Old-Timer incidence question.

[rcav8r2]

I am almost done with framing up a 96” old-timer; the RCM Spook. There is no mention of the wing incidence on the plans or in the instructions. It shows the tail 0 w/ the datum. It “looks” like the wing has some positive. When I built the fuse, it lined up with the plans exactly, but…..
Engine/motor has a fair amount of down and right. I extended the nose a bit to help have a better chance of obtaining a correct CG and to have some room for the batteries
I am lining up the wing now and I get 3 degrees + in relation to the tail. Is this normal for Old-Timers? I’m used to flying 0-0 planes, or at the very least +2. I’m sure it will be flyable with no issues as after all it is an old timer, and a large one at that, but just curious.

As an aside, I plan on using a Dewalt Brushed motor with a 3:1 GB on 18 cells. Good choice?


THANKS in advance.

[Ollie]

Old timers were trimmed to glide at minimum sinking speed, just above a stall. this trim produced maximum duration after the limited motor run in free flight competition. These models were typically trimmed by shimming the wing incidence for a very slow glide and then the thrust line was adjusted for a steep climb. Little reliance was put on the initial incidence shown on the plans. Two or three degrees of decalage does not seem extreme to me.

[BMatthews]

Ollie, you missed the bit about the rearward CG's on old timers that you usually posts so I'll chime in ..... :D

The free flight versions were also trimmed to operate with a more rearward CG than what we are usually acustomed to. To set up the model for the best duration setting you can use the same dive test as the sailplane fliers use. Work the CG back and retrim the elevators and keep doing the test until you notice that the natural pullup from a dive is a nice LOOOOOONG smooth recovery. The funny thing is that the model not only glides slower and longer but the high speed flight when you trim a bit of down into it will be better too as there is less losses due to trim drag.

You'll probably find that once the dust settles that the final CG position is back around the 50 to 60% point of the wing. Somewhere along the way you will find that you want to reduce the elevator throw but there should be no ill effects beyond that.

It's the long tail moments and large tail areas that allow all this to happen as described. A model of more conventional lengths and areas would be divergently unstable if you tried to copy the same CG positioning.

I have an electric Record Hound that is currently operating with about a 40% CG and I really should take the time to modify it. It doesn't fly badly but it could certainly benifit in the thermalling department with a bit of internal surgery to let me move the battery pack back further. I could add tail weight but the model is overweight as it is so I'm loathe to do so.

[rcav8r2]

THanks guys...

The way the wing is mounted (hooks into the fuse at the LE, I won't easily be able to adjust any more positive into it.

Glad to hear that the CG can be adjust rearward a bit. I don't have the motor yet, but with 14 cells (all I have at this time) in the nose, it ballances slightly aft of the TE of the wing. Another reason why I'm chossing the more bulky Dewalt over a brushless. THis is also without covering, that will add more weight in the tail. The one in the magazine was flown with a Laser .60 Four Stroke, and the guy needed 13 ounces of led in the nose ...Yikes.

I will probably start at 33% just to be safe ( I think the plans show 25%). The only other old timer type plane I had was an RCM Whimpy. Not a true old timer, but with the CG around 25-30% it was very snappy. It snap rolled and spun better than most trainers, and this was with very little movement on the tail feathers. Sold it off when I went all electric.
Also the H.Tail has a symetrical airfoil. Does this effect the CG also... i.e., can it be even more aft?

THANKS again.

[Ollie]

What affects the CG location is the tail moment arm in relation to the wing chord and the tail area in relation to the wing area. The bigger the tail and the longer the tail moment arm the more aft the CG can be. When the tail area is as big as the wing area, the CG will be almost half way between the wing and tail and well aft of the wing chord. The airfoils have little to do with where the CG should be. The airfoils have a lot to do with the decalage to achieve pitch trim.

[Radioguy]

Bruce:

Check your PM's. I replied to your note from mid-December.

Regards,
Lee Smith

[Bill Harris]

This explains some things I've noticed on a current project.

I'm preparing a Great Planes Yard Stick park flyer to carry a 3 ounce digital camera. Being the curious sort, I measured the incidences:

tail=0
wing=+5.3 at root, +3.5 at tip (washout)
motor=-6.6 (but reported to climb better w/ a downthrust of -3 deg)

balance point @33%, but it is reported to fly better at 38%.

This seemed odd to me, but as a slow flyer with a large tail, it may be set up like an old-timer.

--Bill

[Robert Stinson]

I have an electric Record Hound that is currently operating with about a 40% CG and I really should take the time to modify it. It doesn't fly badly but it could certainly benifit in the thermalling department with a bit of internal surgery to let me move the battery pack back further. I could add tail weight but the model is overweight as it is so I'm loathe to do so.

Bruce, if you're still monitoring this thread, could you share some more info about your Record Hound? I'm looking to build one from Bob Holman plans and would be appreciative of your advice...

[BMatthews]

Well, I hadn't been monitoring for about a YEAR until you brought it up.... :D

I can tell you that mine balances at around the 45 to 50% mark and if I was brave enough to start doing major surgery I'd like to see it back at around 60% or so.

Mine was built from Model Builder plans but I've seen some of Bob's laser cutting (built a laser cut Sparky from his kit) and the parts are well done.

The model is a dream to fly as an electric but power would not upset the applecart too much unless you're planning on a Nelson 40 or something equally stupid for contest work. Sorry, if that offends but I can't see the point of old timers with modern high powered glow engines set for Saturn V like performance.

When trimmed with a strongly rearward CG the anhedral tail tends to make the straight ahead flight a little sketchy and the model tends to fall over into a turn to either side. With my forward of prime CG location this effect is minimal but as the CG moves back it'll become more prevalent due to the anhedral and to the spiral stability of the design. Two that were built for Free Flight, with the CG agressively to the rear, that I know off show signs of this tendency.

Looking forward to hearing more about your build. In the meantime here's a teaser. The red one is mine for electric RC and the white one is pure Free Flight with a big sparky in it. Super Cyke if memory serves.....

[Robert Stinson]

Hey, thanks for the response.

I haven't ordered the kit yet, but I will in a month or so. I'll start a build thread, and let you know so you can chip in. It'll be electric, brushless & lipo for weight advantage. Do you have wheels on the tail, or skids, or?

Here's a Playboy Jr. I have, uses an Astriflight 05 Geared.