H9 33% Pawnee Tug Conversion

[Xroadie]

This is not a new idea obviously.....but all our other threads about it are gone......for now!

I just started working on a commission to build a H9 33% Pawnee for aerotowing. The model will have a DA100, with the standard pitts mufflers. The customer also supplied a tow release mechanism for the tug. Is there anything that needs to be modified or upgraded on the Pawnee for aerotowing? I plan to go with as big a gas tank as possible...100 oz...if it will fit! What about the landing gear...do the springs need to be upgraded?

Thanks for any input!
Pete

[JimD]

Very timely questions Pete, I am getting ready to do one of these too. Mine will have a DA-120 and canisters so I have some additional questions.

[lenb]

Hi Pete,
The Pawnee is an excellent tug, as we know from the Horizon events. A 120cc is a much better choice with more power for the 30lb + models.
Either way, the airframe is a pleasure to tow with.
The tow release area is the turtle deck, behind the canopy. I will post pictures here of the mods I did to mine. The landing gear upper shock bolts and plywood they go through will fail. They should be upgraded to a larger size with smooth shank where the fitting passes through.
Upgraded springs would be great, except we have sold out of them a while back. I did give some to Peter G, he may have them if he never installed them in the tug.
At the recent Cumberland event, a 100cc Pawnee was towing and did well, until it came to the larger planes. With expert flying skills a 100cc Pawnee is fine with larger ships, however when the sailplanes get larger, the skill level and experience must get larger too.

The stock wheels are way too soft for continuous operation, they'll peel off quickly.
The Doors to the cabin must be reinforced on the inside or they'll blow off in flight ( been there, seen that )
The stock tail wheel is also a fail point over a short time frame.



Above is the inside of the turtle deck viewed from below,
I used an old socket and heated it up to melt the foam where I wanted the release



Plywood angles were epoxied into place to provide some strength to the turtle deck and the release was attached to the new bulkhead




Added the servo and rod.

[kd4jaz]

I bought new springs from McMaster, took the gear apart to replace/upgrade the springs. The stock springs are made of stainless and appeared in good order. Put the OEM's back in place. I do think I'll build up the upper gear bearing supports, raise the top support just a little, this will give just a bit more ground clearance for the prop. I also replaced the 10-32 upper bearing bolts as they were slightly bent. Over 3 yrs of takeoffs and landings on those, so I can't complain too much. I picked up two 3/16" stainless steel pins from McMaster-Carr for the upper LG supports mounts. I'm going to build a bulkhead for those and install at a later date. I'll post pictures when I do.

[lenb]

Some more photos of the strut attachment upgrade and the bent upper LG strut bolts

[Xroadie]

Thanks for the input Len...I would go with a DA120 if this was for me, but it's what my customer supplied for the model. Most of the models he will be towing will be in the 1/3 scale range.

I was about to say....I got springs for the Carbon Cub and not the Pawnee...when I realized you meant the other Peter G LOL

So...basically beef up the landing gear plywood and bolts, at some point get beefier springs and a tailwheel, and replace the main wheels.

What wheels did you go to and will a 100 oz tank fit?

Also...where are the doors failing...at the hinge or at the latch?

Pete

[Xroadie]

....just saw your second post Len.....where did you get the new strut hardware....something from McMaster Carr?

Pete

[Xroadie]

....and what am I seeing in this photo?

[lenb]

Pete, The photo above was not supposed to be included in the upload, however its showing how the throttle servo came back and hit the bulkhead resulting in a burned out servo at full throttle.

With a DA 100, an 80 or 100 oz tank is great.

The parts for the struts were from McMaster Carr. I had to use a splice collar and then a threaded piece to attache the clevis.
I believe these were the ones

M4 × 0.7

2448K41

[Xroadie]

The parts for the struts were from McMaster Carr. I had to use a splice collar and then a threaded piece to attache the clevis.
I believe these were the ones

Those clevis rod ends are slick! But there is no failure problems with the what came with the kit....right?

I like those....it's got me thinking about trying those on the Carbon Cub to speed up assembly!

[lenb]

Pete, they were installed to speed up assembly of the Pawnee.

Len

[Xroadie]

Len....why did you use the splice collar and threaded piece on the end of the strut, before you added that clevis? I noticed that the clevis is already threaded for 4mm...which is what comes on the end of the strut...did you need added length with the new clevis?

Also...what fails on the cabin doors....so I know what to reinforce?

And.....does the plywood shock mount plate break free or do the bolts that attach the shock to the plate tear out? I assume if the plate breaks free, I will need to reinforce the plate mount. (triangular stock etc.)..if the bolts are tearing out then the answer would be a thicker/stronger plate....right?

Pete

[lenb]

Pete,
The threads were not long enough on mine, not sure if they were cut to length previously ( I bought it second hand ) You'll have to see.
The cabin doors glue joint on the lower corners fail. Leaves the piece of wood with hinges hanging on the airplane as the door flutters away.
The bolts bend and hole in the wood will rip out.
I put an 1/8 ply piece on the outside of each side of the box and replaced the bolt to a 10-32

LEN

[Swiss1]

Len....why did you use the splice collar and threaded piece on the end of the strut, before you added that clevis? I noticed that the clevis is already threaded for 4mm...which is what comes on the end of the strut...did you need added length with the new clevis?

Also...what fails on the cabin doors....so I know what to reinforce?

And.....does the plywood shock mount plate break free or do the bolts that attach the shock to the plate tear out? I assume if the plate breaks free, I will need to reinforce the plate mount. (triangular stock etc.)..if the bolts are tearing out then the answer would be a thicker/stronger plate....right?

Pete

As Len said Pete, double up the shock box mounting plates and increase the bolt size, the very same happened to Snatch's Pawnee at the Summer event in Cumberland. One bolt ripped through the plate and the other was bent to a 90 degree angle!

Jeremy and Ben
SCC AAA TT TN

[Xroadie]

Thanks for the info gentleman!

Len.....I'll check the strut, threaded rod length and if I can add those clevises with out having to lengthen the rods, then I will suggest to my client that we get a set of four for his Pawnee. I'm all about making model quicker to assemble

On the subject of quick assembly ....I have helped the Horizon guys assemble their Pawnees before and putting in the wing bolts in, was a real pain. I am thinking of trying the "snap slide latches" that Asher and Swiss1 have tried....or using the quick latches that Red Wing RC sells.

I took a look at the hatch doors and I can see what you mean about the glue joints....on this model the wood where the hinges mount are pretty securely attached, but the latches at the top are not, so I will secure those and give all the other glue joints a once over.

So as far as the shock mounts go... it sounds like I will remove the existing blind nuts, add a ply doubler to both the front and rear shock mount plate, to keep the bolts from shearing out, add some triangular stock....or FG cloth, to keep the plates in place and upgrade to a 10/32 bolts for the top shock mounts. I will also replace all the bolts on the LG assembly that have threads riding on the bearing surface.....and from the photo you supplied earlier make sure that the ply doublers do not interfere with the throttle servo arm.

Thanks again!
Pete

[Snatch]

Pete, we doubled the shock mounting area with 3/16 ply front and back and beefed up the blind nuts to 5mm and used 5 X 55 mm DA prop bolts and this has solved the problem.

[Asher Carmichael]

Pete,
The snap slide latches work great and help make assembly a "snap." The next best thing you can do to help speed assembly is..... get rid of the struts....

For those not familiar with the snap latches....



Here's a new take on wing retainers. I am repositioning the fuel tank over the wing rod and CG and decided I needed a different retaining system since it would be problematic trying to install the plastic thumb screws supplied in the kit. I fumbled around for awhile and stumbled across these snap latches at McMC. One per side near chord center should be plenty. I used a 10-24 screw to hold the stub and installed a plate with T nut in the root of the wing. The pics and video should explain it.

https://www.youtube.com/watch?v=ZvIbIjWdzyY