Cessna 172 Weight

[deanoaz]

Newbie here! Can anybody out there give me an approximate weight range of an unloaded Gillows Cessna 172 (no motor/ESC/battery/servos/etc). I say range, because I know the covering can make a great difference by what you use, but I see a number of posts of 172 builds on this forum and I know someone has weighed their plane before loading it with the electronics.....well, maybe the servos are in but I can deal with that. Right now I am trying to define my electronics system and that would be a good starting point.

[Bill Gaylord]

Mine has a sheeted fuse, so it wouldn't be an accurate estimate, but the goal is to be able to accurately estimate the finished AUW of a model. After doing this for a number of years, I can usually hit within grams, even with scratch builds. That really helps in optimizing a model, if you want to keep it as light as possible and still perform well. There is a lot of flexibility in modern gear, where anything sized within reason will work reasonably well. Gear selection becomes more of an issue when an ultra efficient build is planned, with ample power but no more. To accomplish that, a person has to have enough build experience to accurately estimate AUWs. I've been constantly moving in that direction, since lighter models are more enjoyable to fly, but it's a lot simpler to just use a 300-370 class motor for a model like this that will work fine, than to achieve a notably light AUW that will fly well with a 250 class motor. That said, my focus would be much more on the airframe than gear selection. Just far more important on the list of concerns, to have success. My 172 has a minimal washout/warp issue causing the left panel to effectively have a hair more washout. It flies notably better if the right panel covering is re-shrunk before flight, to correct the issue. It would be better if it was the opposite scenario, given effect of p-factor. It flies fine, but the issue does cause a low speed landing approach wing drop tendency, which can be abrupt. Those types of issues are much more along the lines of the concerns I would have.

The 172 is not a bad flying model at all, but is not quite the trainer that the C-150 is, as a 3ch r/c electric with ample dihedral. I'm convinced I could hand the transmitter to a complete beginner with that model, give a bit of pre-flight instruction.

[deanoaz]

Well, the 172 kit is what I picked up, so that is what I have to work with. I am vassilating on whether to sheet it or not. Any advice (since you did sheeting on yours)?

[kittyfritters]

Built Out-Of-The-Box, as a rubber powered model the die cut version will typically weigh about 78 grams. It has about 80 square inches of wing area so at this weight, built straight and trimmed properly, it will fly decently. The current laser cut version will weigh about 52 grams. It flies a lot better. With a little judicious lightening the current version will weigh about 40 grams. I've seen a rubber powered one with radical lightening technique weigh in at 28 grams. Obviously, with a sheeted fuselage, and loaded with electronics your mileage may vary.

Does that answer the question?

Howard

[dirk gently]

Wait, is that the 36'' wingspan cessna? Does it only have 80sq inches?

[Mitch]

I was wondering the same thing... I looked at the Guillow line up and the only 172 is the 800 series model. I did some calculations and I think the wing area should be 98 square inches.

Mitch

[davidchoate]

If your converting to RC take advice of Bill gay lord. I find that building the parts and weigh them before ass. Then weigh the gear to get a idea of AUW will be. Then you can pick a good motor prop to use. 9

[Bill Gaylord]

I use 1/32" sheeting, and try to select sheets of medium to light weight. There's a large variation in weight, of various grades. Off top of head, a 6x36" sheet weight 14gms or less is the range you should be shooting for.

[dirk gently]

According to my calculations, the wing area of the model is 175sq''

It is 1/12 scale, with the full scale plane having 175sq feet of wing area. So the model is
175/(12*12) = 1.215 sq. feet, or 175sq''

[deanoaz]

According to my calculations, the wing area of the model is 175sq''

It is 1/12 scale, with the full scale plane having 175sq feet of wing area. So the model is
175/(12*12) = 1.215 sq. feet, or 175sq''

You just gave me some good backup to my rough estimate of 180 sq. in., measuring from the plans. I appreciate all the good input I am receiving.

[deanoaz]

If your converting to RC take advice of Bill gay lord. I find that building the parts and weigh them before ass. Then weigh the gear to get a idea of AUW will be. Then you can pick a good motor prop to use. 9

I have been adding the weights of everything I am thinking of using (from their spec's) and adding as I go. Just the un-coated fuselage comes in at 1 oz and I keep adding other things (i.e. a steerable nose gear). I have another post where I put in my assumptions for everything and came in at 23 oz. I am bringing that down because I got input there. Thanks, everyone.

[kittyfritters]

Oops! My bad! For some reason I thought you were talking about the 300 series Cessna 170. The 800 series, 36" span, Cessna 172 was a good R/C conversion even before today's light weight electronics. I've never built this one so I don't know what it weighs, but I've seen several of them converted successfully.

Sorry,

Howard

[Bill Gaylord]

Steerable nose gear shouldn't add much weight, if any. One thing I did, and see new modelers often doing is "over hardwaring" their models. I've slowly removed things like pushrod sleeving, which is almost 100% unnecessary in most cases. A steerable nosegear should use the rudder servo, only adding the weight of a steering clevis and short pushrod. I generally avoid purchase clevis parts, especially metal, as they really weight. A reasonable pivot for the gear strut, small stop collar of some form, and light steering arm should be all that's needed. Done more recently, I would have used lighter screws than in the photo below. The small nylon block works well for the strut pivot, having a good bit of flexibility to absorb shock. The small Maxx or Dubro spoked foam wheels are some of the lightest, in this size range.

[deanoaz]

My plan is to tie the rudder and steerable nose gear together. I am in the throws of trying to figure out how to route the wire back to the rear of the fuselage and the rudder. Any guidance of that task?

[Bill Gaylord]

I generally use the fuse formers to route tail feather pushrods, often crossing them in the fuse. The crossing allows for the pushrods to meet the servo horn at 90 degrees for linear response, while also making the routing easier. Whether they are routed straight or cross, depends on the fuse shape and what is most practical based on desired servo placement. At most, I may need one or two light balsa sheet stips across a fuse former with guide holes, to properly support a .032" wire. They don't need sleeving and can easily run up to 5" or even more without support, and shouldn't buckle, if they're routed reasonably straight. I take the assembled fuse frame and map out the desired pushrod path, doing this first from the fuse side and then from either the top or bottom. The idea is to hold the pushrod against the fuse in the desired path, and then eyeball and place a mark on the fuse formers, where the pushrod will route. After doing this from the top or bottom (whichever is more convenient) and then from the side, you should have a cross + mark on each former, which will then be poked through. After a bit of practice, you should find that the pushrods can often be dropped right into place, straight through all the holes and dead on the desired hole or e-z link on the servo horn.

As a rule, I mount servos where they are used as needed ballast, as well as concealing them for scale looks. You never want to mount a servo in a location only to simplify the linkage, but to then need nose ballast to counter it's aft weight. That Sipa jet I just built was a good example of routing tail feather pushrods with a few interesting "s" bends, in order to mount them in a hidden and practical location. No servos sticking out of the wing or fuse for me, as well as adding lead to the nose to balance.