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Old Aug 14, 2012, 04:39 PM
LSF V aspirant
tewatson's Avatar
United States, CA, Orange
Joined Oct 2006
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...Or just open the servo cutout a bit more to the side to line the pushrod up with a shorter horn. Last I checked, Samba was using plywood servo trays (not tungsten) and thus the customer can configure the servo position as desired with simple hand tools.

Tom
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Old Aug 14, 2012, 04:39 PM
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USA, KS, Wichita
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I just installed servos in a Perfection on Sunday and I did not encounter any of the issues Vespa is referring to both Elevator and Rudder have no play whatsoever in them. I also have seen many Perfects launched very hard in high wind conditions (normal flying conditions for KS) and have yet to see a rudder or elevator flutter even once. So once again I do not see the issue.

See Ya,

Pat
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Old Aug 14, 2012, 05:36 PM
Fit to CFIT
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Thousand Oaks, CA
Joined Mar 2004
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OK, you guys are right. The servo cutouts are in the perfect location.
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Old Aug 14, 2012, 05:42 PM
F3B and F3K
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United States, TX, Dallas
Joined Mar 2009
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A mechanically ideal linkage ( according to Prof Drela, but it is not hard to derive it yourself) should be designed like this:

1) adjust ratio servo arm to surface control horn such that you use all servo travel to get the necessary surface deflections

2) while keeping the ratio determined in 1) constant, maximize the absolute length of the surface control horn.

1) minimizes gear train induced slop and maximizes the momentum transfer
2) reduces linkage slop and forces on hingeline and radial force on servo

The pictures suggest that neither 1 or 2 was considered, but I would not blame Samba. Everyone who
is picky about this can modify it. Unfortunately most don't care/know.

FYI I run ratios of 0.5 ( servo arm half as long as control horn) on my F3B planes. I appreciate the control authority on a F3B launch and precision in a speed run.

Does one need it for a J plane: you could get away with a servo with half the torque with an optimized linkage, should be appealing to the weight fetishists.

Reto
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Old Aug 15, 2012, 02:48 AM
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[QUOTE=vespa;22446180]

Bob, your pic looks good but I'm guessing you can only use about half of the rudder servo travel.



Bob used my fuselage installation picture (the one with the yellow fuselage) and it works just fine. I can get 60mm rudder each way and more but of course this amount isn't needed so rudder travel isn't compromised by this set up. All linkages are very slop free and very direct but as I mentioned before - it is good practice to glue the ends of the snake outers to the fuselage sides. Jojo pointed it out to me I hadn't done it at the Worlds and it was this that caused my rudder flutter and nothing else. You have to bear in mind a fast direct tow into 25mph+ winds with thermals blowing through is about as tough as you can get. There were no Pike Perfection failures during the Worlds wheras other manufacturers had some problems......
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Old Aug 15, 2012, 02:53 AM
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Quote:
Originally Posted by stefanoc2 View Post
Neil,
how much ballast were you carrying in your C68?
I understand that there are no ballast provision in the fuse and all the ballast is in the wings.

Stefano
Mine were the early versions. They can carry 4 x brass slugs in total. Each slug is about 160g. Later versions have slightly longer ballast tubes I believe.

Often in windy slots I flew with full ballast (approx 640g) because the thermals were quite strong and the journey home very long
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Old Aug 15, 2012, 10:17 AM
Detail Freak
target's Avatar
Harbor City, CA
Joined Oct 2003
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Quote:
Originally Posted by tewatson View Post
...Or just open the servo cutout a bit more to the side to line the pushrod up with a shorter horn. Last I checked, Samba was using plywood servo trays (not tungsten) and thus the customer can configure the servo position as desired with simple hand tools.

Tom
This appears to be a simple solution to a small (if any) problem.
The manufacturer cannot possibly know what servo the builder will use. Most manufacturers try to make the install of gear as easy as possible so as not to hear the belly-aching of us lazy Americans....

I personally prefer a blank servo tray unistalled for this very reason. My next choice is a servo tray pre-installed with very small or no cut-outs.

Regardless, a $1 piece of G-10 plate attached to the stock tray would allow ANY size cutout you want, and also may add even more strength to the nose.

__________________________________________________ ____________________________

Dear Vespa:
PLEASE stop assuminging that everybody is of lower intellegence than you are.... I know that you don't mean to sound that way, but.....While this may or may not be true, it won't make you any friends.

I know that you are trying to get Samba to modify the trays as an improvement, but its doubtful that Samba will see the tray as flawed when it really is usable as is. That is one area that even I (and I fancy myself as a bit meticulous) will just deal with. I used a G-10 plate on my Xplorer to fit thinner servos than they had intended builders to use (6125H's), and its fine.
Yes, servo installs should be proper, but if they aren't PERFECTION, that doesn't mean the plane won't fly perfectly for the intended use.

Very Respectfully,
Target
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Old Aug 15, 2012, 10:56 AM
MSgt, USAF Ret.
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USA, WA, Spokane
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Target,

Some people just like pole vaulting mouse turds and landing in fires!!!

Jim
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Old Aug 15, 2012, 12:25 PM
Fit to CFIT
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
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Target,

I didn't assume to be smarter than anyone and I don't see which post would have led you to believe that. I did, however, prove to have a better knowledge of geometry than Samba in this case, but I did so with a concrete mathematical equation. There were no assumptions, no opinions, no egos, no preferences, no belly-aching, just pure math.

I'm often baffled by the blind loyalty some people have to these products. Here I explained how Samba's error contributed to Neil's flutter and Jojo's install trouble, and even they are against me for doing so, insisting instead on taking the blame themselves. I'm on your side guys. I know that very few people will get involved with the math and no one likes to hack in spacers or adapter plates, so I'm just suggesting that the manufacturer take a few minutes to update their servo tray layout. It won't cost them a dime and it will help us all.

P.S. Neil, flutter is the result of a vast number of factors ranging from the weight distribution of the rudder itself to the grommets supporting your servos. Your unsupported pushrod run was surely a huge contributor but every little thing matters and it's possible that the long servo arm was the straw that broke the camel's back. So it can't be said that the rudder arm caused your flutter but it's an absolute fact that it contributed, as every imperfection in the system does.
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Old Aug 15, 2012, 12:36 PM
LSF V aspirant
tewatson's Avatar
United States, CA, Orange
Joined Oct 2006
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There's something I can agree with...why use servo grommets in a sailplane? This adds tremendous soft slop and can only make things worse.

Tom

Quote:
Originally Posted by vespa View Post
Neil, flutter is the result of a vast number of factors ranging from the weight distribution of the rudder itself to the grommets supporting your servos.
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Old Aug 15, 2012, 12:48 PM
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Joined Mar 2010
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I agree tom grommets wth. But hey Vespa told me my servos wouldnt work in my xploder and oh yea he was wrong... basicly a tool who like to stir the pot.....
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Old Aug 15, 2012, 12:49 PM
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Quote:
Originally Posted by vespa View Post
Target,


P.S. Neil, flutter is the result of a vast number of factors ranging from the weight distribution of the rudder itself to the grommets supporting your servos. Your unsupported pushrod run was surely a huge contributor but every little thing matters and it's possible that the long servo arm was the straw that broke the camel's back. So it can't be said that the rudder arm caused your flutter but it's an absolute fact that it contributed, as every imperfection in the system does.
Agreed. I used to be a Formula 1 engineer so understand the variables of resonance and harmonics in undamped systems. Yes grommets, despite being done up tight may not be the most rigorous solution mea culpa but it has never caused a problem before and I've had plenty of Pike Perfects without a problem.

That said I think the flutter was down to 3 main reasons: 1) Not gluing the snakes at their endings 2) Using grommets not a hard servo mount 3) Direct towing in winds and thermals so strong it was silly.



It is worth pointing out again that the Perfection was one of the only planes not to suffer a structural failure during the Worlds. I think the servo tray is fine just take care of points 1) and 2) and there will be no problem.
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Old Aug 15, 2012, 03:19 PM
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............
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Old Aug 15, 2012, 05:47 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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Quote:
Originally Posted by vespa View Post
Target,

I didn't assume to be smarter than anyone and I don't see which post would have led you to believe that. I did, however, prove to have a better knowledge of geometry than Samba in this case, but I did so with a concrete mathematical equation. There were no assumptions, no opinions, no egos, no preferences, no belly-aching, just pure math.
You are PROVING my point, by making YOUR point again....
You are ASSUMING that SAMBA designed their servo tray layout with what they thought was the best geometry; I doubt that they did that.
I'll bet they picked what they thought was easiest for the majority to install, easiest to produce, least expensive; not what was the best for the ultimate geometry performance....
That's MY assumption, but I would bet that is correct.

Yes, your installation may be optimum, but others will work just fine.
Some, at the far extreme, can benefit from a "tune up", but most people won't fly a plane hard enough to find out.
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Old Aug 15, 2012, 07:36 PM
kdt
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Joined Feb 2007
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This worked for me

Happy to see my setup was the similar to Neil's. I trimmed the tube a bit and switched the servo position so the elevator would be shortest. Adding a small piece of bass wood to support the tube to the fuse was a must for both rudder and elevator. No flutter problems at the WC with either my LS or ST80...
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