I have built and flown an 18.5% scale Farman Biplane (1909 version) and have crashed it on 2 of its 3 flights due to a severe pitching instability that develops shortly after a beautifully controlled take-off. It seems that as a certain speed is developed the front (controlled) canard starts to "fight" the rear stabilizers (fixed). The plane goes into ever-increasing poipoising pitches until it noses in.
I have tried the CG between 33 and 40% of the chord. The final dive is steep, making me think it is nose heavy (the rear stabilzers are fully lifting sections), but I really don't know.
The main planes and the rear stabilzers are set at +3 deg. The motor is zero down-thrust, zero side-thrust.
The forward canard is generally very responsive, except in the final nose-over dive, where it seems unable to overcome the final nose-over dive.
The airfoils of wings and rear-stabilizers are thin curved plates, the canard a very thin Clark-Y type, just as per original.
Can anyone help please?

My 18.5% scale Farman Biplane (1909) suffers from severe pitching instability that develops shortly after take off. Maybe I have set the incidences incorectly, or maybe its the CG position that is wrong. The front canard seems to fight the rear stabilizers, once a certain flying speed has developed.

Can anyone help?

Robin:
One problem could be the airfoiled stab. As speed builds up over a certain point the authority of the lifting stab gets stronger proportional to the speed. as the speed builds,and causes the nose down attitude, the canard doesnt have enough authority to overcome the the lift of the rear stab. Some negative incidence in the rear stab will help to bring the nose up with speed. More forward CG would probably help too.

Hello Flypaper 2,
Thanks for your reply, which makes good sense. I appreciate your writing in so quickly.
Regards, Robin

Your welcome. As you probably know, back then not much scientific aerodynamics had developed yet. sort of cut and try methods. Many planes then were very unstable. Let us know how it works out.

Robin

The Farman is sort-of a strange setup aerodynamically. I have some experience with a 3-surface (non-scale) model design, and aerodynamically it is quite interesting. Being a grad aero-student I have the advantage of having access to a lot of aero-software, which helped me to calculate all the correct incidences and most importantly, finding the correct CG and even then it took me a lot of work to get it all right. However, the fact that you have no problems during take-off tells me that you are pretty close. Your model is probably somewhere very close to neutrally stable, and your incidences are probably also not too far off.

The behaviour you describe sounds like a classic dynamic instability. It is probably there during take-off also, but since you are flying slow, you are able to correct it (maybe even unconciously), until the speed goes up and eventually the oscillation starts running away from you. Something that probably also helps initially is that all the horizontal stabilizer area (front and back) cause a lot of aerodynamic damping, which means that although it is unstable, the damping prevents the oscillation from running away immediately.

I recently co-authored a paper for an AIAA conference dealing with computer simulations of the 1903 and 1905 Wright Flyers, and these have behavior very similar to what you describe. Both of those airplanes are unstable (CG behind the neutral points), but both are flyable due to (a) the large amount of damping and (b) the low flight speed. The 1905 flyer however was much easier to fly than the 1903 Flyer because it used a bigger canard that increased the aerodynamic damping to the point where the pilot could easily control the oscilation himself. If he stopped countering the oscilation with canard inputs it would still run away from him though. The way we typically "crash" the Flyers on the research sim is when we let the oscillation run away, and eventually when reaching the top of an oscillation the canard stalls. The canard stall causes a severe nose pitch-down which is unrecoverable as you never have enough altitude to wait for it to start flying again.

OK, that is what I "think" is wrong with your airplane. Of course, it is just a guess based on your description of how it flies. Remember that, since there is a canard, the neutral point of the airplane moves forward from where you normally expect it to be. Most conventional airplanes balance with nice positive stability around 25% to 35% chord (which means the neutral point is at around 35% to 45% chord) - these numbers all depend on the specific airplane of course. However, "normal" canards (not 3-surface airplanes) have to be balanced much further forward due to the canard pulling the neutral point forward. In some cases, when the canards are very big, the correct balance point may even be in front of the leading edge of the wing. Your model has both a conventional tail and a canard. The wing(s) aerodynamic center is at 25% chord. The tail moves the neutral point further back, and the canard moves it further forward again. So, your airplane probably has a neutral point around 25% to 30% chord. Your CG has to be in front of that point or it will be unstable. As you can see, both CG locations you have tried are behind that point...

I suggest you try to move the CG somewhere between approx 18% to 23% chord. I am not sure if you need to adjust your horizontal tail incidence, since you have never actually flown the model in a trimmed condition and therefore you don't really know if it is bad now or not. With the CG where I suggest, you may need to change the incidence on the horizontal slightly nose-down. If it was me though, I would simply move the CG to around 20% and then try to fly it. If you need a lot of elevator to get it off the ground, then you will probably have to adjust the rear horizontal slightly nose-down or you can change the canard slightly nose-up.

I hope this helps. As I said, without actually seeing the model and going through the calculations, I cannot guarantee anything. However, moving the CG too far forward is generally much safer than having it too far to the rear. Also, because of what you observed in regard to the strong pitch-down once the canard stalls, I would suggest that you try never to stall it without having lots of altitude to work with.

Good luck!

Cheers,
Bennie

Just saw your location. Should have said "Sterkte!"

I missed this line earlier:

I forgot that the forward canard is what you actually use as an elevator on this airplane, rather than the tail. I think that confirms my suspicion on the canard stall (and makes it even closer to what I know about the Wright Flyers) - once you enter the dive your normal reaction is full up elevator. Full up elevator actually keeps the canard in a stall, which is why it seems it doesn't do anything.

As I said, if you do stall it intentionally in future, make sure you have lots of altitude. To recover from the stall, don't try to use full up. Just use a small amount of up and let it gently come out of the dive.

Please let us know if you get it to work (with a picture of the model of course)!

Cheers,
Bennie

Hello Bennie,
Thank you for your ideas and comments. Thanks also for the South African greetings too!
I like your thought that the front canard may have stalled in the final dives. It makes sense to put more nose weight in, but I am already carrying over a pound of lead in the canard already! I wonder if the forward struts will carry more. Maybe I need to beef them up.
When I look at the original prototype of 1909, I doubt whether it's CG was more forward than perhaps 50% - on the grounds that the Engine weighed about 75kg, which must be nearly the same as the pilot - and the canard looks no heavier than the stabilizers.
But then, the 1909 pilot had the advantage of being able to feel things developing before they happened, whereas we RC guys have to wait until we see the problem, which may be too late, so maybe I have no choice in the matter, but to make it more intrinsically stable than the original, by shifting that CG.
I need to read your reply a few more times because it contains a lot of thought and then I need to experiment again, but this time with perhaps a bit more understanding.
Thanks for writing in, I'll let you know when I have tried.
With kind regards, Robin

You're welcome, Robin. I learn so much about modeling from this site, that I am always glad when I can also contribute a bit. Even more so when it is to help out a fellow South African

I can see your dilemma in having to carry that much weight in the canard. Also, you may be quite correct about the CG being further back than it should be in the original. As someone else here posted, this was the time when those great pioneers were still just starting to understand flight, and at that time much of the focus was on lift, propulsion and control, with stability not always getting as much attention as it should have. As I mentioned in the earlier post, several of the airplanes of the Wrights were unstable, but due to other factors, including as you mention being able to feel things develop when you actually fly in it, they were quite flyable with some practice. If you built a RC model exactly like a Wright Flyer, including the CG, control actuation, etc, you would have a very tough time flying it. All the successful models of those airplanes that I know of, had certain subtle changes made to them to make them more flyable. The most common "improvement" is usually the use of a more reasonable CG location. Luckily the CG is not something you can see visually on the airplane, so it doesn't spoil the scale looks (unless of course you need external weight for balancing).

I apologize for the technical rambling. I can go a bit overboard sometimes[sm=spinnyeyes.gif]

I had one more thought about your canard stalling out - as I mentioned, once you move the CG forward you may have to trim the rear horizontal with a slight nose-down incidence. If you add a bit of extra nose-down on the rear horizontal, the canard will unload a little and not have to lift quite as much. It may help a bit in delaying the canard stall. However, try to make small changes at a time if possible. First try to get the CG right so you at least don't run into the oscillations.

Cheers,
Bennie

No need to apologize, Bennie, Helps everybody on this site with a more detailed explaination.

THanks fellows for all your comments,
I am really enjoying this site - this is my first experience!
Before my next flight test, I'm going to calculate the whole thing out using lift/drag data for the airfoils and the various moments that I can measure on the model. I'll post my results in a day or two - that is - if I get anywhere and the results are worth anything.
I see that I'm allowed 6 photos on this site, so I'm trying to put a photo on right now.
Regards,
Robin

Robin

You can upload images in your post anytime, and I don't think there is a limit except for the file sizes that have to be below a certain size. If you click on "reply" at the bottom of a post, you will get a text box very similar to the "Fast Reply" box on the bottom of the page. However, it also adds a link on the bottom left of the text box that says "Upload Images!", that will bring up a new window that allows you to select images on your harddrive to upload. The images will appear at the bottom of your post.

Thanks Bennie,
Here's a photo
Robin

Wow, absolutely fantastic, Robin. I hope you get the problems sorted out. Nice scenery also! I have now been away for 5 years but soon I will get to hand in this darn thesis and return to the land of Biltong and Rugby, and maybe I will then get to see her flying myself one day...

Cheers,
Bennie