Categories: all aviation Building a Biplane bicycle gadgets misc motorcycle theater

Sun, 18 Mar 2018

Building a Biplane: The Landing Gear Problem

Marquart Charger forward view

The Marquart Charger is generally very well designed, but one of the problem areas over the years has been the landing gear. Thinking about this problem is years ahead of where I am in the build, but it's been bugging me for a while, and I wanted to get some ideas for a fix before I start building any fuselage pieces.

The Charger landing gear is a cantilever design that uses a box-frame leg, which pivots at the lower outside corner of the fuselage frame, and operates a sort of rubber shock absorber/spring under the passenger's feet. It's very clean-looking, since it only has one "tube" going down (most small biplanes have several tubes, forming a kind of 3D triangle).

Because of this single tube design, the pivot point for that tube has to be very strong. In practice, it hasn't ended up being quite strong enough. On top of this, the rubber shock absorber starts out a little bit too stiff, and only gets stiffer with age. The result of these problems combine to show up as cracked fuselages near the landing gear attachments.

Finding a Solution

The problem really breaks down into two issues:

  1. The rubber shock absorber is too stiff, and doesn't provide an effective energy buffer for the gear
  2. The fore-and-aft attachment points for the gear are pretty close together, meaning that braking and rolling-resistance forces are very high

Fortunately, there is a reasonably simple solution for problem #1, which is to replace the rubber spring with a different material. Univair sells the SK-35 Belleville spring kit for the Ercoupe, which is a drop-in replacement for the rubber donuts (also from an Ercoupe according to the Charger plans), and which by all accounts provides a much smoother ride.

The solution to the second problem, though, is one which required (and still requires) a bit more thought.

Marquart Charger gear attachment diagram
The Marquart Charger gear attachment pivot

To this end, I solicited help on the FATPNW Facebook group, and got a couple offers. I just finished up meeting with one gent, B. (to be named later if he is amenable), who discussed the gear situation with me.

A surprising amount of our time was spent getting the existing structure adequately described for him. It's far too easy for me to forget that I've been thinking hard about this subject for a long time, and how clearly I have it visualized. For the purposes of this discussion, I will refer to the part numbers where they exist, from the diagram above.

The chief problem we ended up on was that the -479 part is a little bit too narrow where it meets the fuselage. In the left-hand section of the diagram, you can see the -479 piece edge-on, and you're looking from the side of the plane in toward the inside of the fuselage. In the right-hand section, you see it looking forward. This setup is about the same for both the forward and aft gear attachment points.

Looking at the red arrow on the diagram you can see how the faded red line is a straight line from the surface of the vertical tube to the beginning of the curve around the bottom of the -479 piece. The plans show that divot inward, and B. said that this is likely to be an area of high stress. Flattening out that curve, and making it more tangential to the vertical tube's surface will help to remove that stress concentration. Doing a similar change on the right-hand side of -479, do smooth the transition between -479 and fuselage was also recommended, although it's not drawn in.

The other change he suggested was to increase the length of the strap (the red faded shape on the left side of the drawing), to increase the welded area that attaches the gear pivot to the fuselage.

Charger gear leg diagram
The Charger gear leg

We discussed and discarded a number of other ideas:

These ideas were either pointless (changing part thicknesses), or too complicated (moving the gear attach points).

Generally, the idea he had was to increase the amount of material bearing the stress of the landing gear pivot. As he said several times, it's pretty intuitive. Adding a little bit more steel, and a little bit more weld area, can pay big dividends. It was a gratifying conversation in a way, since that was the idea I'd had as well, but I didn't know if it would negatively impact the stresses in the fuselage in some non-intuitive way.

I particularly like that his proposed solutions amount to a couple ounces of additional weight at most. They're not big changes, but should make a difference in the longevity of the landing gear (assuming I understood the problems correctly; a possibly big "if").

Posted at 14:32 permanent link category: /charger

Categories: all aviation Building a Biplane bicycle gadgets misc motorcycle theater