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Jan 10, 2018, 12:28 PM
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Balance on rudder

I'm putting together another vintage 70's Astro Jeff . It's a big gas bag and I like it . The picture shows a very large
aerodynamic balance on the rudder .
My question is this , was that design element just a big thing back then or can I just make the balance permanent to the fin and use the rudder alone ?
Thanks in advance
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Jan 10, 2018, 12:47 PM
B for Bruce
BMatthews's Avatar
There are a few reports from even back in the days of these models where folks altered them to change over to a more conventional rudder and found either no difference or in a couple of cases that they get even better rudder performance from such a change. This isn't a guarantee mind you. But I saw it mentioned in the magazines at the time enough to suggest that you'd be OK and might even like it better.
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Jan 10, 2018, 01:10 PM
Registered User
Thanks , I appreciate your fast response .
I've heard exactly the same opinion some time ago but that settles it , off comes the balance .
Some of my friends who fly pattern experienced some unexpected bad performance when the balances on their ships got overdone .
Jan 10, 2018, 01:53 PM
LSF303-AMA Fellow
tkallev's Avatar
The biggest trick to flying the Astro-Jeff is to get the balance point back in the 35% CG range and to fly it at a decent airspeed, do NOT simply float around like a giant gasbag, it has size and legs ... use them. The designed rudder is plenty reactive as long as you don't float around at minimum airspeed.
Jan 10, 2018, 02:25 PM
HerkS's Avatar
A regular fin with a separate rudder is like a small wing with a flap. A wing with a flap can generate more lift than a wing with no flap. That tail is more like an attempt to create an all moving tail like an all moving horizontal stab. Most of the RES type models that have used an all moving vertical tend to wallow a bit at times because apparently that type of vertical provides less yaw damping than one with a fixed portion. A rather extreme example is the original Hobie Hawk. But - they fly ok so no need to make big changes unless you have a problem with its handling. If there is a problem with wallowing or related issues, try a dorsal fin from the top of the fixed portion down to the top of the fuselage and forward five or six inches.
Jan 10, 2018, 03:27 PM
Registered User
Herk, your description of wallowing or Dutch walking is present in my Aquila ,pictured here ,which has an even more pronounced all moving rudder , as speed increases the effect also increases.
That trait is exactly what I'm trying to stop.
Jan 10, 2018, 03:50 PM
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Don Stackhouse's Avatar
The other issue with that huge counterbalance is "snatching".

As Herk said, most of that tail resembles an all-flying control surface.

On all-flying control surfaces it's important to hinge them at their "Aerodynamic Center" ("AC"), which is about 25% aft of the leading edge at the surface's Mean Aerodynamic Chord ("MAC").

If the hinge is ahead of that point, you get larger control forces that put more load on the servo. However, with today's servos that usually nt difficult to overcome.

OTOH, if you have the hinge line significantly aft of the AC (and it looks like that tail definitely does, by a large margin), it is overbalanced. As such a surface begins to deflect away from neutral, the force from the counterbalance portion will try to "help" the servo, but by too much. Combine that with even a tiny bit of flexibility or slop in the linkage (and remember, NOTHING is perfectly rigid), and the surface will deflect farther than you asked it to, maybe by quite a lot. If you try to deflect it back the other way to correct for this overcontrol, it will be sluggish coming back to neutral, but as soon as it crosses neutral, the forces on the counterbalance will reverse, and the control surface will "snatch" in the other direction.

The net result is a plane that has a mind of its own, with very erratic control response, sluggish initially but then suddenly overcontrolling all by itself. NOT a good way to try to control an airplane!
Jan 10, 2018, 04:07 PM
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Larry Jolly's Avatar
I have some very recent experience with this having just modified my Astro Jeff. The AJ is a wonderful flying glider IMHO except the vertical is to small. I did and would suggest you modify it as I did, and is shown in the following pictures. In addition it could use an additional 1 dihedral in each wing tip. Otherwise leave it as Jerry drew it. Really a sweet heart. LJ
Jan 10, 2018, 05:38 PM
Registered User
So here's what I think will be a good compromise .
1-eliminate the balance
2-enlarge the rudder
Jan 10, 2018, 05:54 PM
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Larry Jolly's Avatar
Do as you see fit. I see no need to get rid of the balance. I have at least 2 hours on the AJ since Christmas. It is sweet and actually benign. That being said I like the Hobie, Aquila, and Sagitta. However do increase the height. I modified mine because I saw 2 accidents on Tow with the stubby Fin. Larry
Jan 10, 2018, 06:03 PM
Registered User
Thanks Larry , it was my intent to increase the height .
The term " stubby " seems to fit perfectly , always wondered about it .
Jan 10, 2018, 06:04 PM
Registered User
Nice colour scheme Larry.
Jan 10, 2018, 07:18 PM
Registered User
Larry Jolly's Avatar
Race thanks but it is not mine. Good friend Brian Agnew built and campaigned this one. I know it flew in the 87 Nats. Brian knows my love of the old birds and decided I needed it for my Hangar. But I agree this is a good one and flys very well. L
Jan 11, 2018, 05:32 PM
Registered User
This is just a stick mock up of the 2" taller fin and rudder without the balance .
I'm liking it .
Jan 11, 2018, 05:40 PM
Registered User


As I was told back in the 70's, a balanced rudder was so that the "weak" servos we had then, would be able to move the rudder against the wind stream. We do not need a balanced rudder today with 60+ oz-in servos that are 1/10th the size that we had to deal with then

I suggest in my AJ kits, that a balanced rudder in not needed. The tail feathers, then, look really very full size look to them.

Nostalgia class probably would not allow that much deviation from original................


Originally Posted by jetmaven
I'm putting together another vintage 70's Astro Jeff . It's a big gas bag and I like it . The picture shows a very large
aerodynamic balance on the rudder .
My question is this , was that design element just a big thing back then or can I just make the balance permanent to the fin and use the rudder alone ?
Thanks in advance

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