm just trying to design a fuselage/tail that will work with the stock wing and fly as a RES. Then later work with a bigger 120in wing.

Here's a picture of the new fuselage, ignore the location of the fin, it's only taped on in front of the stab to get and idea of the balance.
bennetbr is online now Find More Posts by bennetbr
Last edited by bennetbr; Yesterday at 01:10 PM.
Reply With Quote
Old Yesterday, 11:54 AM
bennetbr is online now
Find More Posts by bennetbr
Registered User
United States, WA, Puyallup
Joined Nov 2012
108 Posts
More pics
bennetbr is online now Find More Posts by bennetbr
Reply With Quote
Old Yesterday, 12:22 PM
kwmtrubrit is offline
Find More Posts by kwmtrubrit
Be an organ donor
Little Rock, AR. U.S.A via U.K
Joined May 2004
1,314 Posts
Do a search for "Planepainter." He did a pod and boom on a Riser 100 and extended to wings to 112" He may have added ailerons too, but not sure.
kwmtrubrit is offline Find More Posts by kwmtrubrit
Reply With Quote
Old Yesterday, 03:38 PM
mawz is online now
Find More Posts by mawz
Registered User
Canada, ON, Toronto
Joined Nov 2015
781 Posts
Quote:
Originally Posted by bennetbr View Post
I should have included a picture of the new fuselage. I'm kinda doing a incremental upgrade of the airframe. New longer fuselage and tail, then build a new wing with Drela airfoils, flaps and ailerons. For the moment I'm just trying to design a fuselage/tail that will work with the stock wing and fly as a RES. Then later work with a bigger 120in wing.

Here's a picture of the new fuselage, ignore the location of the fin, it's only taped on in front of the stab to get and idea of the balance.
I like it.

I'm planning on doing something very similar to the 2m Riser.
mawz is online now Find More Posts by mawz
Reply With Quote
Old Yesterday, 07:39 PM
bennetbr is online now
Find More Posts by bennetbr
Registered User
United States, WA, Puyallup
Joined Nov 2012
108 Posts
Think of it like a super Riser 100 with a full flying stab longer tail boom and eventually ailerons and flaps. So I guess when I get done I might not have a Riser 100.
bennetbr is online now Find More Posts by bennetbr
Reply With Quote


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Question Washout and spiral stability aeronaut999 Free Flight 23 Mar 06, 2009 09:38 PM
Discussion Fokker D VIII Spiral Stability DLC Modeling Science 4 Jul 15, 2007 07:44 AM
Discussion Spiral Stability - Blaine Rawdon´s Paramenter for a Poly DLG cesarf3j2 Hand Launch 0 Mar 06, 2007 12:53 PM
Discussion How does washout reduce spiral instability or promote roll stability? steveseibel Modeling Science 26 Oct 09, 2006 06:36 PM
Spiral Stability nad Vertical C.G. Location DLC Power Systems 5 Apr 12, 2002 10:53 AM