Hi
I Have been flying for a few years now but always fly artf models.
I have started to build my first large model and could do with a little help please.
my wing span is 72" with a wing area of 792sq" i think .
The wing chord is 11" on calculation the whole weight for the model will be 7.5-8lbs , this will be a high wing model with semi-Symmetrical wing.

will this give enough lift for a 8lb model and what size engine would i be looking at to give slow docile flight

hope someone can stear me in the right direction

thanks

This is basicly what you're designing, it's a nice floater.

http://www.hobby-lobby.com/telemaster6_6879_prd1.htm

It won't be that much of a floater
A typical 800 sq in pattern plane from say-1990 weighed 8 lbs -done right -Ibuilt lots n lots of em and customers also did the same
They land at say 30 mph.
a 45 glow engine will fly that setup
or a 90 four stroke also is a good nice cruise setup
Here is the bottom line
It is physically impossible for you to make this setup too light.
ideally it would weigh 5-6 lbs at that size for really docile flying. as a newcomer to all this you will likely overbuild the model so be careful! That extra weigh all goes into making the plane more sensitive to speed and flying angles

You just can't beat lightweight for getting the best results
the airfoil use should be based on what will build into a strong structure The actual airfoil number means little. go for an average of about 12-14% thickness
Take a look at the old Seniorita kits by SIG - this structure is good for a 800sq in model and all up flying weight is 4-5 pounds.

Dick is spot on with his assessment in my expierence. It will be a nice but not really a slow model. Using a smaller engine won't make it slow and docile. The wing loading is the only thing that will do that. This same model built to around 6 lbs would be a slow and docile flyer much along the line of an old timer free flight with radio. At 8'ish lbs it'll fly and handle just fine but not be considered a slow model.

If you're only part way through the build and would share some pictures of the existing wing and the plans you're using to build the model from we could provide some critique on wood sizes. Also in particular if this design of yours uses a lot of big sheet wood parts then be super careful to weigh and select your wood for lightness. Just that alone can make a pound of difference on a model of this size.

Often times new designers over build by quite a bit in the hopes of making a stronger model. But this generally becomes a self fullfilling prophecy in that the heavier and stronger models often end up getting "tested" where a lighter model would not have stalled and spun or would have pulled up tighter or any other variation of the extra weight being the cause of the troubles where the lighter model would not have gotten into the situation in the first place.

If the thread takes a turn to concentrate on structure choices I'll move it over to the Scratch BUilding forum.

Most ARF's have fairly high wing loadings, and good marketing calls them floaters, trainers and gentle flyiers. With this wing area, 6 pounds is at the upper limit of a light wing loading.

HI again


Loads to think about there then,,
If i see what your are saying then the weight is the key here and need to get it down to around 6-7lb max
i was looking at 60 rcv four stroke in the plan but that is a heavy power plant in its self .the wing could be a foam core and that might save a bit of weight, also undercarage and wheels could loose some weight i was planning to cast a glass fiber undercarrage myself so not sure on the outcome weight of that ,,

my estermated weight of 8lbs is an absolute max flight weight fully fueled up and ready to go.
so i will be keeping a close check on the build weight and try to get it lower


thanks for your infomation.

Yea, I missed that when I said it would fly like a Telemaster.[sm=red_smile.gif]

You'll find that it has the power of a weak 40, the weight of a heavy 60, the cost of a 120, and will take forever to break in, but other than that I enjoyed mine,[sm=spinnyeyes.gif] but I love engines.

Having been through all of this many times -
some more food for thought
The landing gear
You simply can NOT beat wire for strength to weight and shock absorbing
fibreglass is the worst.
carbon fibre is light but lousy on shock absorbing
Foam wing - don't try i unless you have done a bunch of em - for this type plane, the open structure is easily best weight to strength Ihave personally done a least 1000 foam wings -of all types (I built n sold custom stuff fo almost 30 years.)
For power - the setup on the Seniorita shown is a .20 K&B with stock muffler as quiet as most electrics - That model is 4 pounds flying weight and all kit wood was used.
A really nice easy flyer and will loop at full speed in level flight (just barely) The airframe -without any hardware -but completed was - 3 lbs.
You can easily do the plane you wnt at 4 lbs
then add hardware
You can NOT add expensive lighter components to make a lighe model
Many try it and just blow a lot of money for very little return.
You start with a set of scales and a plan for the finished weight -
Hope, just won't do the job. The Cap in the picture is 13+pounds and 1100 squares with a 40ccengine and a carbon fibre tuned exhaust-and 7075T6 cystom made gear - it has solid foam panes on all surfaces and an all balsa fuselage it LIGHT by any standards.
That construction was done for again-best strength to weight - The model is an all out 3D /pattern setup
I mention this to show the different approach-for different tasks