Aircraft Maintenance

[JimD]

I had forgotten about that one! The plywood flap servo mount broke apart allowing the flap to droop on one wing. Had to take the wing off, epoxy the mount back together and then continue flying.

[JimD]

We got to test Desert Aircraft's advice at the "Great American Aerotow" event in Delaware last week. My Porter (number three in a series) is just over one year old. The first two were powered with single cylinder DA-85 engines and had a large external canister attached to the underside of the plane. My goal with Porter #3 was to get the smoother power of a twin cylinder engine plus have internal canisters for a cleaner look. We also wanted to have as quiet of a setup as possible so we decided to go with a 3-blade prop.

We ended up with a DA-120 engine (which weighs less than a DA-100 or DLE-111), internal MTW-75k front dump canisters, and a Mejzlik 27x12TH prop. This seemed perfect, clean lines with headers and canisters hinden internally, minimal prop noise, and good power. The problem with this setup appeared over time as the power output gradually decreased until it became painfully obvious that something was terribly wrong at the Sky High Aerotow event in Muncy, PA. The engine would start easily and idle fine, but had no top end, no power to tow even the smaller sailplanes.

After changing spark plugs, rebuilding the carbeurator with new diaphragm and gaskets, replacing the Hall sensor, changing out the entire ignition system, and re-plumbing the fuel tank and fuel lines, we came up with the same results. Thinking that there must be an engine problem, we replaced the DA-120 with another complete engine and ignition module...SAME results! This meant that we had to look elsewhere and the only thing not replaced was the canisters.

A call to Desert Aircraft gave us direction. The conclusion was that our choice of canister model, prop size, and application were not a good match. While all of these components were recommended for use with our DA-120, the combination did not allow the engine to reach efficient rpm levels during towing operations. The result was a greasy wet exhaust output instead of a more powdery dry output. This caused the canisters to clog up and restrict exhaust outflow with a corresponding reduction in top end power. We tried cleaning out the canisters with lacquer thinner and some power improvement was achieved, but still not what it should be, so we ordered new 2-blade props as recommended by DA. Just to make sure, we also ordered in a new set of MTW-75k canisters.

We got a Mejzlik 28x10 EVO and a Falcon 27x11. The 28 inch diameter Mejzlik proved to be too big and we caught a prop tip on landing and broke the prop (oh well, there went $100). The Falcon was bolted on and we never looked back...the engine came alive and the power went up exponentially! This setup is awesome, but the downside is noise...you don't get something for nothing. With the higher rpm's comes increased engine and prop noise.

Bottom line, I guess if noise is a major issue at a certain venue, one could run the 3-blade 27x12TH prop for a brief time. We are also going to try a smaller diameter and/or flatter pitch 3-blade.

[stevekremer]

I spoke to DA today. For the 120 with cannisters and towing, they recommend 3 blade 26x10TH. So I ordered one. I will try it on the Pawnee.

Steve K

[Steve P]

Was that quote based on using the small cans? The 75K are not the usual spec on the 120 as I recall?

[stevekremer]

I told DA that those were the cannisters (75K) I was using

Steve K

[Steve P]

Cool. I have one and will try it on mine too.

[JimD]

For a 3-blade, that makes sense...I was using a 3-blade 27x12TH...the recommended prop is not only a smaller diameter, it is also a flatter pitch. This should help to get the rpm's up while keeping the prop noise down. This will be the next prop that I plan o try.

[Charlie]

All I know is this combination tows My half scale like a rocket ship
Good Job Dr Dolly!

[JimD]

MTW has updated the 75k and 110k series to new versions...check this link

http://www.mtw-daempfer.de/en/silencer/silencer-with-in-and-outlet-in-front-short.html

[RShelby]

MTW has updated the 75k and 110k series to new versions...check this link

http://www.mtw-daempfer.de/en/silenc...ont-short.html

These appear to be 10 -20 mm longer than the TD 75k depending on whose measurements you believe.

[stevekremer]

Jim,

Doing some maint on Porter #1
There is a wheel collar at the bottom of the landing gear strut - after significant measuing, looks like I need a 4 mm wheel collar. Assuming that the OD of a 4mm wheel collar is 9mm.
Can you verify this for me?
Where can I get them?

Thanks

Steve K

[JimD]

Steve,

The original parts had brass spacers in these joints with a 4 mm bolt holding things together. The brass inserts were quickly worn out by pivoting on the threads of the bolts. As the parts failed, I substituted wheel collars that were close in size. Using a large milling file, the thickness of the wheel collar was reduced and the opening in the strut was drilled out slightly to match the diameter of the wheel collar. I think I even had to drill out the hole in the collar to allow the 4 mm to pass through.

I got longer 4 mm bolts that were not fully threaded (like a shoulder bolt) and cut off the excess length. This gives a smooth shaft for things to pivot on.

Seems to me that the wheel collar size was 1/8".

[stevekremer]

The wheel collars you put in were pretty worn. I had already started filing to get the thickness correct. The OD was too big and I did not want to open up the hole in the strut. So, I locked the wheel collar on a 3/16" drillbit, put the drill bit in the drillpress, then, held a file against it until I got to the correct diameter. Then drilled out the set screw as the hex pattern was gone. Worked pretty well. Thanks Jim

Steve K

[lenb]

Make sure the bolt that goes through the collar has smooth shank where it passes through otherwise the threads act like a file and chew up the metal.


Len

[stevekremer]

Thanks Len. Did that!

Steve K

[lenb]

Steve,
If you can give me the outside diam of the collar and the inside, plus thickness, I can turn a few for you out of bronze. I have some bushing stock that would work well, I have some oilite bushing stock too.

len

[stevekremer]

Len,

OD=9mm
ID=4mm
Thickness=5mm

Thanks

Steve K