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This may seem like a silly question but I was wondering if any of the classic Sonex builders with the manual trim tab could help. In my plans it states you must use the dial-a-trim for elevator control. I want to use the manual trim for my
model-B and my question is: are you still required to put springs on the elevator control? As of now with no pressure on the control stick it stays in full down (forward) position resting against the wing spar tunnel. Is there any specs in the plans on whether the classic Sonex needs springs and where they are located? Just not sure if those springs are only required for the dial-a-trim setup. Sorry for long winded question and thanks for the help.

-Tim


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The original plans had a spring specified under the seat attached to a tab on the control tree that pulled the elevator near neutral. I have never installed the springs and the elevator works just fine. I use the trim tab from the original design. A friend has the dial a trim on his Onex and likes it.

Tim,

The A-model plans show a balancing bungee, with the note to adjust the bungee so that the elevator is more or less held in a neutral position. This helps serve to counteract the force of gravity pulling on the elevator. Gravity pulling of the elevator functions in much the same way as a down-spring does (down-springs have long been used in order to increase the stick forces, thereby creating a way to fine tune the handling qualities). Adding the bungee will lighten the stick forces.

The dial-a-trim springs are typically adjusted to hold the elevator in the same orientation as the bungee would (with the dial-a-trim in the middle of the travel, of course). In this respect, the springs replace the bungee and provide the same function. You wouldn't use both together.

The bungee is a standard part of the Sonex design, but it's an interesting choice. The elevator stick forces are so light that I wonder if we really need to make them any lighter. In any case, flying one's airplane with the balance bungee installed and then again without will help you decide what handling feel you really want. It's interesting for those that haven't done it (I obviously haven't done this on a B-model, but I'd expect it to be essentially the same as the A-model in this regard).

Jeff

While at the builders seminar at Osh 2004 (!) I recall someone specifically asked the Sonex staff about this. I seem to have understood that the bungee under the seat was there to act as the elevator mass balance: like the ailerons it should be tensioned causing elevator trailing edge to level out 1/4’’ or so over the horizontal with the plane in a level/flying position.

Problem with the bungees I am using is that they tend to loose their pull after while and have to be retensioned every 6 months or so. I got tired of this and resorted to the use of a suitably strong steel spring.

Kai

I don’t recall that my plans (#465) called for any springs or bungee for the elevator, only springs for the rudder pedals. Therefore, I have nothing for the elevator and have been flying it that way for over 14 years. I only have the trim lever, no trim dial.

Elevator bungee on my Sonex TD, has been in place for at least 7 years now, and still is as effective as day one. I use the original lever to control the trim tab and would not change for the dial-a-trim. The lever is very quick to set, and can just flick it from normal cruise setting to full nose up when putting in flaps on landing. A small bit of nice wood screwed on the tip of that lever makes it look less crude.

David A.

Kai wrote:While at the builders seminar at Osh 2004 (!) I recall someone specifically asked the Sonex staff about this. I seem to have understood that the bungee under the seat was there to act as the elevator mass balance: like the ailerons it should be tensioned causing elevator trailing edge to level out 1/4’’ or so over the horizontal with the plane in a level/flying position.

Problem with the bungees I am using is that they tend to loose their pull after while and have to be retensioned every 6 months or so. I got tired of this and resorted to the use of a suitably strong steel spring.

Kai

My plans (#95) called for the bungee as Kai has stated. My Sonex also has the manual trim but I've since replaced the bungee with a pair of springs in tandem with an aluminum strap in between and of the proper length to be able to adjust the static tension of the springs. The two springs I've used are the same ones used in a dial trim system. This works well and doesn't require the periodic adjustment of the replaced bungee to maintain the desired tension.

Art...............Sonex #95 taildragger. ............Jabiru 3300 #261

Thanks for all the replies guys! I think I’ll leave it without any springs for now and add them later if I feel it’s necessary. I too like the idea and simplicity of the manual trim and don’t think the dial-a-trim works for my application. Thanks again!


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DCASonex wrote:Elevator bungee on my Sonex TD, has been in place for at least 7 years now, and still is as effective as day one. I use the original lever to control the trim tab and would not change for the dial-a-trim. The lever is very quick to set, and can just flick it from normal cruise setting to full nose up when putting in flaps on landing.

My experience exactly. Love the manual trim. Bungee still doing its thing after over 5 years.