rudder lock

[AVOCET]

I've heard at various times the term Rudder lock , but I carn't seem to find any explanation as to what it is and how it works aerodynamically speaking .

 

can any one please explain in detail .

 

much appreciated .

 

Mike

 

 

[Roscoe]

I've heard at various times the term Rudder lock , but I carn't seem to find any explanation as to what it is and how it works aerodynamically speaking .and scrwewedcan any one please explain in detail .

much appreciated .

 

Mike

Ive seen them on Cessnas. Two pieces of wood like material attached to either side of the Rudder interconnected and screwed tight to prevent the Rudder from moving in strong wind. Just dont forget to remove during pre flight!!!

 

 

[Roscoe]

And only seen them on Aircraft parked outside for long periods of inactivity

 

 

[AVOCET]

And only seen them on Aircraft parked outside for long periods of inactivity

sorry , not rudder lock ,

I should have said , rudder reversal in flight

 

 

[djpacro]

Rudder lock is also a common term for it. Have you asked google? Search also for dorsal fin (as a fix).

 

 

[AVOCET]

Rudder lock is also a common term for it. Have you asked google? Search also for dorsal fin (as a fix).

yep , doesn't really give any theory

mike

 

 

[kaz3g]

Safari books online "Airplane Stability and Control" Ed 2

 

Kaz

 

 

[djpacro]

or http://nptel.ac.in/courses/101106043/downloads/025_Chapter%205_L18.pdf

 

 

[facthunter]

Looking through that, I would have a couple of concerns with the terms used. Some of the assertions should not be taken as absolute. Nev

 

 

[Guest Maj Millard]

I have an old PBY Catilina pilot operators manual and it states in there that rudder lock to one side was possible in certain flight profiles, and because the rudder was so large and manually operated by cable it may not bepossible to reverse the condition.

 

Only suggested solution was to drop the nose and put more than 80kts on the aircraft and 'blow' the rudder back !....Not a lot of help if your at 200ft....which may have accounted for a lot of low level Cat crashes..........Maj.....

 

 

[facthunter]

If "rudder lock" refers to when the rudder cannot stop the plane turning it seems a silly term. VMc (a) and VMc(g) are more understandable terms. relating to control in the air and on the ground respectively. VMc(a) can be varied by power change or lowering the good engine .( Banking towards it. )It also changes with different engine(s) out, unless they are rotating in opposite directions on a twin. I have never heard the term used, so I should have crashed long ago, no doubt. This is a bit strange since about 90% of my flying has been in multi's . Nev

 

 

[AVOCET]

or http://nptel.ac.in/courses/101106043/downloads/025_Chapter 5_L18.pdf

thanks for that , it seems that those little dorsal fins make a lot of difference .

cheers mike.

 

 

[AVOCET]

it seems that if the v/fin reaches the stall it creates the rudder lock ,

 

I was warned not to go to full rudder deflection in the Avocet until a "no rudder lock situation could be established ".

 

mike

 

 

[facthunter]

Well that would be a pretty poor design. I have felt that happen. The Fokker F-27 had a dorsal fin fitted later as did the P-51 Mustang, to activate the air in the region if the rudder. Lighter aircraft would pick it up in the sideslip situation perhaps. Nev

 

 

[AVOCET]

Well that would be a pretty poor design. I have felt that happen. The Fokker F-27 had a dorsal fin fitted later as did the P-51 Mustang, to activate the air in the region if the rudder. Lighter aircraft would pick it up in the sideslip situation perhaps. Nev

I don't quite follow ,

are you saying the P51 / F 27 are a poor design , or just the Avocet ?

 

mike

 

 

[facthunter]

No . Just that something has to be done if there is that kind of problem and the problem is not uncommon.. The rudder has a job to do, particularly in multi's

 

There are other aspects to this. Stability as distinct from controllability. Compromise is of the essence in aircraft design, but there are "must have" features. The P-51 was in service for many years with people still learning about some of it's flight charactistics, mainly relating to controllability in yaw and roll at high power and angle of attack, which were not dissimilar to a twin. Nev

 

 

[AVOCET]

I'm trying my third v/fin & dorsal fin combination to improve the stability .

 

the first one was improved by adding the dorsal fin , the second attempt with a fatter v/fin went the other way ,

 

the third attempt was to add more area than the-first attempt ,

 

what do they say ; third time lucky!

 

also I added 1/2 deg more incidence to wings so the the tail doesn't sag .

 

mike

 

 

[facthunter]

That will change your inflight attitude in pitch. That's a pretty complex aircraft. Does power change pitch trim much.? Nev

 

 

[Head in the clouds]

Mike - I don't see very much of a dorsal fin at all but I do see that you've added a ventral fin at some stage. That might help directional stability and/or spin resistance but won't do anything for rudder effectiveness. A dorsal fin acts like half a delta wing lying on its side and when there is any non-linear slipstream, as in a slip or skid or landing in a crosswind, it produces a leading edge vortex which flows over the fin/rudder and helps to keep the airflow attached to the control surface.

 

Have you flown it with the new fin yet?

 

 

[AVOCET]

That will change your inflight attitude in pitch. That's a pretty complex aircraft. Does power change pitch trim much.? Nev

there are no sudden pitch changes , however, as the power is reduced , suddenly , the nose doesn't drop , but doesn't pitch up either , same for power on , the pitching moment isn't extreme , it's quite docile , it's a vey easy plane to fly compared to a jab .

 

I would expect if it was a tractor those pitching effects with power would be more pronounced .

 

mike

 

 

[AVOCET]

Mike - I don't see very much of a dorsal fin at all but I do see that you've added a ventral fin at some stage. That might help directional stability and/or spin resistance but won't do anything for rudder effectiveness. A dorsal fin acts like half a delta wing lying on its side and when there is any non-linear slipstream, as in a slip or skid or landing in a crosswind, it produces a leading edge vortex which flows over the fin/rudder and helps to keep the airflow attached to the control surface.Have you flown it with the new fin yet?

yea, got that wrong , ventral , not dorsal , sorry.

cross winds ok , have demonstrated about 10 knts so far , .

 

it's different sitting up the front , having no t much of a reference .

 

have only taxi tests so far on new v/fin , still trying to get my nerve back .

 

I have to say at this point I'll be starting the test flying again from scratch.

 

mike

 

 

[Head in the clouds]

yea, got that wrong , ventral , not dorsal , sorry.cross winds ok , have demonstrated about 10 knts so far , .

it's different sitting up the front , having no t much of a reference .

 

have only taxi tests so far on new v/fin , still trying to get my nerve back .

 

mike

Yes, I far prefer sitting up front, love gliders and Drifters for that reason but as you said, there's less reference - good reason not to clean every bug off the screen.

 

I've always found a yawstring to be an excellent reference device, one of my students years ago improved on it even more. He had a bit of trouble with pitch reference tended to porpoise along a fair bit. He bought his own Drifter and promptly fitted a yawstring onto a base made from a telescopic radio antenna. Then he adjusted the length so that the yawstring was on his visual horizon, he was pretty tall so he couldn't just stick it to the windshield, but something along those lines might help you perhaps?

 

Every sympathy with the getting the nerve back thing, not always as easy to get back on the pronking horse as we might wish. I'm sure you're aware that every forum member - and on HBA - is wishing you well with the forthcoming testing though!

 

One step at a time and you'll soon be up there again. All the best to you, Alan

 

PS - in the photos Avocet is looking as good as new, you've done a remarkable job of the rebuild.

 

 

[AVOCET]

Yes, I far prefer sitting up front, love gliders and Drifters for that reason but as you said, there's less reference - good reason not to clean every bug off the screen.I've always found a yawstring to be an excellent reference device, one of my students years ago improved on it even more. He had a bit of trouble with pitch reference tended to porpoise along a fair bit. He bought his own Drifter and promptly fitted a yawstring onto a base made from a telescopic radio antenna. Then he adjusted the length so that the yawstring was on his visual horizon, he was pretty tall so he couldn't just stick it to the windshield, but something along those lines might help you perhaps?

 

Every sympathy with the getting the nerve back thing, not always as easy to get back on the pronking horse as we might wish. I'm sure you're aware that every forum member - and on HBA - is wishing you well with the forthcoming testing though!

 

One step at a time and you'll soon be up there again. All the best to you, Alan

 

PS - in the photos Avocet is looking as good as new, you've done a remarkable job of the rebuild.

thanks Allan , this forum is a great source for knowledge , and inspiration , to say the least ,

 

the pitot tube is out the front and visible as a reference , I'm glad I put it there ! as it's really needed , I might put a string on it to aid further .

 

mike

 

 

[djpacro]

Rudder lock refers to when rudder pedal forces reverse so need to push the other side to reduce rudder deflection - a more accurate term would be rudder overbalance. If a big aeroplane with high forces then it can "lock" such that the pilot is unable to get the rudder back to neutral. Not acceptable for certification. I've encountered rudder overbalance on a couple of first flights of production aircraft.

 

 

[Dafydd Llewellyn]

Thanks, DJ - about time somebody answered the original question. There's a subtlety to it, though; let me see if I can explain: Rudder lock occurs when the rudder hinge moment reverses, which can happen at sufficiently large yaw angles - we define control surface hinge moments in two ways: Firstly, the tendency of the control surface to "blow back to the neutral position" that one might measure in a wind tunnel, where the aircraft is always pointing directly into wind. Secondly, the tendency of the control surface to blow hard over against its stops, if the aircraft is sufficiently yawed. If you apply, say, left rudder, keeping the wings level, the aircraft will yaw to the left, and a rudder that has no aerodynamic balance will tend to be driven to the left. Where the rudder actually ends up, if the pilot takes his feet off the pedals, is called the "free-floating angle", and it's the consequence of the balance between these two effects. If the second effect predominates, the rudder will want to go hard over, so the aircraft will remain in the yawed position, unless the pilot forces the rudder back to its central position. The second effect actually acts to reduce the effort needed to yaw the aircraft.

 

This is what is called "rudder lock"; and it's not actually a consequence of aerodynamic balance of the rudder, but rather due to the lack of it.

 

Suitable forms of aerodynamic balance can increase the first effect and reduce the second effect, to the point where the free floating tendency of the control surface acts to increase the resistance to yaw. You will often see rudders that have a horn balance; it may come as a surprise to discover that they take more effort on the rudder pedals to make the aircraft yaw, than does a simple rudder without any balance.

 

A lot of small aircraft have poor control harmonisation - mainly in the form of too-light rudder forces - simply because they lack any form of aerodynamic balance on their rudders. This may possibly contribute to stall-spin accidents, because the rudder is too light for the pilot to realise that he has too much rudder applied.

 

The design of aerodynamic balanced control surfaces is not simple, and there's an element of "black art" in it; so this is NOT a good area for an amateur designer to play around. However, the most common "fix" for rudder lock is a dorsal fin, especially one with a sharp edge along its top; any time you see one of these, it's a fair bet the aircraft either had, or the designer anticipated, rudder lock.