Aerodynamics discussion re Pitts Special

[Graham Davies 3]

On 07/10/2023 at 20:45, Paul Marsh said:

Got this cheap today. Not the FMS Pitts, but another Pitts, the Eflite one, now have a micro UMX , this one, the FMS and the big 60cc balsa Pitts!

 

 

Nice Paul.

 

Question for you; do any or all of your Pitts do odd things on Rudder? Both my scratch builds have a very pronounced pitch down when rudder is applied; most noticable in knife edge. Do your do this, or is it a function of me following full size dimensions?

 

Thanks

 

Graham

[Peter Jenkins]

1 hour ago, Graham Davies 3 said:

 

 

Nice Paul.

 

Question for you; do any or all of your Pitts do odd things on Rudder? Both my scratch builds have a very pronounced pitch down when rudder is applied; most noticable in knife edge. Do your do this, or is it a function of me following full size dimensions?

 

Thanks

 

Graham

Hi Graham

 

IF you look at the arrangement of the rudder on the Pitts, you will find that there is far more area above the horizontal axis of the aircraft taking that as the line drawn through the motor centre line.  This is very similar to a Wot 4 or Acrowot arrangement.  What this does is to provide a rolling moment around the horizontal axis as there is more rudder area above than below the axis.  I would think that you would find that application of rudder also causes a roll to develop very easily and as soon as the aircraft rolls, unless you increase wing AoA, then the lift vector is spread between supporting weight and accelerating the aircraft into the turn.  The outcome is the aircraft drops its nose as its natural stability will try and return to the required lift for level flight and does this by increasing airspeed.

 

The KE situation is also fairly typical of this type of design.  On purpose designed aerobatic aircraft, the rudder area is designed to be the same on either side of the horizontal axis.  Even then, unless you move the CG quite a long way aft you will almost always get the nose pitching down in KE and also either the aircraft trying to over or under roll.  If moving the CG doesn't do the trick, then we mix aileron and elevator to the rudder as master to try and fix this problem in KE.  It can mean that you make the problem worse in level flight though so putting the mix on a switch will allow you to check this behaviour.

 

On my Wot 4, it was almost impossible to fly a slow roll using the rudder and elevator to maintain level flight during the roll.  As you rolled left and started to apply right rudder this caused a reduction in the roll rate you had selected because right rudder was inducing right roll.  Once past the inverted stage you start to need left rudder and this induces a left roll that adds to you roll rate and speeds up the roll rate.  By mixing opposite aileron to the rudder so that a yaw in level flight produced just a yaw and no roll, solved the problem.  There is nothing unusual in this behaviour unless despite the designers best efforts he has failed to resolve the problem caused by the unequal distribution of rudder area above and below the aircraft's horizontal axis.

[Peter Miller]

I am a bit puzzled by this.  All aircraft have more rudder above the horizontal  axis.  Now I have never noticed it in any of my designs but then perhaps I don't fly very well. I know that my CAP flew the most perfect knife edge and I don't remember ever needing any aileron correction and one needed a LOT of rudder for that. and I can't say that I have ever heard of it in any full size aircraft.

After all, the amount of differential leverage on  a rudder considering that most rudders are tapered with more area below the centre line will be quite small.

[Peter Jenkins]

Peter, 

 

I agree that most aircraft designed for aerobatics have the rudder area split 50:50 either side of the horizontal axis.  But take a look at the Wot 4/Acrowot types and almost all the rudder is above the horizontal axis.

 

The other point if not noticing it is because your flying skill might be automatically compensating for this design feature.  However, if you fly precision aerobatics, especially in competition, you are particularly attuned to any minor "defect" in the way an aircraft flies.  It comes as a surprise to most club pilots to use rudder to correct the aircraft's heading when "flying the line", indeed to using the rudder knce airborne.  It may not be noticeable in their style of flying.

 

A lot of time is spent on top rated aerobatic designs to get them absolutely spot on - 50 - 100 flights is not unusual.  You notice these things when you look for perfection because you know the judges will penalise you for every minor misdemeanor in the purity of a manoeuvre.

 

Best regards

 

Peter

[Peter Miller]

Actually rereading Graham's statement he is  saying that when he applies rudder it is having the same effect as DOWN ELEVATOR.  If it is the effect of having more rudder surface above the datum line in theory it should make the aircraft PITCH UP.  but his pitches down.

 

I still don't believe that the small difference in area so close to and above the datum line would make the model pitch down.

 

I do appreciate the super fine tuning of pattern ship and the need for ultimate precision  of everything but here Graham is talking about a basic model and an extremely wild effect theoretically going the wrong wy if the larger area above the datum line is the cause.

 

[Neil R]

My Acrowot has always had quite a pronounced pitch down with (wings level) rudder inputs, with negligible noticeable roll. After speaking to a few aerodynamics bods, the best explanation seemed to be that the commanded sideslip causes the rear fuselage to blank part of the 'downwind' half of the tailplane, and since the tailplane is normally providing downforce, a reduction in same causes the aircraft to pitch down.

 

I have a vague recollection that there was a known modification to the Acrowot, involving changing the vertical position of the tailplane, which I think (?) might have been related to this issue.

 

For what it's worth the same bods also explained that you also need to think about tailplane vertical position - and how much it is thus influenced by the increased downwash rearwards of the wing when deploying flaps - to better understand the pitching effect of flaps. High wing with low tailplane (i.e. right in the downwash region) being most likely to pitch up, the opposite extreme being low wing with T-tail.

Edited October 9, 2023 by Neil R

[Peter Miller]

Lets be quite honest You can go on with all these theories and ideas but in fact slight errors in suposedly identical models will affect things.. The only way you are going to solve these various effects is with a proper wind tunnel.

 

I will give you an interesting event to think about.  Early in my air force career I worked on the last Lancasters in the RAF and was able to go in three air tests after major servicing.

 

Part of each air test involved a stall test which involved throttling back,pulling back on the controls until the nose would drop in the classic stall. Then open the throttle and resume normal flight.

 

On one air test the aircraft refused to stall. IT just wallowed along nose high and throttled right back.  The pilot tried this four or five times with the same effect. After that they just gave up.

 

Explain that because no one else could.

[Peter Jenkins]

Well, I don't believe that what Graham has said is that application of rudder will cause the aircraft to pitch down without any yaw and roll.  

 

You know that the secondary effect of yaw is to cause a roll in most cases this is on the direction of rudder travel but, occasionally, against rudder.  As soon as the aircraft starts to roll, the lift force is split between a vertical and horizkntal elements and the aircraft starts to descend as the vertical component of the lift force is no longer fully supports weight while the horizontal component of the lift force provides the acceleration to turn the aircraft.

 

As the aircraft rolls, the rudder force now has a sideways and downward component and that will accentuate the loss of the vertical component of lift and push the nose down.  It doesn't have to be much of a roll for this to happen.

 

A yaw also increases drag, slows the aircraft and reduces lift as the airspeed is reduced.

 

As regards the Lancaster failing to stall, it is a clear indication that there was insufficient elevator force to pitch the wing to its stalling AoA in this particular case.  I have heard of a situation where the tailplane had been fitted upside down causing some poor handling but it was years before this was noticed and rectified.

 

As regards increased downwash causing different effects with where the TP is mounted I disagree with as at low speed the downwash effect is felt quite far away from the aircraft as the flow is treated as incompressible at those speeds.  Your aero boffin may have not been quite so knowledgeable.

 

 

Edited October 9, 2023 by Peter Jenkins

[Graham Davies 3]

Thank you Peter(s).

 

 

 

Graham

 

 

[Martin Harris - Moderator]

New thread created as per request

[J D 8]

On 09/10/2023 at 10:23, Peter Jenkins said:

Well, I don't believe that what Graham has said is that application of rudder will cause the aircraft to pitch down without any yaw and roll.  

 

You know that the secondary effect of yaw is to cause a roll in most cases this is on the direction of rudder travel but, occasionally, against rudder.  As soon as the aircraft starts to roll, the lift force is split between a vertical and horizkntal elements and the aircraft starts to descend as the vertical component of the lift force is no longer fully supports weight while the horizontal component of the lift force provides the acceleration to turn the aircraft.

 

 

 

As regards the Lancaster failing to stall, it is a clear indication that there was insufficient elevator force to pitch the wing to its stalling AoA in this particular case.  I have heard of a situation where the tailplane had been fitted upside down causing some poor handling but it was years before this was noticed and rectified.

 

 

 

 

  I also read of the rouge Lancaster, it belonged to 617 squadron and disliked by those that flew it. Eventually it was discovered that the elevators were reversed and so the were also upside down. 

[MattyB]

On 09/10/2023 at 09:49, Neil R said:

My Acrowot has always had quite a pronounced pitch down with (wings level) rudder inputs, with negligible noticeable roll. After speaking to a few aerodynamics bods, the best explanation seemed to be that the commanded sideslip causes the rear fuselage to blank part of the 'downwind' half of the tailplane, and since the tailplane is normally providing downforce, a reduction in same causes the aircraft to pitch down.

 

I have a vague recollection that there was a known modification to the Acrowot, involving changing the vertical position of the tailplane, which I think (?) might have been related to this issue.

 

For what it's worth the same bods also explained that you also need to think about tailplane vertical position - and how much it is thus influenced by the increased downwash rearwards of the wing when deploying flaps - to better understand the pitching effect of flaps. High wing with low tailplane (i.e. right in the downwash region) being most likely to pitch up, the opposite extreme being low wing with T-tail.

 

Yes, this is a well known on the Acrowot, and has affected full sized aircraft too. We have discussed it before a number of times:

 

Possible explanation from the (now defunct) RCMF forum...

 

"The fishtailing is the result of having a flat-plate fin, which has a low lift-slope (and a bit of hysteresis) at low angles of attack. The yaw-pitch coupling is the result of unequal keel areas above an below the tailplane, so that when it's yawed there is a "pressure trap" under the out-slip side (right-hand-side if yawed left) underneath the tailplane but not above it. It's a well-documented phenomenon and was potentially lethal on certain full-size aircraft. The Miles Magister couldn't be side-slipped at low levels because large law angles produced a nose-down pitch that was so powerful a 150lb pull on the stick wouldn't correct it. The primary function of the large fin-strakes you see on a lot of full-size aircraft is to correct a problem of this nature which wasn't discovered until flight testing.

As to why it occurs to one side but not the other it's probably something to do with the fin blanking some spiral propeller slipstream effects. To prove this see if it behaves the differently power-off to power on, but at similar airspeeds."

 

On 09/10/2023 at 10:09, Peter Miller said:

Lets be quite honest You can go on with all these theories and ideas but in fact slight errors in suposedly identical models will affect things.. The only way you are going to solve these various effects is with a proper wind tunnel.

 

I have flown 5-6 Acrowots of different types, and they all exhibited this characteristic to a greater or lesser degree. It is consistent and demonstrable, and definitley a characteristic of the design, not a building error.

Edited October 12, 2023 by MattyB

[Neil R]

I did find some old stuff on the Acrowot and this pitch down tendency - in this very forum! - link below - scroll down slightly see the post from MattyB. The embedded quote also explains that the full size Miles Magister had a similar characteristic. I don't buy for one moment that this particular effect is due to rolling (change in bank angle), but it is of course a yaw (more correctly sideslip) effect.   

 

I've obviously no idea how well this translates to the Pitts in the original question though!  

 

 

Edited October 12, 2023 by Neil R
Just noting Matty beat me to it!

[Graham Davies 3]

Hi All,

 

Thanks for all the input; really interesting.

 

Peter J, you are right; the [itch couple is not the only 'issue', There is also a huge roll couple, but I was kind of expecting that. I did indeed set some mixers, but it will take a while to get them optimised, partly because I have to really gird my loins before attempting knife edge flight as it really 'kicks off' when the rudder is applied! I get a couple of goes per flight as that's as long as I can hold my breath...

 

It's good to hear so much understanding of the issue. I can accept traits of a design; that comes with the territory with scale models in my mind. 

 

Graham

 

[Shaun Walsh]

2 hours ago, J D 8 said:

  I also read of the rouge Lancaster, it belonged to 617 squadron and disliked by those that flew it. Eventually it was discovered that the elevators were reversed and so the were also upside down. 

I never knew they applied makeup to Lancasters, you live and learn!

Edited October 12, 2023 by Shaun Walsh

[ron evans]

This topic has a touch of de'j'a vu about it for me.

 

Back in the last century this was discussed on the other channel, and my thoughts at the time were that when the rudder and elevator hinge line coincided, the deflected rudder would spoil the tailplane producing downforce. and the model would pitch down.

 

Booth my aerobatic gliders at that time had rudders well aft of the tailplane, and did not pitch down with rudder application.

 

I also cited a review of a CAP scale model that did not pitch down with rudder application ( might have been the P R model ). The CAP aeroplane has the fin and rudder well aft of the tailplane and elevator.

 

 

 

 

 

 

 

[ron evans]

This topic has a touch of de'j'a vu about it for me.

 

Back in the last century this was discussed on the other channel, and my thoughts at the time were that when the rudder and elevator hinge line coincided, the deflected rudder would spoil the tailplane producing downforce. and the model would pitch down.

 

Booth my aerobatic gliders at that time had rudders well aft of the tailplane, and did not pitch down with rudder application.

 

I also cited a review of a CAP scale model that did not pitch down with rudder application ( might have been the P R model ). The CAP aeroplane has the fin and rudder well aft of the tailplane and elevator.

 

 

 

Meant to put P.M. model.

 

 

 

[john stones 1 - Moderator]

Cermark Pitts, great model and flew it a lot, would pitch to canopy in K.E. elevator at neutral. Used no mix to disguise it, but nice feeling when I occasionally got it right.