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Why not a "Rhino Rudder"? Why can`t it work?

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CobraCar11

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I am new to aviation. I have a technical background, worked as a mechanic to put myself thru college. I have read about the "Rhino rudder" that Rutan tried, and the reasons why people say it's not a good idea.

The mechanic in me sees this kind of like `4 wheel steering` and would give some more control. Why couldn`t this work in concert with a standard rudder? I have read how some people have mentioned the force being in front of the CG, and it's kind of like throwing a dart backwards, and I understand the analogy. My question is that as I sketched out a map of the forces, why couldn`t it work WITH a standard rudder, and turn the plane with more control like how 4 wheel steering works in vehicles?

I know Iam comparing two industries, and two environments with tires pushing against solid ground instead of air. Couldn't it work like a canard system, how it shares some of the lift? For example, the Rhino Rudder could act like front wheel steering and carry 25% of the turn, while the Rudder carries the 75%?

4 wheel steering allows vehicles to make tighter turns. I understand about the instability of the plane trying to swap ends if the "Rhino rudder" worked by itself, but couldn`t it work in a system.

I searched for other threads, but did not find lots of other threads that went beyond the fact that it wouldn`t work because it was ahead of the CG, etc.

It works great in my head (ha ha) so hopefully I didn`t just give away a patentable breakthrough! Somehow, I think I didn`t....... :)

Thank you in advance for your patience!

Safe flying.
 

Turd Ferguson

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I am new to aviation. I have a technical background, worked as a mechanic to put myself thru college. I have read about the "Rhino rudder" that Rutan tried, and the reasons why people say it's not a good idea.
I'd be interested in hearing why it's not a good idea? I know some were critical of the appearance but it worked just fine. It's a form follows function thing.

The purpose of the rudder is to provide motion around the yaw axis. That can be accomplished with one, two, three or more external surfaces. How well effective or ineffective the rudder is may be dependent on where it's located, but then again, maybe not. At any rate, I think a good design goal is to have the control perform it's intended function while being as simple as possible.

The mechanic in me sees this kind of like `4 wheel steering` and would give some more control. Why couldn`t this work in concert with a standard rudder? I have read how some people have mentioned the force being in front of the CG, and it's kind of like throwing a dart backwards, and I understand the analogy. My question is that as I sketched out a map of the forces, why couldn`t it work WITH a standard rudder, and turn the plane with more control like how 4 wheel steering works in vehicles?
There is no doubt one could design a control surface ahead and behind the c.g. to create the required moment that fulfills the yaw control function. But why make it more complicated than it needs to be?

I know Iam comparing two industries, and two environments with tires pushing against solid ground instead of air. Couldn't it work like a canard system, how it shares some of the lift? For example, the Rhino Rudder could act like front wheel steering and carry 25% of the turn, while the Rudder carries the 75%?

4 wheel steering allows vehicles to make tighter turns. I understand about the instability of the plane trying to swap ends if the "Rhino rudder" worked by itself, but couldn`t it work in a system.
Sounds complicated. What happens when something fails? Only 25% of yaw control available? Or since they are linked, total failure of yaw control? Why can't a rhino rudder work satisfactory as a stand alone control? I don't see any reason why it can't. A rhino rudder by itself can't be blamed for destabilizing the airplane enough to "swap ends." If what you said is true, than why doesn't the elevator on a canard airplane cause it to flip end over end when nose up or nose down is applied? The canard is destabilizing because pitch moment is being applied ahead of the c.g.

In either case, yaw or pitch, if it's possible for the plane to depart controlled flight while in the normal operating envelope, somebody failed big time with other design parameters.
 

ultralajt

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Firstly: aeroplane dont "stear" by rudder. It uses all controls simuntaneously, coordinated and deflections of them vary trough all process from start to end...
And secondly: If these rudders will be good, all high efficiency competition sailplanes will have one....
 

TFF

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Yes but the rudder and dihedral induces the third axis. If you are trying to turn a plane like a car, it is not a good idea. I know someone who was, I believe, the first rhino rudder Defiant builder. It did its thing. One thing about control is most of the time you don't need much. Its easy to see the ailerons on most planes and although they may deflect 25 deg at full stop, to roll a plane into a turn takes 2-3 deg. Control is a balancing act like ballet, it is not running a road grader through some ground. If you have a rudder in the front and one in the back, the biggest problem is harmony. for the pilot, he/she does not want to know there is any fighting of the controls; you want just pure bliss of the plane doing what you want. Working out that harmony would be miserable with front and back rudders. Creating a cool plane does not necessarily make a great plane.
 

Joe Fisher

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Back in the 1960's some one put a small canard wing on the front of a Cessna 182. They called it a Wren conversion. Normally the tail pushes down to lift the nose so these folks used the canard to lift the nose. I think they actually got it FAA approved but nobody that flew it liked it and the prototype was the only one.
 

Riggerrob

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We don't have rhino rudders because Burt Rutan tried a rhino rudder on his Defiant and concluded that it did not work very well.
Part of the problem is that any vertical surface ahead of the aerodynamic center destabilizes the airplane in yaw ..... requiring you to add more vertical stabilizer to the tail to regain stability.
Just look at floatplanes with all their after-market dorsal fins, ventral fins,title extra fins bolted to the outboard edge of the horizontal stabilizer, etc.

Secondly, 2 rudders (rhino and conventional aft rudder) add complexity, parts count, weight, drag, cost, maintenance, etc.

Finally, multiple small surfaces are less efficient because of low Reynolds numbers, so the math is non-linear.
 

Norman

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Two reasons I can think of this early in the morning: Efficiency and visibility:

Efficiency:
Airplane fuselages are normally longer behind the center of gravity than in front. This means that control surfaces on the nose will have to be larger to produce a given moment than surfaces on the tail. this means more wetted area and therefore drag.

visibility:
It is a critical area. It's conceivable that you could be on a collision course with another airplane and not see him until he's too close to avoid. Imagine driving your car around in a parking lot with lamp posts and a flat screen TV box on you're hood.

The stability problem is simple just make the tail fin (not the aft rudder, just the fixed part) bigger to compensate for the increased area up front. The stability problem is pretty straightforward. All surface area contributes to damping but only fixed area aft of the CG contributes to static stability. Area forward of the CG subtracts from static stability.
 
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Topaz

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I'd be interested in hearing why it's not a good idea? I know some were critical of the appearance but it worked just fine. ...
That's the thing. It didn't work "just fine." Rutan tried it twice - on a LongEZ and then later on the Defiant twin. In each case, rudder authority was weak in some flight modes (high AoA, if I recall), and adding the rhino rudder near the nose meant either reduced yaw stability or truly massive vertical stabilizers to the rear. In the end, Rutan concluded that the added weight and drag the system created wasn't worth the reduction in control system complexity, especially with the yaw-control deficiencies.

Also, there are some misconceptions going around in here. Rutan's "rhino" rudder existed instead of aft rudder surfaces, not in addition to them. The rear vertical stabilizers didn't have any moving surfaces, except for a very brief period on Defiant when it was being transitioned to conventional aft controls. The hope with the rhino rudder was to eliminate long rudder control runs from the pilot's feet back to the rear of the aircraft. This was about the same time Rutan was experimenting with elevons on the canard, for the same reason. If they'd worked, the longest control surface run in a LongEZ would've been about a foot and a half. But they didn't work, and so the LongEZ and Defiant reverted to conventional aft control surfaces for the roll and yaw axes.

The idea of having both a forward and aft rudder pops up every now and again, usually with someone who is struggling with their cross-wind landings. The idea of "direct side-force" seems appealing. However, in the end, it's a needless addition of weight and complexity compared to the much easier solution of simply learning and practicing crosswind landings with conventional controls. Direct side-force has little actual use in aviation.
 

Dan Thomas

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CobraCar11

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Two reasons I can think of this early in the morning: Efficiency and visibility:

Efficiency:
Airplane fuselages are normally longer behind the center of gravity than in front. This means that control surfaces on the nose will have to be larger to produce a given moment than surfaces on the tail. this means more wetted area and therefore drag.

visibility:
It's is a critical area. It's conceivable that you could be on a collision course with another airplane and not see him until he's too close to avoid. Imagine driving your car around in a parking lot with lamp posts and a flat screen TV box on you're hood.

The stability problem is simple just make the tail fin (not the aft rudder, just the fixed part) bigger to compensate for the increased area up front. The stability problem is pretty straightforward. All surface area contributes to damping but only fixed area aft of the CG contributes to static stability. Area forward of the CG subtracts from static stability.
Thanks Norman! I had learned a lot on this issue thanks to you. It was some of you comments that I had referenced in my original post. I had not considered the authority it had due to its distance from the CG. Great point. So basically, it COULD work, but it would have to be larger in front than a rudder in back. Weight and complexity.

Also, I had not considered the visibility aspect. If it's damaged, the only way to tell would be the pilot feeling less controllability, and by that time, you may be in serious danger.

Thank you for your comment!
 

Midniteoyl

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Also, there are some misconceptions going around in here. Rutan's "rhino" rudder existed instead of aft rudder surfaces, not in addition to them. The rear vertical stabilizers didn't have any moving surfaces, ...

Yep, rhino rudder only:


Defiant.jpg


5403d1262569247-why-not-upside-down-rudder-100_5709_re.jpg
 

CobraCar11

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Also, there are some misconceptions going around in here. Rutan's "rhino" rudder existed instead of aft rudder surfaces, not in addition to them. The rear vertical stabilizers didn't have any moving surfaces, except for a very brief period on Defiant when it was being transitioned to conventional aft controls. The hope with the rhino rudder was to eliminate long rudder control runs from the pilot's feet back to the rear of the aircraft. This was about the same time Rutan was experimenting with elevons on the canard, for the same reason. If they'd worked, the longest control surface run in a LongEZ would've been about a foot and a half. But they didn't work, and so the LongEZ and Defiant reverted to conventional aft control surfaces for the roll and yaw axes.

The idea of having both a forward and aft rudder pops up every now and again, usually with someone who is struggling with their cross-wind landings. The idea of "direct side-force" seems appealing. However, in the end, it's a needless addition of weight and complexity compared to the much easier solution of simply learning and practicing crosswind landings with conventional controls. Direct side-force has little actual use in aviation.
Forgive me. I did not mean to say that it had a rhino rudder AND a normal rudder. I was just making a suggestion of how it might be beneficial. I see what you mean about why Rutan tried it. Shorter control runs means less weight and complexity. The longer the run, the more potential for issues to occur over time.

As always, I appreciate your comments.
 

CobraCar11

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Thanks to everyone who replied to this thread. As I am new to aviation, I am learning so much by reading and absorbing info from others. Many thanks.

I had seen this mentioned that it was a bad idea. I just wanted to see the deeper reason of "why" it was so bad of an idea. Coming from other industries, I tried to compare it to something I knew and understood.

I appreciate all the knowledge and feedback. Thank you for the education. I am learning fast!
 

Turd Ferguson

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That's the thing. It didn't work "just fine." Rutan tried it twice - on a LongEZ and then later on the Defiant twin. In each case, rudder authority was weak in some flight modes (high AoA, if I recall), and adding the rhino rudder near the nose meant either reduced yaw stability or truly massive vertical stabilizers to the rear. In the end, Rutan concluded that the added weight and drag the system created wasn't worth the reduction in control system complexity, especially with the yaw-control deficiencies.
The "prototype" Long-EZ also had the original wing from the Vari-Eze that was extended by the center section. It didn't work either, partly contributing to the control and stability problems. This original airplane was described in newsletter CP-23, as was the final version, which was substantially different. If you recall, even the "square" winglet rudders in the Long-EZ plans did not prove to be optimal. Taller rudders were later designed to correct weak yaw authority and those became the standard. Besides, it would be hard to "kneel" the plane with a big rudder under the nose - lol.

The rhino rudder on Burt's Defiant is what was released in the plans; Fred Keller, builder of Defiant #2, stated rudder performance was excellent, however, some would find it objectionable from an asthetic point of view. He was correct but I don't think the winglet rudders on the Defiant were ever supported by RAF.

Also, there are some misconceptions going around in here. Rutan's "rhino" rudder existed instead of aft rudder surfaces, not in addition to them. The rear vertical stabilizers didn't have any moving surfaces, except for a very brief period on Defiant when it was being transitioned to conventional aft controls. The hope with the rhino rudder was to eliminate long rudder control runs from the pilot's feet back to the rear of the aircraft. This was about the same time Rutan was experimenting with elevons on the canard, for the same reason. If they'd worked, the longest control surface run in a LongEZ would've been about a foot and a half.
Actually, elevons were used on the Vari-Eze prototype, N7EZ. It was flown for ~100 hrs in that configuration and was one of 4 major changes made before building the second prototype, N4EZ. The elevons provided poor roll control at slow speeds so conventional ailerons were added to the rear wing. At any rate, this was ~4 yrs before the Long-EZ was built and I'm not aware that elevons were re-visited for the Long-EZ. Can't seem to find any data on it.

But they didn't work, and so the LongEZ and Defiant reverted to conventional aft control surfaces for the roll and yaw axes.
Again, Fred Keller's Defiant, the second built had a rhino rudder and I think the plane is still in that configuration today based on photos around the web. Johnny Murphy , one of the early Defiant builders was the first to put rudders on the winglets. I think if you were to see a Defiant at an airport today, there's a 50-50 chance it will have the rhino rudder. Not all builders deviated from the RAF plans. The original Rutan Defiant, N78RA was/is displayed at the Hiller Aviation Museum, San Carlos CA, still supports the rhino rudder. I can't believe Burt would have allowed it to fly ~1500 hrs if the rudder didn't measure up to his desires.
 

Victor Bravo

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The traditional fin/rudder is much further away from the center of gravity than the Rhino rudder. So it can be smaller and lighter while providing the same amount of control.
 

PTAirco

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I would have thought that (as was pointed out) that the single biggest reason not to use them is that it destabilizes the airplane. Which might be useful in a modern fighter that is so unstable (and hence maneuverable) that only a computer can fly it, but on a homebuilt?

I don't put fletching on the front of my arrows either.
 

Mad MAC

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I have always wondered what the trade study would look like of an all moving active rudder on a Supersonic transport i.e. no vertical surface aft of the CG. It would appear to give a 75% weight reduction in fins weight and little increase in fuse weight although it would have to be on the under side of the fuse.
 

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