wizardprang

Hi all, I am drawing up plans to build a 1/5 scale Mitsubishi A5M Claude. Should I increase the size of the rudder over the scale dimension or leave as is? It is semicircular in shape and has no counterbalance area ahead of the hinge line. Any thoughts, Thanks in advance, Rob.

HighPlains

My Feedback: (1)

Leave it as scale. Prop on a RC model is smaller diameter than scale, so it is not as destabilizing.

Rotaryphile

Wouldn't hurt a bit to enlarge the rudder and fin, while keeping the same shape. Nobody will notice, and the airplane will be more resistant to inadvertent snap roll/spin, with better controllability close to stall. Remember the words of the famous test pilot, Frank Courtney, "No airplane was ever designed with a big enough vertical tail."

HighPlains

My Feedback: (1)

The need to enlarging the tail surfaces is an old canard from the days of rubber powered free flight. With rubber powered models, the propellers are much larger than scale, and since most of these designs are tractor, an oversized prop is destabilizing. Thus the need for larger vertical surfaces in rubber powered models is quite real.

However, glow and gas powered models typically do not have enough prop disk area to cause any problems. Further, RC systems compared to smaller full size aircraft with unboosted flight controls, have rigid control systems, with no float of the moveable surfaces, again adding to the overall stability.

Frank Courtney. I had to look that one up. A man born in the 19th century may not have flown too many RC models in the 21st century.

cyclops2

I always cheat on both tail surface areas. Impossible to pick up by eye.
I use 10 % increase of both as I fly in gusty weather.

Rotaryphile

A lot of light aircraft, in my opinion, could benefit from larger vertical stabilizer surface, particularly light twins. NASA used R/C models of common light aircraft in attempts to determine whether handling would be improved by adding some vertical tail area. The results were that in most cases, handling was improved when vertical area was added. WW1 fighters were a flagrant example of insufficient vertical tail area that resulted in a horrendous stall/spin accident rate. It would appear that designers mainly just copied one another, and slapped on vertical tails that 'looked right'.

Look at the commuter turboprop twins for an example of vertical tails that were designed for good single engine controllability right down to stall airspeed. Their vertical tails frequently have as nearly much area as their large horizontal tails, adding weight and drag, but the designers know only too well that crashes don't help sales, and all pilots sometimes have bad days.

I have managed to improve some poor-handling R/C models by adding area to their vertical tails. For a quick check, you can simply tape on a cardboard extension to the trailing edge of the rudder to check the effect, then make a more permanent modification later, reducing rudder travel in proportion to the area added, if your rudder already had enough authority.

I am inclined to think that Reynolds number effects tend to make vertical tails relatively less effective as scale is reduced, by reducing the velocity of the air moving over the tail surfaces, although I don't know of any wind tunnel tests that attempted to relate handling to vertical tail area.

cyclops2

The SAAB Draken J-35 was a classic example of ..........speed rules.......... It had to have a small vertical stabilizing fin added behind the cockpit after instability in spin pullouts became a killer. pilots died because of ridding out spins a little to low. Fantastic plane. Except when into a full flat spin.

P-40 DRIVER

Are you talking about increasing the rudder area for more control or increasing the verticle stabilizer area including the rudder for added stabilty. There is many factors involved in the stability issue. To little tail, you get dutch roll because there isn't enough stability. to much tail and the airplane will want to go into a spiral dive etc. usually some adustment is required and there are methods for doing this. Keeping the CG forward (25%) will help as well, which may be all that is required. just like CG on the lateral axis there is a center of lateral area on the verticle axis which needs to be maintained as well.

Rotaryphile

Free-flight models and R/C trainers are caught between limits of vertical tail area, as P-40 driver points out. Too little vertical tail, and you get dutch rolling. Too much, and you get spiral instability. However, in models intended for serious aerobatics, spiral stability is usually not all that important, since such airplanes are not intended for hands-off flying.

I have experimented with adding progressively more and more vertical tail area in the form of taped-on cardboard extensions of the rudder on R/C sport models, and found that snap roll resistance got progressively better as the vertical tail area is increased, while reducing rudder travel to suit. A really outsized vertical tail made the airplanes controllable far into deep stall by using ailerons and elevator only. I like to use vertical tail area of about 15% of the wing area on my own design taper-wing bipes, with the area distributed about equally between rudder and fin. A bipe with constant-chord wings can probably use even more vertical tail.

Models intended for intense 3D flying probably should have about twice as much rudder area as fin area. Some non-scale sport bipe models can benefit greatly from increased vertical tail, since bipe tails tend to be blanketed by turbulence from cabane struts and the upper wing. The Ultimate is a good example of a bipe with adequate vertical tail area.

I also like to use subfins and subrudders to get stronger yaw resistance when airspeed approaches stall territory. At high angles of attack, the subfin/subrudder flies in much cleaner air and really seems to help beef up snap roll resistance at low airspeed, without noticebly hurting control response for deliberate snap rolls at higher speed.

cyclops2

I upsize a tail by the "I think that will fly straight in a gusty breeze".

Bozarth

My Feedback: (15)

Wouldn't that make it worse?

Kurt

cyclops2

Possibly.
I fly very consertivly. Low wind & under 1 pound per square foot of wind loading.
I can circle 10' above the water until I see a break in the severe ripples. Then dive in for a fast landing with enough speed in reserve to maneauver. Speed is always my friend on landing.
I have tried to land slow & was forced into a panic to go to WOT and still flop onto the surface. Dumb luck with no skill.
My larger planes do not accelerate fast enough to make up for varying gusting speeds.

Rich

Lnewqban

wizardprang,

This on-line calculator can help you decide on the tail of that Mitsubishi:

http://www.geistware.com/rcmodeling/model_calc.htm

Regards!

cyclops2

Hi kurt.

Are you concerned about a wind taking control of the tail while the plane is doing a down wind return ? That could become iffy. If the plane is only 5 or 15 mph faster than the downwind speed. At about 45 degrees into the return the wind could push the rudder side ways & cause a loss of speed above stall speed.
I had this happen to several GWS warbirds on near stock setup in VERY deep field grass.
Fall down. Need glue.
Speed is my friend.......

Rich

cutaway

My Mk1 Mod 0 eyeball thinks the rudder may be just big enough, but the stab/elevator might need scaling up a tad.

pimmnz

The Claude will do very well at this scale without enlargement of the tail surfaces. Keep the balance well forward (25~28%) and you will have no trouble. Enlarged tail surface were used on small F/F models with the classic 33% balance for 'single speed' flight. Radio models don't need the mods. Take no notice of the 'wind taking control of the tail during downwind turn' theorists, if the model is flying it does not know which way the wind is blowing, and any wind there is always comes straight at the nose. The airplane makes a nice model, the ailerons are pretty big and effective but it looks very pretty in the air. Keep the tail end light as the nose is a bit short.
Evan, WB #12.

wizardprang

Hi gang. Well thanks for all of your input. I think for now I will just replicate the scale rudder and fin. Keeping the tail light is going to vital as the firewall sits right up against the L.E. of the wing.. Will be using a Z45 mag ignition. If I'm going to add weight it may as well be doing something. Hi Evan, agree with everything you said. Whats the WB#12 stand for ?.
Cheers, Rob.

pimmnz

Waco Brotherhood number 12.
Evan.

ARUP

Just a thought. Build it scale size. Make an extra rudder that is oversized for the first trimming flights. Who cares what this rudder looks like. When satisfied with flying trim replace rudder with the scale sized version.

BMatthews

My own Mk1 eyeball also says that the Claude will not require any fudging. The long'ish tail moment will allow those surfaces to do just fine.

And a hearty "ME TOO!" on keeping the tail light. If by some miracle it comes out a hair nose heavy it's an easy thing to add an oz or two to the tail. But an ounce or two tail heavy will require 8 or 16 oz in the nose.