acro.wot nose drop

[skylark3f]

What ho chaps,

I wonder if anybody has any experience here with the acro.wot, designed by Chris Foss. Or more specifically, does anybody have any experience with the apparent nose drop that this model has when the rudder is applied in flight?

My model is quite new, only about two years old (i've been told these have been in production since before I was born) but someone mentioned that they have always had this problem with the pronounced nose drop with rudder deflection. I would have thought the designer would have designed this flaw out by now since it has been in production since the early 80s so I gather. Or maybe it's the case that due to a number of complicated and interlinked factors concerning the models dynamics, this flaw must remain to give good handling and performance elsewhere?

For those who might be unaware, the acro.wot is a low wing sports model loosely based (in appearance anyway) on a Zlin of some kind. It's the sort of model that people move onto after they have pranged their first couple of models I suppose it's fair to say. I have mine set up as per the instructions, with an O.S. 46 FX (rubber mounted) turning an APC 11*5 or 11*6 prop with the recommended throws on all the control surfaces and with the CG more or less bang on the recommended point.

The only thing that is wrong is this blasted nose dropping. I have had a quick look over the model and perhaps come up with some reasons for this but I was wondering if anybody else had any ideas?

It's not the biggest problem in the world and only shows up really in the aerobatic manoeuvres where larger amounts of rudder are required i.e. stall turns, flick rolls and knife edges, that sort of thing: in all other areas I can not fault this model one iota. I know I could buy a more pattern-ish model to get round it but not now as I am a student and money is tight.

I have probably gone on for long enough now,

Cheers for any ideas,


Jack

[skylark3f]

hmmm... maybe this is a harder question than first seems. I have a few ideas of my own as to why the nose drops so dramatically when the rudder is applied but I didn't want to put them for fear of sounding daft.

I discounted the prop-wash because the nose drop happens which ever direction of rudder is fed in.

My next thought was that maybe the fin has a blanking effect on the tailplane when any yaw is present, rendering the tailplane less effective therefore causing the nose to drop. Maybe the tailplane is too small too far forward (too small a tail volume)? Perhaps it could do with being larger or further back? Maybe the tailplane is not in the optimum place vertically: perhaps it might be more effective on top of the fin? Does any of this sound plausible?

I realise that the designer has probably gone through all of this but I find it quite interesting anyway. I suppose the thing to do would be to experiment on my second acro.wot which was given to me by a mate... the problem with this being I chopped the orignal tail off to put a v-tail on. Nevermind lol

I have also tried to include a picture of an acro.wot also, just for good measure.

Cheers,

Jack

[JetPlaneFlyer]

Maybe the CG is too far forward? Does the model need any up trim to fly level and does it need quite a bit of down elevator for inverted flight?... If so you could probably move the CG back(do it in small steps).

You could also check the theoretical optimum CG using this web tool: http://www.geistware.com/rcmodeling/cg_super_calc.htm

Steve

[vintage1]

I have to say I don't really understand the question..all the manoeuvers you described will result in rudder being needed as you say in large amounts, because the model is sideways anyway at some point..so nose drop is more about the rudder anyway..it having replaced the elevators as the main means of pitch input.

Are you saying that in knife edge..the simplest case with the model banked hard over, you need more up elevator to hold it level?

Normally you would anyway.. without a lot of forward fuselage side area to give lift..

The acro-wot is a sweet handling good looking lane: ultra precision aerobats are less forgiving and sacrifice looks for neutrality on the controls..and will have longer tails and deeper forward fuselages and normally a mid wing position.

Moving the CG back will tend to make the plane more neutral, at the expense of making it more sensitive..

[hul]

this depends on CG position (more forward will require up trim and drops the nose less) and location of the elevator (moving it down will make the nose drop less).

Mix it out if you use a computer TX, rudder -> elevator mix (apply up with rudder).

Hans

[skylark3f]

Sorry for the confusion Vintage1, clarity in written English has never been something at which I have ever excelled.

The problem may simply be described thus:

flying in cruise at about 40% throttle (estimate), elevator trim in the middle with stick more or less central (some input needed for attitude correction in rough air etc etc). The elevator itself is in the neutral position. Rudder is applied and almost instantaneously, the nose drops. If no correction was made to the flight path, the wot would hit the ground. CG is in the limits suggested incidentally. It has a nose tank so I have to wangle it so full tank the CG is ahead of optimum CG suggested and empty tank the CG is behind. I had guessed this is what everyone does?!

For a more complicated example, the stall turn. Feeding in rudder at the top of the ascent, the nose drops again. However, because in this instance the wot began in a vertical attitude, the nose will "drop" forward until the plane is in the cruise attitude and continue to drop until it is very nose down.

In the knife edge, the plane flies well but the stick has to be held back all the time to stop the nose from shooting off.

I suppose non of this is a problem as such, its just a little frustrating. I thought the problem was quite an interesting one though.

Thanks for looking,

Jack

[Mel Duval]

Check out the post from the 3D master, Gary Wright....

http://www.rcgroups.com/forums/showthread.php?p=1240841&highlight=knife+edge+belly#post1240841

[vintage1]

Sorry for the confusion Vintage1, clarity in written English has never been something at which I have ever excelled.

The problem may simply be described thus:

flying in cruise at about 40% throttle (estimate), elevator trim in the middle with stick more or less central (some input needed for attitude correction in rough air etc etc). The elevator itself is in the neutral position. Rudder is applied and almost instantaneously, the nose drops. If no correction was made to the flight path, the wot would hit the ground. CG is in the limits suggested incidentally. It has a nose tank so I have to wangle it so full tank the CG is ahead of optimum CG suggested and empty tank the CG is behind. I had guessed this is what everyone does?!

For a more complicated example, the stall turn. Feeding in rudder at the top of the ascent, the nose drops again. However, because in this instance the wot began in a vertical attitude, the nose will "drop" forward until the plane is in the cruise attitude and continue to drop until it is very nose down.

In the knife edge, the plane flies well but the stick has to be held back all the time to stop the nose from shooting off.

I suppose non of this is a problem as such, its just a little frustrating. I thought the problem was quite an interesting one though.

Thanks for looking,

Jack


Ok..yaw->pitch coupling is what you are saying. It is odd..yaw-> roll is common. I suspect its associated with dihedral and sweepback.

Oh, and reading Grays post..the swept back rudder line will add 'UP' elevator when full rudder is appllied..curiouser and curiouser.
.

[Mel Duval]

..........................

Oh, and reading Garys post..the swept back rudder line will add 'UP' elevator when full rudder is appllied..curiouser and curiouser.
.


Hi Vintage,
"Back in the day" I had a "Galloping Ghost" system that used the rudder pulse frequency along with a swept back rudder hingeline to give elevator control without the need for an additional actuator. Pretty elegant set-up, actually.....

later,
Mel D.

[astirman]

Tailplane area ! On the only model I used to experiment with ways to solve this other than mixing, the only change that made any difference was to increase the horizontal tailplane area ! Once this was 25% of wing area the problem dissapeared. Only on this particular model of course, others will probably be different. Interesting never the less.

[vintage1]

Increasing tailplane area has the effect of needing to move the CG aft to maintain stability..perhaps that is really the key..to move the CG aft to get more foward fuselage area, and to do that and keep stability, you need a bigger tailplane!"

[Work in Progress]

My next thought was that maybe the fin has a blanking effect on the tailplane when any yaw is present, rendering the tailplane less effective therefore causing the nose to drop.

Well done, and have more faith in your deduction - you're exactly right.
Same thing happens on an awful lot of full-size aeroplanes. The hurricane does it a lot, and the Miles Magister is notorious for it, for example. Even the humbple Cessna 172 and PA-28 do it a bit.

Maybe the tailplane is too small too far forward (too small a tail volume)? Perhaps it could do with being larger or further back? Maybe the tailplane is not in the optimum place vertically: perhaps it might be more effective on top of the fin? Does any of this sound plausible?


It's not that it's too small, it's just that in stable 1G flight it's providing a download to balance the nose-down pitching moment of a cambered airfoil operating at a positive coefficient of lift. So when you blank part of the tail with the fuselage turbulence in a sideslip, of course the nose goes down. A bigger tailplane might help a bit, especially if longer span.

The real culprit is the location of the tailplane in relation to the deep fuselage. Sometimes the fore-aft location of the fin in relation to the tailplane can change the effect.

To be honest it's such a typical feature of many full size types that I don't really consider it a flaw, just a characteristic.

[Work in Progress]

For a more complicated example, the stall turn. Feeding in rudder at the top of the ascent, the nose drops again. However, because in this instance the wot began in a vertical attitude, the nose will "drop" forward until the plane is in the cruise attitude and continue to drop until it is very nose down.
Jack

Do you always stall-turn in the same direction, i.e. "with" the engine (to the left with a standard prop rotation?
If so (and most people do) this could be a gyroscopic effect. The Zlin 526 and Yak 50/52 exhibit it very strongly. So does my Formosa when it has a big heavy prop on it.

It's one reason why I only used to do stall-turns at a reasonable height in the Yak. I don't like being near the ground with zero speed, full rudder and full up elevator to prefent the nose from tucking under!

It may also be that you're not properly unloading in the vertical upline before you whack the rudder in - to hold a perfect vertical upline will need the stick to be a little way forward of where it is for level flight - and it's also possible that reducing the power slowly at the top of the manoeuvre will help. Don't put the rudder in too soon, it leads to all kinds of complications.

[Work in Progress]

Increasing tailplane area has the effect of needing to move the CG aft to maintain stability..perhaps that is really the key..to move the CG aft to get more foward fuselage area, and to do that and keep stability, you need a bigger tailplane!"

Increasing the span in particular of the tailplane and elevator can have the effect of reducing the proportion of the area that is blanked by turbulent flow off the fuselage in unbalanced flight. It improves the chances that the tip on the inside of the slip is still in clean air.

[skylark3f]

Hey,

Thanks for all the posts and comments chaps. It sounds to me like I need a larger tailplane then. Increase the tailplane and elevator span but making sure to keep the AR below that of the wing right? So the wing will always stall first? I will go for the 'at least 25% of the wing' like astirman says and see how things go.

Regarding the direction of stall turns...
to build up precision and accuracy my instructor (astirman) suggested I fly the manoeuvres in front of me in pattern styley. I try to always do the turn do the nose falls aways from me, this way its a LH stall turn at one end and a RH stall turn at the other.

Still dont think I have the time of the rudder inout quite right yet, sometimes its too early and sometimes too late... lol.

Thanks for your help again,


Jack

[JetPlaneFlyer]

It's not that it's too small, it's just that in stable 1G flight it's providing a download to balance the nose-down pitching moment of a cambered airfoil operating at a positive coefficient of lift. So when you blank part of the tail with the fuselage turbulence in a sideslip, of course the nose goes down. A bigger tailplane might help a bit, especially if longer span.


This is the bit that makes me curious... On most properly trimmed aircraft the tail does not provide a downforce... it provided positive 'upward' lift. Only on an aircraft where the CG was ahead of the centre of pressure (25% MAC) would the tailplane force be 'down'... this would be a very inefficient way to trim an aircraft.


This is why i suggested that the CG may be too far forward way back in the second reply ;)

Note also that the Acrowat has an un-cambered airfoil.

[Work in Progress]

That might be the case for certain specialised free-flight models but it is simply not the case for a typical conventional aeroplane. On 19 out of 20 aeroplanes with positive pitch stability flying in the sky today, or any time since about 1914, the tailplane is providing downforce in the cruise and in every other 1G configuration, for that matter. (I exclude the modern dynamically unstable 'fly-by-wire types' and possibly certain uncertificated competition aerobatic types.) You can observe it by tufting or on some of the more oil-drizzling types, like the Yak 50/52 or Chipmunk, by observing the way the oil stains flow around the tail.

In full size aviation it is unhelpfully the case that CG location is generally referred to by a simple measurement from a datum point rather than as a %age of MAC, but the two I can think of offhand (because I've worked them out by hand) the Yak-52 and Hurricane, both have the CG range well ahead of 25% MAC. It may be that modellers, operating without control feel, purely by visual reference and with irreversible powered controls, are routinely able to operate with far more relaxed pitch stability than full size types. They don't have to worry about stick force per G or about a positive correlation between stick force & position and trimmed airspeed throughout the speed range. That would certainly explain why so many modellers are reluctant to trim well back to slow down to a stable low speed before starting a landing approach! It might also explain why so many RC models have very small elevator deflection angles compared to equivalent full size types.

My copies are 200 miles away at the moment but I'll dig in to what Darrol Stinton has to say on the subject when I get back to them. His books "The Anatomy of the Aeroplane" and "The Design of the Aeroplane" are standard texts for the Empire Test Pilots School course and widely recommended as part of ATPL theory - they've been bibles for me for the last 30 years. Not cheap but some serious libraries have them.

The predominantly downward-acting force of the tailplane is why aeroplanes with slotted leading edges in the stabilator have the slots up-side-down (Cessna Cardinal, F-4 Phantom etc)! It is also the reason why every full-size aeroplane I know that *doesn't * have a symmetrical tail airfoil has a downwards-lifting one.

You can see download on the tail quite clearly in some of the flimsier light aeroplanes, if you look out the back. The PA-38 Traumahawk is a good one as you can see the T-tail clearly out of the back of the bubble canopy. Accelerate to Vne and you can see the tailplane loading, tips down, just as if you'd loaded it with sandbags on the ground. When tailplanes fail in flight due to overspeeding the airframe, they always fail tips down. And in accounts of WW2 bombers losing half a tailplane, or having the fabric burned off part, the pilots are always having to pull like mad to make up for it, not push.

[JetPlaneFlyer]

I can’t comment too much on full size practice because I have little experience in the area, so I accept what you are saying in regard to some full size aircraft, in certain trim configurations. All aircraft have a range of acceptable CG and I’m sure for many the forward extent of the range would result in a negative lifting tail. From my modelling experience, the CG, at least with an empty tank, is rarely ahead of the 25% MAC point (with the obvious exception of tailless aircraft and canards)
I've checked a couple of RC plans I have at hand and they have CG located just behind 25% MAC indicating a slightly lifting tail (camber effects excluded). It would be interesting to see some designs where this was not so.

There is an online calculator that works out the neutral point and optimum CG positions here: http://adamone.rchomepage.com/cg_calc.htm
Note that the CG according to this tool is usually behind 25% MAC which would indicate a 'lifting' tail. I appreciate that wing camber comes into play too but for the model in question in this thread there is no camber.

[vintage1]

Increasing the span in particular of the tailplane and elevator can have the effect of reducing the proportion of the area that is blanked by turbulent flow off the fuselage in unbalanced flight. It improves the chances that the tip on the inside of the slip is still in clean air.

It also..continuing your theme..leads to a tailplane with no downforce, or possible even upforce on it..so it would seem to work for all of the mechanisms thus proposed.

[vintage1]

It is certainly possible for any aircraft to have a lifting tailplane..it just has to be large enough and/or far enough away. In airliners where fuel economy is crucial, I suspect it is more the norm than in less commercial planes. The downside is the plane gets sluggish in pitch..the extreme case is the canard, where to achieve a decent response the elevators are moved to the front wing..and the CG is somewhere about the leading edge of the 'tail' plane..

The typical modern pattern plane DOES have a long tail moment..extreme pitch sensitivity is not required. It also has a deep fuselage that gives lift in knife edge, and a carefully balanced lateral area all over.

You get a smooth performing plane, but not one that can be thrown into violent manoeuvres.

It's personal choice I suppose.

[JetPlaneFlyer]

I think maybe the difference here is largely between the set up of full size aircraft with their typical thick cambered wings and something like the Acrowot with its symmetrical wing section.

Typically a symmetrical wing model like the Acrowot will be set up with both wing and stabiliser at 0 degrees. The only 'decalage' is provided by a touch of 'up-elevator' trim. This works out something in the order of 1 deg decalage between wing and stabiliser, much less than most full size aircraft.

To be producing negative 'lift' the AoA of the stab must be negative. If there is only one degree between wing and stabiliser then for the stab to be at negative AoA the wing clearly must be operating at a positive AoA of less than one degree. A sub 1 degree AoA on a symmetrical wing would give a Cl vale of about 0.1 or less... i.e not nearly enough to generate sufficient lift for flight unless the model is incredibly light or incredibly fast.
In reality the wing must be operating at (say) 3 or 4 degrees AoA, or more. This means that the stab must also be operating at a positive AoA and therefore MUST be generating positive lift.

To bring this back on-topic... as the stab is (according to my deductions above) producing positive 'upward' lift then blanking of the stab can’t be the issue... If blanking was occurring then the nose would go up, not down.

Or at least that’s how it looks to me :)

Steve

[Work in Progress]

I must say I can't fault that logic. The only thing I've flown with a symmetrical airfoil, the Extra 300, felt exceptionally weird and un-aeroplane-like, more like a ride-on video game than a proper aeroplane, no real control feel. I only had about half an hour in it and didn't really come to terms with it at all, it didn't tell me what the airflow was doing or how the aeroplane was sitting in the air. So maybe that's the key to the difference.
Meanwhile, ho-hum, this poor chap still has his nose going down with rudder. Next question is it going down as a function of sideslip or as a function of yaw? In other words, does the pitch effect vary with the rate at which the nose moves off line, or with the angle to which the nose is off line? If the first, then suspect gyroscopics. If the second, suspect aerodynamics.

And just as a wild card, I suppose there is no possibility whatsoever of the controls interacting mechanically, is there? Under air loads maybe there are pushrods clashing and interfering with each other?

[rorywquin]

What you describe is typical of an Acrowot. You can mix it out with rudder causing slight up elevator and opposite aileron. I did this on mine and it would yaw almost perfectly. I lost my first AW in a mid-air and when building the second, found a thread that recommends installing the tailplane about 1" lower than given in the instrustions. I never finished the build and sold it on and cannot comment it this works.

I also had an Extra Wot which, exhibited the same tendencies even with the relatively bigger horozontal stab area!

[vintage1]

.

To bring this back on-topic... as the stab is (according to my deductions above) producing positive 'upward' lift then blanking of the stab can’t be the issue... If blanking was occurring then the nose would go up, not down.

Or at least that’s how it looks to me :)

Steve

How about WING blanking..

[skylark3f]

hey guys, thanks again for all of your input, it's appreciated. As for the controls interacting mechanically with each other, I don't think this is the case. The rudder is on a closed loop system and the elevator is operated by the hard balsa push-rod, as per the intrustions. I have just checked (with a torch and mirror on a stick) down the fus. to make sure though lol.

To me, it reads like I need to move the tailplane lower, increase its area or both.

Thanks again, Jack