Neutral point calculation accuracy?

[skj]

Hi All,

My semi-scale Bleriot XI is too heavy and flies very bad.
See this video:
http://hem.passagen.se/skj/familjen/BXI960730Start1.WMV
And data:
http://hem.passagen.se/skj/engelska/models.htm

Now I have reduced the weight from 1800 to 1600 grams.

Also I tried to calculate the neutral point using three different methods/programs.

Results:
Method NP
-----------------------------------------------
Martin Simons book 31 %
http://ciurpita.tripod.com/rc/notes/neutralPt.html

WinLaengs4 72 %
http://home.arcor.de/d_meissner/schwerp.htm

SPP 52 %
http://zzmanu.club.fr/spp/spp.html


Why are the results so different?
Can you recommend any other program like AVL?

The original drawing recommends CG at 50 %
but I use another airfoil.

Regards
Staffan

[vintage1]

Do you have ailerons?
More dihedral would help, if you haven't.

It looks like the roll/yaw coupling is dire, and possibly the fin is to small as well. More forward CG will help there a bit.

[hul]

try FLZ_Vortex: http://freenet-homepage.de/frankranis/flz_vortex.html

Hans

[HerkS]

Clearly this aircraft has too little vertical tail surface. It looks to me also, as if what vertical tail you have is being affected by turbulence from the fuselage. Enlarge the vertical tail to see if this helps. Also -- if you do this -- you should reduce its movement.

The balance may be off slightly but does not look so bad in the video. None of the calculation methods you use appear to take into account the effect of downwash from the wing. This is a rather strong destabilizing effect with low aspect ratio wings and low mounted tail. All NP calculation methods are approximation. Once the aircraft is flying you must then always adjust balance based on the flight characteristics.

[skj]

No ailerons.

I have now reduced the dihedral from 5 to 3 degrees.
The reason is that I had big problems to take off, due to a dutch roll movement.

I used the following to calculate a dihedral suitable for roll control:
http://ocw.mit.edu/NR/rdonlyres/Aeronautics-and-Astronautics/16-01Fall-2005-Spring-2006/3781B0CA-CE4E-4A9C-954D-FD8108ED21B1/0/spl8.pdf
Section : Dihedral Sizing – Roll Control

My calculation says that 0,34 degrees dihedral is a good value.

[kcaldwel]

Staffan,

None of those sites is using the entire longitudinal moment equation to calculate the neutral point. The first site is the closest, but it still leaves out aero centre height of the wing and tail relative to the CG. Your CG is quite low, and that will affect the results. And you will notice there are lots of assumptions to be made on things such as the tailplane effectiveness and lift curve slopes of the wing or tail.

Those assumptions have been made in the other two sites, and obviously not the same numbers were used!

These all just calculate the static stability margin, not the dynamic pitch response.

AVL would give far better results than any of those sites.

The aircraft doesn't actually look pitch unstable, just like it has bad roll/yaw behaviour, not great dynamic pitch response, and the stall looks pretty vicious too.

The airfoil will not make much difference to the CG location, since the wing planform will mostly determine the lift curve slope of the wing.

All these early aircraft are barely flyable, even at full scale. The early Wrght flyers were actually pitch unstable, and relied on the pilot's reflexes keeping up with a very light airplane.

Can you imagine what it was like teaching yourself to fly, with no idea how it should be, on airplanes that behave that badly. Those guys were good!

Kevin

Hi All,

My semi-scale Bleriot XI is too heavy and flies very bad.


Also I tried to calculate the neutral point using three different methods/programs.

Results:
Method NP
-----------------------------------------------
Martin Simons book 31 %
http://ciurpita.tripod.com/rc/notes/neutralPt.html

WinLaengs4 72 %
http://home.arcor.de/d_meissner/schwerp.htm

SPP 52 %
http://zzmanu.club.fr/spp/spp.html


Why are the results so different?
Can you recommend any other program like AVL?

The original drawing recommends CG at 50 %
but I use another airfoil.

Regards
Staffan

[skj]

try FLZ_Vortex: http://freenet-homepage.de/frankranis/flz_vortex.html

Hans
Thanks hul !

Is this similar to AVL by Drela?

[skj]

Clearly this aircraft has too little vertical tail surface.
Yes, I think so too. That's why I have reduced the dihedral ( to match).

It looks to me also, as if what vertical tail you have is being affected by turbulence from the fuselage. Enlarge the vertical tail to see if this helps. Also -- if you do this -- you should reduce its movement.
Yes, I have already built and flown a "fin enlargement" but without improvements. I believe it will fly better with the reduced weight. (hope)

The balance may be off slightly but does not look so bad in the video. None of the calculation methods you use appear to take into account the effect of downwash from the wing. This is a rather strong destabilizing effect with low aspect ratio wings and low mounted tail. All NP calculation methods are approximation. Once the aircraft is flying you must then always adjust balance based on the flight characteristics.
Two of the methods claim that they account for downwash.
Hopefully the creators of the methods will join the discussion.

[JetPlaneFlyer]

Looks to me like it needs a bit more dihedral to improve rudder/turn co-ordination plus MUCH more vertical stab area placed vertically out of the fuselage wake (and/or fill in some of the rear fuselage open structure)...Possibly also some washout to fix what looks like a nasty tip stall... And it does fly like its very heavy.. what's the wing loading?


Steve

[JetPlaneFlyer]

try FLZ_Vortex: http://freenet-homepage.de/frankranis/flz_vortex.html

Hans

Is FLZ available in English?

[skj]

Thanks Kevin !

I have used AVL for another model so I will try that.
When I made the last flights (in 1996 !) the CG was at 33 %.

It is disappointing to see the big differences in my calculation results.

Regards
Staffan

[skj]

Thanks Steve

The wing loading was 70 grams / dm2
the new wing loading will be 1600 / 26 = 62 grams / dm2

Regards
Staffan

[JetPlaneFlyer]

My calculation says that 0,34 degrees dihedral is a good value.

From the formulas on that page I calculate that you would need somewhere in the region of 6-7 degrees of dihedral. Which should put you in the right ballpark for neutral spiral stability plus adequate rudder response.
I've assumed scale overall dimensions and a Cl of around 1. This also complys with my the good old TLAR (that looks about right) rule
I think you need to check your figures.

Steve

[vintage1]

On a shoulderish wing vintage model, 2-3 degrees is generally not bad, and gives a model that will stay in the turns forever,,3-5 is preferred. Each side of course.

Looking at the flight, I'd also say its a tad tail heavy, and could benefit from washout if you are going to operate that close to the stall.


What it LOOKS like is a model, near the stall speed, with insufficient dihedral To turn you get yaw on, but instead of rolling the model into the turn it just slips..slows the inner wing and causes it to stall.

Another thought is that the fin is small and in the propwash. Since the wash turns with the plane, this makes the tail pretty ineffective as a stabiliser. Try how it behaves on the glide..This may be the dutch roll problem.

[hul]

Is this similar to AVL by Drela?
I believe so, but it's easier to use, as far as I can tell
Is FLZ available in English?
I have not seen an English version.
FLZ-Vortex maker Frank Ranis posts on RC-Network: http://www.rc-network.de/forum/showpost.php?p=1236791&postcount=108, pm him.
Or contact Markus here: http://fliegerforum.embeh.de/viewtopic.php?f=22&t=93, he seems to have good connections.

A helpful person checked my Vortex calculations with this expensive professional software: http://www.hanleyinnovations.com/multisurface/, results were virtually identical

Hans

[skj]

From the formulas on that page I calculate that you would need somewhere in the region of 6-7 degrees of dihedral. Which should put you in the right ballpark for neutral spiral stability plus adequate rudder response.
I've assumed scale overall dimensions and a Cl of around 1. This also complys with my the good old TLAR (that looks about right) rule
I think you need to check your figures.

Steve

Hi Steve,

Yes, I found a mistake in my calculation.

Now my results are:
marginal roll control (Vv * B = 0,1) dihedral 3,3 degrees
very effective control (Vv * B = 0,2) dihedral 6,7 degrees

Thanks
Staffan

[biber]

FLZ_VORTEX is based on the vortex lattice method as AVL is.
However, both are different implementations, independent from each other.
FLZ_VORTEX does come with a graphical user interface that makes working with it way more easy for noobs.
Frank is constanly working on his software to add new features and debug it.
Last time I checked FLZ-Vortex was quite beta.
However, Frank is open to feedback anytime.
I think there is no english translation of it published yet, but Frank has my offer to translate it, and I already translated parts of the software.

biber

[skj]

Hello vintage1,


On a shoulderish wing vintage model, 2-3 degrees is generally not bad, and gives a model that will stay in the turns forever,,3-5 is preferred. Each side of course.

The result from WinLaengs4 is very different if the fuselage area between
the wings is included. After some more work my results are:

Method NP
-----------------------------------------------
Martin Simons book 35 %
WinLaengs4 39 %
SPP 37 %

This is nice. I select 35 %, which is on the safe side, and a static margin of 4 %. Thus the CG should be at 31 % or less.

Looking at the flight, I'd also say its a tad tail heavy, and could benefit from washout if you are going to operate that close to the stall.
Washout is already there, maybe 5 degrees.

What it LOOKS like is a model, near the stall speed, with insufficient dihedral To turn you get yaw on, but instead of rolling the model into the turn it just slips..slows the inner wing and causes it to stall.
I looked again on the video and I have to disagree. The plane banks very efficient and fast when the rudder is changed, almost too much.

Another thought is that the fin is small and in the propwash. Since the wash turns with the plane, this makes the tail pretty ineffective as a stabiliser. Try how it behaves on the glide..This may be the dutch roll problem.
Yes, I will try when springtime arrives (May ?)
Thank you for your interest! RGDS Staffan

[Bernd Brunner]

Stefan,

do you can post the winlaengs file here?
Or, if not, please post all the dimensons from the plane.
What airfoil are you using for wing and elevator?
Where is your CG now?

Let´s see what we can do..

Bernd

[skj]

Stefan,

do you can post the winlaengs file here?
Or, if not, please post all the dimensons from the plane.
What airfoil are you using for wing and elevator?
Where is your CG now?

Let´s see what we can do..

Bernd


Yes Bernd, here we go:

My first calculation was done without including part of the fuse in the wing area. See Frida1.jpg and Laengs4data.zip.

My last calculation was done including part of the fuse in the wing area. See Frida2.jpg and Laengs4data.zip.

Wing airfoil is an Eppler 214 modified for nose radius 0.4%. See vinge0001.jpg.

Stabiliser airfoil is an SI 64009 (Sigurd Isakson). See stabbe0002.jpg.

The CG is now on 33%

RGDS
Staffan

[hul]

I think the lifting tail makes the difference. I have zero experience with lifting tails but I think the CG needs to go much further back than normal.

I put you plane into FLZ-Vortex. Not sure what to think of the results though.
I get no instability warning with the CG at 150mm even though static margin is negative (first pic).
With the CG at 88mm (33%) the elevator stalls (second pic).

Used the BE50SM airfoil.

Hans

[vintage1]

I think the lifting tail makes the difference. I have zero experience with lifting tails but I think the CG needs to go much further back than normal.


No!No!No!. You have it back to front!

You need lifting tail section when there is net upforce on the tail.

You get net upforce on the tail when the CG is aft of the center of lift of the wing.

You get a stable condition with an aft CG BECAUSE the tail volume is large.

A 'lifting tail' does not cause anything. It is the result of everything else!

[hul]

like I said, I have no idea.
Neutralpoint is at 103mm according to Winlaengs, CG of the original design at 50% (130mm). Don't know if it flew, but if it did, CG was behind NP.

Hans

[hul]

changed the elevator section to a NACA0012, wing unchanged. It flies normal with the CG at 33% (88mm).

Hans

[BMatthews]

Try this one and see how it works out. A lot of folks have been reporting good results with it.

http://www.geistware.com/rcmodeling/cg_calc.htm

As for the dutch roll I found that a more forward CG results in a reduction in the need for vertical tail area. The translation for that is that if you're on the edge of dutch roll and your model hangs tail low in the turns like yours does that a forward CG will compensate for too small a tail area up to a degree. But if it's way too small then there's no cure other than to make it larger.

Aerodynamics was in its infancy when the Bleriot was designed. It shouldn't come as a surprise that they set some parameters really close to the edge.

The best way to improve your model will probably be to make a new rudder with about a 10% size increase to avoid the tail low in the turns.

[CloudyIFR]

Another easy to use MAC / Balance Calcultor is an MS Excel Spreadsheet available at www.TailwindGliders.com

Look for Sailplane Calc in the Articles/Files tab.

Curtis
Montana

[skj]

Try this one and see how it works out. A lot of folks have been reporting good results with it.

http://www.geistware.com/rcmodeling/cg_calc.htm

Yes I did. See attachment Frida2Geist.jpg for the result.
NP at 40,3 %

A different method for calculation is
http://www.geistware.com/rcmodeling/articles/trim_calc/index.htm

, but I do not know what method this program uses.
They are on the same website though.

Data for my plane, in its new version, are in frida2Prop.jpg.

As for the dutch roll I found that a more forward CG results in a reduction in the need for vertical tail area. The translation for that is that if you're on the edge of dutch roll and your model hangs tail low in the turns like yours does that a forward CG will compensate for too small a tail area up to a degree. But if it's way too small then there's no cure other than to make it larger.

Aerodynamics was in its infancy when the Bleriot was designed. It shouldn't come as a surprise that they set some parameters really close to the edge.

The best way to improve your model will probably be to make a new rudder with about a 10% size increase to avoid the tail low in the turns.

Yes, and then increase the dihedral to match.
RGDS
Staffan

[skj]

Another easy to use MAC / Balance Calcultor is an MS Excel Spreadsheet available at www.TailwindGliders.com

Look for Sailplane Calc in the Articles/Files tab.

Curtis
Montana

Thanks Curtis !
I looked at the documentation but it does not describe the calculation methods.

RGDS
Staffan

[Bernd Brunner]

IMO the neutral point calulated with winlängs is ok. NP = 107mm from LE.
If you set the stability to 10%, you CG should be at 81mm fro LE or 31%.
So far so good...
The other information you need is the AoA for the wing. The E214 has an AoA for Cl=0 of appr. -2.6° . Flying at 11m/sec needs an AoA of 3° ...
With this stability you need a litte amount of negative lift from the elevator.
But the airfoil is working extremely bad around Cl=0 . Its made for cl=0.5 to 1.2 !

What i would recommend is building a new elevator with a flat-plate airfoil (or naca0009, or Ht14) and let the CG on 33% (don´t change).

Bernd

[JetPlaneFlyer]

What i would recommend is building a new elevator with a flat-plate airfoil (or naca0009, or Ht14) and let the CG on 33% (don´t change).

Bernd
I second that notion... a cambered airfoil stab is a bad idea on a model such as this.

Steve

[hul]

agree, it's also what FLZ-Vortex says (see also posts #21 and 24). The circled red bar indicates the elevator stalls (is trying to create more down force than it can).

Hans

[skj]

Hello Bernd,

IMO the neutral point calulated with winlängs is ok. NP = 107mm from LE.
This means that the area between the wings should be included in the wing area when using Laengs4 ?

If you set the stability to 10%, you CG should be at 81mm fro LE or 31%.
So far so good...
The other information you need is the AoA for the wing. The E214 has an AoA for Cl=0 of appr. -2.6° . Flying at 11m/sec needs an AoA of 3° ...
With this stability you need a litte amount of negative lift from the elevator.
But the airfoil is working extremely bad around Cl=0 . Its made for cl=0.5 to 1.2 !

What i would recommend is building a new elevator with a flat-plate airfoil (or naca0009, or Ht14) and let the CG on 33% (don´t change).
The original drawing shows a stabilizer airfoil with much more camber, however thinner. I guess it is more "to scale".

Yes, I see why a flat-plate is better.
Thank you for your advice !

Bernd

[Bernd Brunner]

Staffan,

This means that the area between the wings should be included in the wing area when using Laengs4 ? Yes!

The original drawing shows a stabilizer airfoil with much more camber, however thinner. I guess it is more "to scale".
Yes, but look at the wing airfoil too. It has much camber and is thinner too. I think the aerodynamic concept of these early planes is a bit different to the current ones - more like canards: all wings of the plane should generate lift.

Bernd

[Mignetman]

Nearly all published calcs for NP, and then CG position, are faulty. If it does not say that the 1st approximation for NP location is at the weighted mean of the effective surface areas, then it is false. Check: tandem wings of equal area, then the NP is at their mid-point; but the expression 'Vbar', usually used, is plainly incorrect.
Further, the distance of the CG ahead of the NP is commonly related to the 'MAC' of the wing. It can be shown quite easily that this is a nonsense. The distance between the NP and the CG is better related to the distance between the 'wing' & 'tail' AC's (commonly called 'Ls'), as can be shown mathematically.

[skj]

Thank you All for your help !

To summarize I can say

The tail has to carry a lot of negative load, because of the wing moment.
The tail profile is a high camber type and flies close to stall.

Because of the negative load I thought I could use a foil with good lift, but turn it upside down.
E.g. Eppler e205 inverted. However the wing should stall before the tail and that made me hesitating.

So I have decided to use a symmetrical foil with better lift than a flat plate.
Then I don´t have to increase the stabilizer area.
NACA 0009 is my choise, if you agree.

The tools I rely on for the calculations are:

Helmut Stettmaier´s Excel sheet.
Very good documentation and it allows me to calculate decalage for a loaded tail.

XFLR5 V4.15
The attachments show some results

RGDS
Staffan