Wing loading - how much is too much?

[RGN]

I'm completing my first scratch build and due to some serious miscalculations on my part it's heavier than I wanted it to be.

It's a 58 inch span model and I had hoped for about 80g/dm2 (26 oz.sq.ft) but it looks like it's going to be closer to 88g/dm2 (28.3 oz/sq.ft).

It's got a Saito 91 so hopefully will be OK for power and I built in 1.5 degrees of washout to help prevent tip stalling and it has flaps for landings but I'm concerned the first flight might be it's last.

Is this too high for safely flying something of this size?

Any and all comments welcome, please.

Thanks

Richard

[macboffin]

I shouldn't think you'll have any problems, especially with flaps. Incidentally, if you have flaps, why wash out? With flaps down, the inner part of the wing has more incidence than the outer ; so there's washout!
Having said that, a washed out wing is nicer in turns ; the inner tip in a turn is moving slower than the outer ; it's the first to tip stall. The outer is moving slightly faster, but the washout reduces the extra lift slightly, so the turn doesn't tend to tighten up. Of course you use rudder as well as aileron in a turn? Aileron-only turns reslt in the plane side-slipping into the turn, and, often, losing height. Rudder only turns make the plane skid outwards slightly, but in a steep turn the rudder is now starting to act as an elevater also, tending to raise the tail.Complex aint it? The lazy way is to use CAR, coupled aileron and rudder, so you only move the left stick but get rudder as well as aileron operating.Remember to cut out the CAR before landing though!
Last point ; correct wing drops on final with rudder, not aileron, reduces the chance of a tip stall and nifty roll into the ground!

[Ollie]

If the wing loading is increased 10% then the air speed in level flight is increased 4.88%.

[BMatthews]

You didn't mention the wing area so I guesstimated at 600 sq inches. That means it's a bit over 7 lbs. for a 58 inch span.

That's as much as I'd want to see it be for sure. It'll fly and the flaps will help a lot with adding some lift and washout for landings and likely for takeoffs but it won't be fun at lower airspeeds.

But it's not far off the charts. Many scale models have long and successful carreers at this sort of weight and loading. Just be careful and don't try to horse it off the ground early. With that much power and torque from the size of prop that the engine will require it would be easy to do and just as easy to suffer a snap roll and crash.

[NX-687]

A 91 blows a lot of air over the wing , that helps , Just setup your landing approaches 1 mistake high to evaluate the stall characteristics of the model , before you try landing , Good Luck

[RGN]

Thanks, all, for the comments - I'm a bit more comfortable now but I'll still be real careful on the first flight!

Macboffin - I have washout to improve the all-round handling as you mention - also it can't hurt when flaps are deployed.

Ollie - thanks - is there a formula to calculate that? Would be very useful!

Bmatthews - it is 58 inch span, 8.15 pounds, 651 sq. inches area (all converted from metric so approximate!).

NX-687 - I definitely will. I'll probably be so nervous I'll need at least 2 mistakes high! I usually try a few stall tests nice and high anyway it's just that I'd rather be safe than sorry.

Will be running a 14x6 or 14x8 prop so should be enough draft, I hope.

The subject is a Midget Mustang - see attached photo (unfortunately still has masking tape holding canopy in place while glue dries).

Thanks again for the help!

Richard

[Ollie]

Weight=Lift=1/2 x air mass density x air speed^2 x area x coefficient of lift.

Read Model Aircraft Aerodynamics by Martin Simons, 4th edition, page 11.

Or

Read Basics of R/C Model Aircraft Design by Andy Lennon, page 8.

My point of view is that you need these books for design or scratch building!!!

[RGN]

Thanks, Ollie - I'll get a copy of them.

Richard