J

jimgreen

Well Known Member
A friend who spends lots of time upside in his 7 posed an interesting question the other day. During some manoeuvers your feet can slip off the rudder pedals and the cables go slack.
Now imagine a large rudder deflection - maybe due to lots of yaw or a tail slide . Say a screwed up wing over. The rudder cable could lodge itself on top of the rudder stop.
Playing with Ken's machine in the hangar it certainly seems easy for it to get stuck on top. When this happens the cable end fitting prevents centering of the rudder and you are stuck with permanent rudder deflection. You could get it back on the runway but it wouldn't be pretty.
Has there been a documented case of this happening or is all this hypothetical?

Internal rudder stops would make this potential problem go away.
 
I put a rudder return spring on each pedal. Mainly did it to keep the rudder from flapping in the wind when parked. No slack when you take your foot off.
 
Webb said:
I put a rudder return spring on each pedal. Mainly did it to keep the rudder from flapping in the wind when parked. No slack when you take your foot off.
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Early plans called for these springs. Not quite sure why it got deleted later.
 
When we were building our RV-8, we found the "swage" on the rudder cable could get caught on the stop... We approached Van's and they showed us their RV-8 rudder stops, which were positioned somewhat higher than the plans ;) We moved ours up to match, and the problem went away. I know of one other UK RV-8 that met the same problem at final inspection. In spin testing a UK RV-6A I got this problem, requiring a reasonable kick to dislodge it, with due rounding of the rudder stop :D The "cure" is to ensure that maximum up/down movement of the cable end on the rudder does not allow the swage to get to the stop - either by moving the stop, and/or using washers on the cable end to limit movement.

In all the above cases it was the swage on the cable catching the stop, not the entire cable looping over the stop. Whilst the latter is possible, I think it unlikely in the air... and if you are concerned, easily prevented by some locking wire from the stop up to the fuselage.

Internal rudder stops would make this potential problem go away.
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I have expressed my concerns on these before. A rudder stop ultimately is to prevent the rudder contacting the elevator during a tailslide (aircraft going backwards at 30K? 40K? more). If the rudder does come into contact with the elevator the outcome can be fairly terminal :eek: The internal rudder stops have ~half the arm of the external, and seem to contact the rudder at a far less strong piece of structure (skin?). If you are into aerobatics, I would look at any alternative to the Van's design in the frame of mind that it needs to be at least as strong in terms of a tailslide before failure...

Andy
RV-8 G-HILZ
 
.....are to be avoided in these airplanes.....gentleman aerobatics :)
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Pierre...

Sorry - I should have made clear ;) From my RAF learning/instructing days and now my instructor stuff, a "tailslide" is not a "manoeuvre" (I know it is to some!), but a situation to be recovered from - in the same ways as a stall and spin are...

I would not consider any pilot (or aircraft) fit for aerobatics unless they can recover from the basic 3 "UPs" [and spins] - low nose / hi speed, hi nose / low speed, and vertical recovery. The last of these can, and if you are learning "stall turns" (Hammerheads) [which the RV does wonderfully!] often does, turn into a tailslide :eek:

Put it another way - if the RV is redesigned (Internal Rudder stops?) such that an unintentional tailslide becomes a prohibited manoeuvre, then IMHO the aerobatic envelope needs to be severely restricted e.g. declare a minimum speed of say 80K?

Andy
 
proper shaping and positioning of the stops....

My experience was similar to Andy's - by proper positioning and shaping, I made it so that the swage can't get caught, and if the cable did get looped over, it would slide back off under tension.

I also agree with Andy about the questionable stiffness of the internal stops. Not specifically because of tailslides. Just because all the parts contact in areas that are not very stiff. A very heavy rudder input, say to check an immenent ground loop, or a very strong gust slamming the rudder against the stop could bend something.

Andy Hill said:
When we were building our RV-8, we found the "swage" on the rudder cable could get caught on the stop... We approached Van's and they showed us their RV-8 rudder stops, which were positioned somewhat higher than the plans ;) We moved ours up to match, and the problem went away. I know of one other UK RV-8 that met the same problem at final inspection. In spin testing a UK RV-6A I got this problem, requiring a reasonable kick to dislodge it, with due rounding of the rudder stop :D The "cure" is to ensure that maximum up/down movement of the cable end on the rudder does not allow the swage to get to the stop - either by moving the stop, and/or using washers on the cable end to limit movement.

In all the above cases it was the swage on the cable catching the stop, not the entire cable looping over the stop. Whilst the latter is possible, I think it unlikely in the air... and if you are concerned, easily prevented by some locking wire from the stop up to the fuselage.

I have expressed my concerns on these before. A rudder stop ultimately is to prevent the rudder contacting the elevator during a tailslide (aircraft going backwards at 30K? 40K? more). -snip- The internal rudder stops have ~half the arm of the external, and seem to contact the rudder at a far less strong piece of structure (skin?). -snip-

Andy
RV-8 G-HILZ
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Webb said:
I put a rudder return spring on each pedal. Mainly did it to keep the rudder from flapping in the wind when parked. No slack when you take your foot off.
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Hi,

I put these tension springs on both the RV's that i have built. I do however think that they may not be a good idea. Lets say that for whatever reason one of your cables becomes disconnected. The cable that is still connected then permanently deflects the rudder by the spring force. It may not be very much, but it doesn't take much deflection to make a difference, such as when landing with any wind. Perhaps you could slip your foot in front of the rudder pedal and pull it back. Perhaps.

And perhaps i am missing something too, but i think that this issue has been talked about before, and may be the reason that Van has cautioned against them (if indeed he has?)

Cheers,
Steve Ciha
 
Maybe I don't have the picture, but....

steveciha said:
...... Perhaps you could slip your foot in front of the rudder pedal and pull it back. Perhaps.

Cheers,
Steve Ciha
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It seems to me that if one spring came off, you would just have to push a little harder on the opposite rudder to compensate. :confused:
 
PCHunt said:
It seems to me that if one spring came off, you would just have to push a little harder on the opposite rudder to compensate. :confused:
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If a spring came off, step on that pedal. No big deal. I would want to land and fix or remove the other spring.

Springs keep the cables tight and rudder doesn't wave in the breeze when parked.
 
The geometry of my RV-3 could possibly, i stress possibly, allow for this to happen at full rudder deflection. Its very unlikely and I had to use my finger to help the rudder cable find its way over the stop, but i'm not going to challenge murphy's law.


I was going to just use some safety wire from the tip of the stop to a hole drilled in the fuse 1.5" above the stop. My dad said that would look "homebuilt" so he came up with these.



They're pretty draggy for my taste (he added the lightening holes after I told him that) but they will do the job.
 
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pierre smith said:
.....Tail slides are to be avoided in these airplanes.....gentleman aerobatics:)

Ask Van,
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I have pictures (not digital) of the aft fuselage of an RV-3 that encountered tail slides. It is not pretty. We had to rebuild the last 12" of the fuselage. The rudder stops wrinkled the skin on both sides.
 
Just be careful..

Andy Hill said:
Pierre...

..... The last of these can, and if you are learning "stall turns" (Hammerheads) [which the RV does wonderfully!] often does, turn into a tailslide :eek:

Put it another way - if the RV is redesigned (Internal Rudder stops?) such that an unintentional tailslide becomes a prohibited manoeuvre, then IMHO the aerobatic envelope needs to be severely restricted e.g. declare a minimum speed of say 80K?

Andy
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A lot of RV's only have .016 rudder skins and if they're allowed to slam against the stops, ugly things can happen. Firstly, I've seen holes in them where the elevator penetrated and secondly fuselage wrinkles.

If the airplane is never allowed to stay vertical, it ain't gonna tailslide....which I've done in Citabrias and Pitts's. Most guys don't realize that if you happen to get unlucky and the airplane starts doing 50 MPH backwards, you could fold your tail, even if you brace both your feet against the pedals and firmly hold the stick so it isn't snatched out of your hands.

Regards,
 
Hi Pierre...

Interesting points... thanks.

...you could fold your tail, even if you brace both your feet against the pedals and firmly hold the stick so it isn't snatched out of your hands.
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Are you saying that an RV has, or could suffer a rudder failure in a tailslide where the pilot, by correct rudder use, essentially kept the rudder central i.e. well off the stops?

A lot of RV's only have .016 rudder skins and if they're allowed to slam against the stops, ugly things can happen
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I think here you seem to be with my concern for these "internal rudder stops". Firstly, they are only ~half the arm, secondly the stop acutally acts against the skin, not the (strong) horn. Whilst, as you state, an extreme tailslide could over power both types of stop, surely the internal one would be likely to fail far earlier?

If the airplane is never allowed to stay vertical, it ain't gonna tailslide....
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I would agree with you.. except this is adding a distinct limitation over and above what many people would choose to operate to. Indeed, it is a limitation over and above those of the manufacturer (where entry speed for a vertical roll is offered)... and finally, even if one intended to "never stay vertical", I have seen students achieve this unintentionally (and sure I did as a student too!). As "advice" it is of course sound, but am afraid for me, Stall Turns and other Vertical M'vres are still "in".

Andy
 
It seems to me that if one spring came off, you would just have to push a little harder on the opposite rudder to compensate.

Hi Pete,

It isn't the spring coming off that is the worry, it is the situation where the cable becomes disconnected. Half of the rudder pedals no longer are connected to anything, and the one still connected, with the spring has no tension balance against it anymore.

Steve Ciha
 
steveciha said:
Hi Pete,

It isn't the spring coming off that is the worry, it is the situation where the cable becomes disconnected. Half of the rudder pedals no longer are connected to anything, and the one still connected, with the spring has no tension balance against it anymore.

Steve Ciha
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The return springs are not strong enough to pull the rudder against the slipstream. They simply keep the cables from going slack.
 
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