MR Aerodesign's 120" Porter

[Germanscott]

I guess I jumped the gun on the engine selection, it's being recommended that I put an 85 cc engine up front. For some of the engineering types out there, the front end has a 1/4" firewall with balsa longerons and balsa sheeting that go from the firewall back to the windshield. I would imagine I'd need replace the 1/4 x 1/4" balsa longerons with spruce and go with a 1/2" firewall and some plywood sheeting around the top area to replace the balsa sheeting too. Any advice will be greatly appreciated ! With a projected 30 lb. weight for the finished model I guess the 85 would be a better choice.
Not sure about prop clearance with a DA 85, I'm hoping it'll have enough for a 25" three blade.

[JimD]

I can tell you that there is not much structure inside the TopModel Porter. Here are some pictures of the nose with the top balsa sheeting removed:





This is inside the fuselage looking forward toward the firewall. You can see the minimal structure:



The top of the fuselage is just 3/32" balsa sheeting with one 1/4x1/4" stringer. The sides are balsa sheeting with an inside plywood doubler with substantial lightening holes. The formers are lite ply with large areas cut out for less weight. We added more stringers and cross bracing to help counter the torque of the larger engine used.

[JimD]

What is the distance from your model's firewall to the front of the cowling?

[Germanscott]

Jim, hey thanks for stopping in,
What we have here is the top and bottom longerongs at the firewall are 3/8" thick and the other attach points are 1/4". The firewall is 1/4" also. If I used 1/4 x 1/4 spruce for the top longerons do you think I could use 3/32" balsa sheeting on top and bottom ?
From the firewall to the spinner backplate will be right at 11".
If everying goes right I hope to buy Len's DA85.

[JimD]

The structure of your plane looks much sturdier than the TopModel design, but filling that 11" is going to take some planning. We have built several TopModel Porters and the one I fly has a DA-85 on it. At this stage of construction, you have some options to consider, like style of exhaust system and also the needed right thrust for the engine mount.

The TopModel Porter is 9-1/2" from the firewall to the spinner backplate (this allows 1/4" gap between spinner backplate and cowling).
The DA-85 engine is 5-7/8" from mounting flange to spinner backplate.
This means that a motor mount extension of 3-5/8" needs to be added to the firewall to get correct spacing.



In your case, this will need to be an extra 1-1/2" ...or an extension of 3-5/8" + 1-1/2" = 5-1/8"

It would be a lot easier to talk about this than type, so I am PM'ing you my phone #

[cmoore806]

Looks great. I'm subscribed tothis one!!

[JimD]

The firewall of the TopModel Porter is offset by two degrees:



The cowling also has this offset built into it:



We made a motor mount extension box out of aircraft grade plywood:



A hole was cut into the firewall for the extension box to slide through:



The box slides in through the opening and is then anchored with hardwood triangle stock around the front and back:

[JimD]

We made an access hatch in the top of the fuselage nose:



Len helped me by making a header pipe modification that allows for a cannister muffler under the belly of the plane:



The problem we ran into was that the header pipe wants to fail at the flex joint. My plan is to rebuild the fuselage bottom so I can place the cannister inside the belly (like AeroWorks does on their DA-85 powered YAK). In the meantime, we are using a Pitts style muffler (but it is rather loud after being used to the cannister). The nice thing is, it fits inside the cowling:

[Len]

Duane,
I would look close at what Jim has posted here. Its an excellent idea to extend the front out a bit, It would allow you to run your motor box through 2 mounting points. It would be very easy to do.

I'm in Elmira for an event, but Jim and I spoke on the phone this morning about this, he will give some excellent advice.

LEN

[Germanscott]

Excellent pictures and numbers you provided Jim ! I like the shot of your model on take off, pretty cool lookin. Len I'll definatley be paying attention to what Jim has to say, he's built some first class models.
I did manage to get the Porter on it's gear for the first time today, with the 6" wheels I dont think prop clearance is going to be a problem so far. Man I got a long way to go building this one, I may as well start the tail feathers and wings while I wait on getting a refund for this R 50 engine. Once thats done I'll be able to shoot Len some cash for his DA 85. Oh btw Len, I'll take you up on the prop offer too.