Rata by name rat by nature.

[J D 8]

The Russian Polikarpov I16 was the first monoplane retract UC fighter to go into service in the world. During the Spanish civil war it was known as mosca [fly] by the republicans and rata [rat] by the nationalists.

My model was built about eight years ago from the TDM models kit and was my first electric plane. Weight came in just an ounce over the target ready to fly and c of g as per plan

This model turned out to be the most unflyable thing I have ever had. Flights would go like this.

A nice straight launch nose up about 20 degrees and it would climb away fine, I did expect it to be a bit touchy fore and aft being so short coupled but it is fine. Lateral control is the problem,you feed in some stick either way but nothing happens,feed in more and it still keeps going straight and getting rather small. Then at full movement the ailerons bite and its over at 90 degrees and you can turn,after that its a chase slapping the stick left to right until you catch it level and can fly on until the next turn is needed and so on.

Some very erratic circuits result and if lucky a final approach can be made before the batt runs out. Any attempt to open up if under or over shooting will result in rolling in.

Most flights have ended with it landing [crashing] where ever it wants to on my farm. The thing has had a charmed life most flights ending in something soft like long grass,heather,bracken ect with little damage.

I tried different levels of expo and differential but the only thing that helped a little was virtually no down aileron. Some six years ago I hung it on the wall but now fancy another go at getting it to work and any ideas would be great.

Have seen video of the same model being flown by others,fast yes but I can cope with that, its the lack of control that's the problem.

The dirty RAT. Cheers John.

[Robin Colbourne]

Hi John,

That's a nice looking Rata. I've had a soft spot for them since building the Revell 1/72 I-16 many moons ago.

Youur experience with the aileron control reminds me of a Precedent T180 that a pupil built. In that case it was the elevator which was ineffective when 'up' was given. I spoke to a number of other instructors who had similar experiences with them. The consensus was that the elevator was moving within the boundary layer, and it was only at full movement that it projected into the airflow to have any effect.

Although it wouldn't look very good, you could try making ailerons of constant thickness chordwise. If any reduction or elimination of the hinge line gap is possible, it can only help. Finally, coupling the rudder with the ailerons would help if the rudder is functional.

[Denis Watkins]

The trouble with Expo JD is that your model knows nothing of it, Expo just alters what your fingers are applying to the surfaces. Now Differerential is another story, and a starting point would be more up aileron than down. The next option would be more down aileron than up. And do mix in some rudder, that tames many beasts.

Edited By Denis Watkins on 17/11/2016 07:37:27

[Piers Bowlan]

I think the problem lies with the tapered strip ailerons and the very short coupled nature of the design. Basically the Ailerons are producing adverse yaw and this is not helped by the short coupled fuz resulting in the model being rolled in one direction but the further effect of adverse yaw causing it to try to roll in the opposite direction. Result is that it carries on in a straight line until you come to a point where you have very large aileron deflection and the fin/rudder area is sufficient to restrict the amount of yaw. As you have already found out aileron differential helps but reducing the down going aileron deflection to zero may be necessary. Robin's final comment of using coupled aileron and rudder is probably the answer, as this will counteract the adverse yaw and the model should behave normally. I am not a fan of strip ailerons particularly where they taper towards the tip. Also if you have a short fuz you usually need a larger fin (also tailplane) but you don't have that luxury with a scale model. Robin's other comment to ensure that you don't have any hinge line gap will also help roll control but the adverse yaw is the culprit of your problem in my view.

[Bob Cotsford]

OK, I'll be the one with the daft questions. How are the ailerons driven? I'm guessing torque rods from it's age and the use of strip ailerons. Strip ailerons have worked fine on all the models that I've used them on, but they do need to have stiff torque rods driving them. The ailerons themselves need to be fairly rigid to prevent them twisting over their length - free hinges help here.

How about the aileron servo - is there any slop in the output arm? Is the servo big enough? Are the control rods slop free in the aileron and servo horns?

[J D 8]

Hi all and thanks for your views on the problem. Robin I did wonder if it was a boundary layer problem and if it was worth doing something like smoothing the wing surface or changing the profile,however that seems a lot of work for possibly no gain.

Denis and Piers the aileron differential is already at set at only a little [3mm] down, done mechanically on the one central servo. Bob the servo is of good quality and in good order and the torque rods are man enough.The ailerons are a bit flimsy at their outer ends where they taper off and need sorting. Hinge gap is as good as can be without changing to a tape type top hinge.

You all recon a working rudder [fixed at the moment] may well be the answer and with just an extra 9g servo needed an easy fix. Wingtips need some repair [cartwheel on its final flight] and we will be good to go.

Repairs / fixes are under way and will let you know the outcome.

Thanks again John.

[MattyB]

Sorry, but I don't think adding a rudder and using coupled ail to rudder mix is going to cut it - you really need ailerons with a deeper chord. Personally I would cut the existing ones in two a third of the way out, glue the inner section back on as a fixed TE, relieve the TE on the outer portion to double the chord then make some ailerons to fit driven by a servo per side. I would be very surprised if that did not fix it.

[Denis Watkins]

I think we are all on the right track, and with models, the ailerons tend to be most effective on the outer 3 to 4 inches of the wing. Looking at JDs model the least amount of aileron is out there at the tip, exactly where it is needed. Coupled rudder will help, as would clear plastic extensions to the rear of the outer ailerons. That does of course spoil the look of a beautiful scale model. I hate to say this, and will endure the backlash, but this model would benefit with the addition of a stabiliser.

Edited By Denis Watkins on 18/11/2016 07:56:09

[MattyB]

Posted by Denis Watkins on 18/11/2016 07:54:54:

I think we are all on the right track, and with models, the ailerons tend to be most effective on the outer 3 to 4 inches of the wing. Looking at JDs model the least amount of aileron is out there at the tip, exactly where it is needed. Coupled rudder will help, as would clear plastic extensions to the rear of the outer ailerons. That does of course spoil the look of a beautiful scale model. I hate to say this, and will endure the backlash, but this model would benefit with the addition of a stabiliser.

A stabiliser might help to improve pitch stability, but only once the aileron response problem is addressed. The stab is going to find it difficult too if the nothing happens til the ailerons reach 75% or so of their movement, at which point it suddenly rolls hard left or right!

PS - I once saw a short coupled slope soarer that flew very well back in the late 80s or early 90s; when I asked how they'd got such nice handling I found out it was set up with a flying wing aerofoil with a very low pitching moment, stabilised by reflex. The tail had no elevator and was essentially just for show!

[Peter Miller]

Personally I think that the suggestion of cutting the ailerons off midway and insetting them into the wing will be the most effective. Wider ailerons at the tip definitely

[J D 8]

Thanks Matty,Denis and Peter for your suggestions. If you look at some pics of full size I16's my models ailerons are close to scale,saying that I have seen models of I 16's on line that have inset ailerons so it may be that others have had this problem.

To modify my wing would not be simple as its a stressed skin construction with only a few ribs inside.[ skin is just 1/16 th soft balsa with tissue covering and believe me it has proved to be tough.] Building a new non scale [ flat sheet? ] wing would likely be quicker .The aircraft is to small to consider having a servo for each aileron in the wing.

Stability is not the problem in any axis and if it were free flight I recon it would keep going until the batt ran out,in fact it did get away into the distance from me a few times and then the only option was cut the power and let it land its self and then start a search mission.

[Former Member]

[This posting has been removed]

[Robin Colbourne]

Funnily enough I was wondering what a bit of reflex would do before I read MattyB's post. It is definitely worthy of consideration.

Another approach would be to add a sliver of balsa to the top surface of the outer part of the ailerons to thicken them up towards the trailing edge. Imagine a bit of trailing edge stock glued on back to front. This may make the 'up aileron' more effective on the outer part. Installed with a couple of drops of cyano, you could experiment to see if it works before going to lots of further effort.

A slightly more complex approach would be to move the hinge line back in the aileron and have a rounded leading edge on the top surface. The upgoing aileron would then put its leading edge down into the airflow giving a spoiler effect, which would assist yawing into the turn.

[Charles Smitheman]

I would also think about adding a rudder, and perhaps a gyro on it.

The model might be side slipping erratically which could dramatically change the way it flies.

Has anyone else tried the small Black Horse Cassutt?

It also has a big fat nose and a small tail fin area.

At low to medium speeds mine was always trying to dive into the ground in the turns rather like John describes. I had to be on the rudder all the time in the turns to keep it in the air.

I fixed it by changing from 3 to 4s and flying it flat out. Then it flew OK until the sc burned out! It survived pretty much, but I got fed up and put it in the loft. Maybe one day...

[Piers Bowlan]

Posted by Robin Colbourne on 18/11/2016 20:34:19:

Funnily enough I was wondering what a bit of reflex would do before I read MattyB's post. It is definitely worthy of consideration.

Another approach would be to add a sliver of balsa to the top surface of the outer part of the ailerons to thicken them up towards the trailing edge. Imagine a bit of trailing edge stock glued on back to front. This may make the 'up aileron' more effective on the outer part. Installed with a couple of drops of cyano, you could experiment to see if it works before going to lots of further effort.

A slightly more complex approach would be to move the hinge line back in the aileron and have a rounded leading edge on the top surface. The upgoing aileron would then put its leading edge down into the airflow giving a spoiler effect, which would assist yawing into the turn.

This model doesn't have a problem with stability, either longitudinal or lateral so reflex is not appropriate.

Yes, thickening the trailing edges of a control service can improve it's effectiveness at model sizes so worth a try?

Frize ailerons (what Robin is describing) are used on many full sized aircraft to counteract adverse yaw. More difficult at model sizes especially when they are small. Coupled aileron and rudder (CAR) is a simpler solution as it has the same effect (counters the adverse yaw which is exacerbated with this short coupled aeroplane).

Building bigger (inset) ailerons is clearly going to work but is not ideal as it requires virtually rebuilding the wings. I think aileron differential will probably still be required.

[Robin Colbourne]

Hi Piers, The thinking behind the reflex on the ailerons in this instance is not to affect the stability, rather to put the trailing edge of the up-going aileron closer to where it will start to be effective; thus having much the same effect as thickening its trailing edge.

By having reflex, the down-going aileron will pass through a zone in which drag will reduce or at least stay constant before it starts to increase. This together with aileron differential, should make the upgoing aileron the most effective one.

Thanks for identifying my description of the Frize aileron. Anyone contemplating using them should read what Wikipedia has to say, as they have their problems: **LINK**

[Piers Bowlan]

Thanks Robin for the clarification regarding reflex, my only thought here is that it's effect is similar to differential but yes, definitely worth to try to tame the Rat. I had a yen to build a model Polikarpov Rata but this thread has put me off! There are some great video of the full sized being flown on uTube here.

Edited By Piers Bowlan on 19/11/2016 06:04:18

[Robin Colbourne]

Here's a surprise; or not, depending on your point of view... A quick look this morning on the 'Incomplete Guide to Airfoil Usage', here: **LINK** revealed what section the full size Rata used.

It states that its the TsAGI R-II for both root and tip, so I looked that up and here it is. Notice anything about it ?

Yes, the underside is reflexed! However, the wing section on the TDM plan is not. **LINK**

 

By the way, TsAGI is the Central Aero Hydrodynamic Institute (Tsentral Aero Gydrodynamichiski Instituta, as they say it in Russia), their equivalent to NASA.

Piers, Thanks for the link to the I-16 flying at Wanaka, a visit there has to be one for the bucket list! If John can tame his Rata, will you give one a go?

Edited By Robin Colbourne on 19/11/2016 11:34:13

[Piers Bowlan]

Fascinating, maybe that explains how they got away with such a relatively small tailplane considering the short fuselage? Helps the longitudinal stability but does it really help with roll? Clever chaps these Russians and I bet they knew how to use their rudder pedals when flying the Rata!

Regarding the video, tragically Mark Hanna was killed the following year flying a Hispano Buchon (Spanish built Me 109) near Barcelona.

There are some large model Ratas on uTube, I wonder how they fly? The aircraft has loads of character but it looks like a challenging aircraft to model. It will probably stay on my 'build list' but it will be a long way down i.e. probably never!

Edited By Piers Bowlan on 19/11/2016 14:41:43

[J D 8]

Interesting about the full size wing section and its possible effects but as Robin points out the TDM model wing is flat bottomed,the only change is some washout sanded into the soft block tips. [ that need repair ]

Easy/ simple things first. So I am going for a working rudder and stiffen / thicken the ailerons and go from there.

Piers, the [model] RATA's I have found on utube appear to have inset ailerons or they are over size non scale.

It is surprising how longitudinal stable the model is,my similar size Spitfire XI is quite touchy. Cheers.

[Piers Bowlan]

It seems that the full sized Rata had a zero pitching moment aerofoil similar to what has been used on flying wings, judging by Robin's findings. The centre of pressure doesn't move with changing angle of attack so therefore there would be little change of trim with pitch. The full size Rata was not popular when it entered service as its high landing speed (no flaps) and poor view over the nose (not helped by the lack of flaps) made it tricky to land especially in a cross wind. The other problem was that it would spin with the least provocation, so wasn't for the inexperienced. On the plus side it was very manoeuvrable and fast for its time, especially when up against Fiats in the Spanish Civil War. The Me 109s however soon put it in the shade.

I did look at the TDM plan some time ago but I wasn't moved to build one. In the thread, 'Peter Miller asks, what next?' I did suggest to him that he build one as a future project. I thought he could 'Millerise' it and produce a flyable model as in some ways it was similar in proportions to his Aug 2004 Culver Dart (see Peter's Avatar). Peter was not impressed with my suggestion however and it got a polite, 'No!' Incidentally his Culver Dart did have inset ailerons and wash out (pretty essential on a tapered wing I would have thought). I just had a look at the plan again and it is 47in span and for a 52FS, if anyone is interested, - not for the inexperienced perhaps.

You are right John, the large Moki radial powered Rata in the video not only had inset ailerons but flaps as well. Not scale at all unless someone is about to prove me wrong!

Anyway, I hope you manage to tame the beast John as it is frustrating building and finishing a model, only for it to fly like a pig, or in this case, a rat.

[J D 8]

Hi Piers, Full size rata's had the ability to droop the ailerons to slow things a bit,I think this was done automatic as the gear was wound down. Many of the early monoplane fighters were not popular with pilots of the biplane era,for the reasons you give and the extra complications of retracts,flaps,variable pitch prop ect.

I16's flying today have better modern brakes and tail wheels to help with crosswinds but this was not so much of a problem when air fields were just that, fields of grass and you could take off and land in any direction.

Rudder has now been made working and wing repairs underway. Cheers John.

[Robin Colbourne]

As with almost all second world war fighters, the I-16 was in continuous development throughout its service. The I-16 Type 24 replaced the drooping ailerons of the earlier versions with separate flaps. It also introduced the tailwheel.

I'm glad Piers has used and explained the correct term, 'non-pitching aerofoil', for the I-16 section, as its a better description for that particular section than 'reflex', which I used.

John, Good luck with the repairs. I'm looking forward to the post-'mid-life update' test flight report.

[J D 8]

The RATA rat has flown and we have control.

Repairs/modifications are working rudder,stiffer thicker ailerons with blunt trailing edges and new wing tips of slightly greater area.

At least we have enough control for me to place it in the sky where I want and in two flights I was able to explore the flight envelope some.

As expected from before it is fast and if it were a cessna would look bloody silly racing around the sky but ok for a fighter I guess,got my heartbeat up a bit anyway.It does not do slow.

Turns can be made quite tight if elevator is fed in steady but a bank and yank results in a sharp flick into a fast spin. This is a trait that the full size had I believe and if this was a good or bad thing for a combat I don't know. The good thing though is it straightens up quickly on centering controls.

However roll rate is slow/poor unless ailerons reach that bite point at the end of travel,in fact most of the time I think the ailerons are just along for the ride with the rudder doing nearly all of the work. [ have done a lot of rudd/elev flying ] As said those I16 pilots must have known how to use the rudder and perhaps we should not be surprised at this after all with most aircraft from the first thirty years of flight rudder was the primary direction/steering control.

So what next ? baring a new [ non scale ] wing with inset ailerons.Extend ailerons out to tip? Perhaps now that we have a working rudder do away with the no down differential and at least with rudder to counter adverse roll the down going aileron would work even if the up going one is blanked in a boundary layer bubble.

[J D 8]