Rans S9 Chaos- Flying Characteristics.

[Mr Ficky]

After completing my first build, and having flown over 20 flights, can anyone help me answer why my Rans S9 Chaos is behaving like it is?

1. When climbing for a stall turn the plane veers off to the left very strongly. In general flight, it flies straight and true.

2. When doing a loop, as it gets to the 12.00 o'clock position, it wishes to close the circle instead and fly off at an angle, instead of keeping to the line of the circle.

3. In order to maintain straight and level flight, a lot of up elevator is required - is this normal? It is balanced well. It flies inverted quite well, and maintains an inverted 45° line quite well with no significant drop.

 

It would be sad to retire this plane to the hangar because of it's annoying aerobatic characteristics - perhaps I need to modify something? Any help would be much appreciated!

[Peter Miller]

Quote

 

First, let me say that you have done a superb job on your model.

 

Those flight characteristics are strange. Mine does none of those things. Nor does the larger one built in the club although the builder never does much in  the way of aerobatics.

 

Actually NONE of my designs show those tendancies and they all use the same aerodynamic layout.i.e 0-0 incidence,m0-0 thrust line,CG at 25% chord.

 

Has any other Chaos builder experienced any thing similar?

 

I assume that the propeller etc is the recommend size and pitch

 

 

[Graham Davies 3]

I can testify to Peter's model's characteristics.

 

Silly question, have you trimmed the rudder? My method is to trim rudder on vertical climbs. After each climb, I then check aileron trim. Can you correct the upline with rudder, and can you steer it over the loop similarly?

 

Otherwise, check the thrustline. All Peter's models are rigged zero/ zero as he mentions above. It may be worth checking your motor line is indeed straight. Pulling to the left is quite common due to the torque reaction, and it's usually a balance between the controls to minimise this. 

 

I'm not sure I'd be too worried about the elevator position; if you can trim it to fly level at your preferred speed (mine are trimmed level at a 3/4 throttle, but most are still close flat out), it's not normally something that would worry me. Moving the CG back may minimise this, but at the expense of other characteristics.

 

Graham

[Mr Ficky]

Thanks Peter and Graham for your comments! Much appreciated! A couple of guys at my club have said to check out the thrust line, but isn't it true Peter that you always have 0% thrust on your models? I am probably going to put some thrust % in, but I'd appreciate your comments on this first Peter, if you would be so kind.

 

The equipment including prop, is all the recommended from 4max.

 

Peter - should the elevator be at the same incidence as the wings? Mine isn't. 

 

Flying it today, it was going vertical for a stall turn with no veering to the left, but there is a LARGE amount of rudder trim on. I only had 2 flights, as I nearly crashed it due to me trying to rectify how it flies. 

 

Any suggestions on what to do is much appreciated. 

[PatMc]

Have you checked that the wings are balanced ?

[Piers Bowlan]

Cracking model you have built Mr Ficky 👍

Approaching a stall turn in a low speed and high power situation, both torque and the prop slipstream will be pulling the nose to the left. This may be exasperated by the Chaos’s short coupled design (not a criticism but an observation). As you say, (and controversially!) you could try a degree or so of right thrust and see if this has any benefit? Peter’s model doesn’t seem to need it though.

Odd that your model flies with a lot of up elevator, could the model be a tad nose heavy, although you say the balance is good?

Does the model drop a wing in a power-off stall incidentally?

[Peter Miller]

My tail planes are always at the same incidence as the wings. 

 

I know that this may sound obvious so forgive me if I am too obvious. The chord line is from the centre of curvature on the leading edge t the trailing edge and this is always on the aircraft datum line. Thrust line and tail plane on the same line.

 

Yes, my thrust lines are always 0 side thrust, 0 up and down thrust. I don't know of any full size aircraft with side thrust and I was interested to see film of American Navy fighters taking off with bootfulls of right rudder.

 

You could try some right thrust.  I would be most Interested to know the effect of that.  The comment on one wing being heavier is interesting but with the skill you have I would doubt that.

 

I do confess that I never remember the check my wings for off centre balance. Haven't had a problem with that to my knowledge.

[Peter Miller]

By the way,My Chaos does not have a motor in it at the moment but when I fit it I will try some more checks.

[Mr Ficky]

Thanks everyone for such good suggestions!

 

PatMc - wings are very balanced, I just checked; but I also built them balanced, even weighing the balsa. 

 

Piers - thanks for your comments! And also explaining what happens when approaching a stall turn! Makes sense! 

% Thrust: In looking closely I notice (with my protractor) that i built the firewall with 2% of down thrust - doh! I have therefore put a couple of washers on to bring it back to 0% down thrust.  But I have put in 2% right thrust - sorry Peter! As a result I have re-trimed the rudder to 0 - but I have shown a picture of the rudder before I changed the thrust line. Is this too much just to enable vertical flight?

 

Peter - nothing is obvious to me as it's my first build! I've took some pictures, perhaps you could advise (if the pictures are good enough) on whether you think the angles are ok?

The next picture is showing the angle of the elevator to enable straight and level flight - is this acceptable? 

 

Battery: As you can see, I have a 3300mah 3 cell lipo, and you can see where it is to obtain C of G. I believe this battery is larger than then recommended. I went for this as originally I built a Rans S9 Chaos (yes, this is my 2nd attempt!), and it really was EXTREMELY tail heavy, so bad that I was afraid to maiden it, and I put it in the bin! When buying the second kit, I thought 'try and build light and get a heavier battery!'

The plane weighs 1910 grams.

C of G is 8cm from the leading edge, (closest to the fuselage). I'll have to check with the article that this is ok!

 

As I've said before, comments and suggestions are really appreciated, and I mean it! Please keep them coming. 

 

 

 

 

 

[Peter Miller]

I use a 3000 Mah battery so the lightly heavier one will be fine.  I just crawled upstairs and measured my CG and it is 8 Cm from the leading edge right on the centre line  which is close enough.  

I am assuming that the wing section is as designed. Not that that would make a vast amount of difference.

I must admit that I am baffled but then apparently perfect aircraft can have kinks.  My latest design which could not be more simple and conventional and trainer like needed lots of rudder to turn right but turned perfectly to the left withot any rudder. In the end I wound in masses more aileron and it was fine.

And as a real illustration of the kinks in aircraft.  I went on three air tests in Lancasters back in the late 50s. One standard part of the test was the stall.

One of the Lancs flatly refused to stall but just wallowed along nose high.  Work that one out!!! The pilot tried it several times.

[Alan Gorham_]

13 hours ago, Peter Miller said:

My tail planes are always at the same incidence as the wings. 

 

I know that this may sound obvious so forgive me if I am too obvious. The chord line is from the centre of curvature on the leading edge t the trailing edge and this is always on the aircraft datum line. Thrust line and tail plane on the same line.

 

Yes, my thrust lines are always 0 side thrust, 0 up and down thrust. I don't know of any full size aircraft with side thrust and I was interested to see film of American Navy fighters taking off with bootfulls of right rudder.

 

You could try some right thrust.  I would be most Interested to know the effect of that.  The comment on one wing being heavier is interesting but with the skill you have I would doubt that.

 

I do confess that I never remember the check my wings for off centre balance. Haven't had a problem with that to my knowledge.

The Zlin Z50L has 3 degrees sidethrust according to the old MAP scale drawing. 

[Peter Miller]

10 hours ago, Alan Gorham_ said:

The Zlin Z50L has 3 degrees sidethrust according to the old MAP scale drawing. 

That is interesting! Come to think of it I seem to remember that someone build a twin engined Piper Cub with the tow engijes side by side attached to the fuselage The left on had left side thrust and the right one right side thrust.

[Piers Bowlan]

Of course there will be exceptions with full sized aircraft, another is the single engined Beach Bonanza S35 with 2 degrees right engine offset. The reason piston engined singles don’t generally have it is the obvious one, namely, they have a pilot sitting in them to keep the aircraft flying in-balance with the slip ball in the middle (hopefully!). This is regardless of engine power-setting or aircraft speed.

With a model aircraft, we can only observe how the model flies, so if we open the throttle in the cruise and the aircraft rolls to the left we might apply right aileron automatically to the right, to correct. Whereas the roll may well be the further effect of yaw from the increase in power, in which case some right rudder would have been appropriate. You will never know which was right!

So engine offset can mitigate some yawing tendencies but will only be optimised for one airspeed (cruising?).

At the end of the day, it is a model aircraft and we can suit ourselves whether to have engine offset or not. 😊

 

[Piers Bowlan]

Mr Ficky, It spears that there is a nice gap between your LiPo and the protruding motor shaft. Also the LiPo looks very securely fastened. However, in the unfortunate event of an unplanned excursion from an aerobatic manoeuvre (a crash), it is possible for the LiPo to break free and impale itself on the motor shaft. This may not only ruin the LiPo but also the ensuring fire consume the model. Don’t ask me how I know 😭.

A couple of small blocks of balsa to act as buffers can prevent this happening. Some people actually cut the end of the motor shaft off but it is a pretty hard material.

Edited August 29, 2022 by Piers Bowlan

[Peter Miller]

To cut the end off the shaft you need a cutting disc on a Dremel. Wrap the motor in a small plastic bag with a tight fitting hole round the shaft and cut the extension off.

 

This is as described by George at 4-Max.

 

I have been doing that for years now.

[Ian Lennox]

Thanks for the suggestion! I will do that, as I do have a Dremel and cutting disc. I think that space may become larger as I think I need to move the battery further back. I moved it back by about 1.5cm from what the picture shows, and noticed an improvement in how it flies. (Even though I'd checked the C of G and thought it was ok).

 

2% Thrust. I returned the rudder to 0 trim, and I noticed that as I flew verticle, approaching a stall turn, there was a significant difference, no veering to the left! So I can't help but think that a small percentage of thrust may help. 

 

[Piers Bowlan]

Sounds like you have it sorted Ian 👍.