Prop Wash VS Rudder

[HugePanic]

My first testflights at my new plane gone completely wrong.

First flights showed NO rudder effect. The rudder was relatively small, so i doubled the size of the rudder.

Second flights showed NO rudder effect too!!!
BUT: after climbing a couple of uncontrolled meters i shutted down the motor. As a surprise, the plane flew nice as a glider... (little nose heavy)

So, whats up??
Is the prop-wash blocking the rudder function??????
If i think of other planes, most of them have some rudder surface out of the prop-wash. My plane has hardly any surface out of the prop wash.

I dont think that the rudder is blocked from the wing. As a glider it had a much hinger AOA than in powerd "flight".

I also think that the rudder-volume is ok!

Any ideas WHY my plane haves like this???


The images gives an idea how my plane looks, EXCEPT the damn rudder. It is now a V-rudder at the end of the fuselage. (not at the wing tips) There are no slots in the wings now.
http://static.rcgroups.com/forums/attachments/1/8/4/0/2/2/a1723683-65-D_1.jpg

[JetPlaneFlyer]

Have you got any dihedral built in?... Without dihedral (or at least dihedral effect) rudder input wont turn an aircraft, it will just produce a yaw and the aircraft will side slip, dihedral is required to roll the aircraft into the turn.

If you dont have dihedral then you need to use aileron to initate a turn.

Steve

[HugePanic]

the rudder is just a vertical stabilizer!!

the plane is controled by elvons.

With "no rudder function" i mean that it flies like without any vertical stabilizer.. it spins around it´s Z-axis without much control...

[JetPlaneFlyer]

Ahh!... i see

Increase the area of the vertical stab further, if the tail is a 'V' type then decrease the angle between the V. Better still move the vertical stab rearwards by extending the fuselage aft of the wing TE. Shortenning the nose should also help increase directional stabilty.

A two or three degrees of right thrust may also help if the model was tending to yaw left.

[BMatthews]

There's a very good chance that the rudder is being blanked by the wing. You'd be suprised at how high an angle of attack you can have on the wing in a climb.

Also when the prop is turning there's a side area effect related to the propellor disc. If you're on the ragged edge for fin area then it may be enough to counter the fins to the point that you loose directional control. The issue being that when the fins are that close to the balance point they need to be HUGE to get the authourity they need. Look at some of the short coupled jet fighters with dual fins like the F15 and F18. The fin area is massive. There's more fin area than there is stabilizer area. They need that much due to the long nose side area as well as because they are so close in to the CG location.

Also perhaps try adding some short and wide subfins to the bottom of the wing. Short in span but long so they don't get cleaned off when landing.

[Brandano]

Make them as long as the wing and you got a couple of handy landing skids. as long as the COG is in front, the plane will be stable, and you get a some nice knife edge flights as a side effect

[Gary Warner]

With "no rudder function" I mean that it flies like without any vertical stabilizer. It spins around its Z-axis without much control...
The rudder is short-coupled, outside the higher speed airflow of the prop and as Ben said, at high AOA much of it can be blanked out.

Solution (one or more than one):

More vertical fin area
Move fins inboard to enter prop wash
Move fins aft increasing the moment arm
Prop for and fly at higher airspeeds
If, like you said the CG is too forward, move it back (a bit)
Add right thrust if turning a large prop

Note that moving the fins into the prop wash should only be done with the addition of rudder control since the prop thrust will vector with the yaw. Also, keep the forward fuselage profile rounded. Flat vertical surfaces forward of the CG will reduce the fins effectiveness.

[HugePanic]

First of all: THANK YOU ALL FOR YOUR ADVISE!!


Some remarks:
The rudder fins are IN the Propwash NOT as given in the picture!! They are at the end of the fuselage!!!

I know that i can solve the problem with a larger rudder-volume (rudder volume is rudder-area multiplicated with ruder-lever), but i don't WANT to!

The rudder works fine in gliding-mode. No dutch-roll or sth like this. This tells me that the rudder surface itself is NOT too small.

The plane already has two landing skids at the bottom of the wing, at the ends of the elvons. I introduced them after the first flights.

The V-Angle of the rudders is fairly small. about 20° i guess.
I will make a few more sketches later today.

I have one more speculation:

Right infront of the rudder (and so inside the prop-wash) sits the battery and the receiver. Is it possible that the high-speed air from the prop gets deflected by the battery and makes the flow so unsteady that the rudder loosens its function???? This could be an answer why the plalne glides well when the enginges shut down.... :confused:

[JetPlaneFlyer]

Certainly any object placed in front of the vertical stab will reduce it's effectiveness but ultimately I think the easiest solution is to increase vertical stab area and/or moment arm. If the vert stabs are as you say, totally in the prop wash, this tells me that they must be rather small, or you are using a huge prop.

The difference between power on and power off behavior is because the prop wash is always alligned with the vertical stab. When the model yaws sideways the relative wind blowing over the fin, which is dominated by prop wash, ramains virtually alligned to the fin so the fin cannot generate the side force that is required for it to correct the yaw. Contrary to previous advice you may actually benifit from moving the fins outward away from the prop wash. Making the vert stab longer (higher) in it's current location so it extends up out of the wash should also work. Rather than two surfaces in a 'V', a single surface of greater height and about the same area as the two surfaces combined would be more effective.

[HugePanic]

i think you are right, putting the rudder out of the prop-wash would solve the problem.

here is a sketch of my current setup.

What i try first is to make a aerodynamic-fairing (tape) around the battery. this is sth i planed anyhow, but i didn't expect it to be necessary.

If this won't work, i will replace the rudder outside the propwash.

The curious thing:
Lots of aircraft have laaaaarge vetical fins due to interference with the wing at hing AOA (A380....), but i never heard of the effect that the propwash is disabling the vertical fin... :o

[BMatthews]

Panic, if your sketch is fairly accurate to scale then those fins are bordering on being too small.

As with many things in life there's no black and white. A successful fin size for any airplane is actually a range of sizes from almost too small to almost too big.

A fin that is on the ragged edge of being too small will cause issues in a climb that won't show up in a glide. I've seen this on some free flight models over the years. OK in the glide but dutch roll'y on the climb. I haven't seen your model fly but your description and the size you show the fins to be says to me that the antics your model was doing are a form of dutch roll. I'm sure the roughness of the gyrations is caused by the fact that your model is a flying wing with a compact layout.

Just make your fins a little bigger and you'll find that your problem is fixed. The fact that you're close as shown by how it glides is evidence that you're close to the right size already but you just need to increase them by maybe another 15 to 20% in size.

Part of why they need to be larger is the side area of the nose on the fuselage. The same reason those fighter jets use such huge fins as I mentioned before.

[JetPlaneFlyer]

The vertical stabs in your sketch look much too small considering the short coupling. The vert stab area appears to not even equal the side area of the fuselage ahead of the CG so it's really no surprise the model wants to spin around. I'd make the vert stabs much bigger, maybe double the current area, and swept back at about 45 degrees so that the area is located as far aft as possible.

BTW... the A380 and other airliners have such large vertical stabs to reduce dutch roll tendency. They tend to dutch roll because they have enormous lateral stability due to a combination wing sweep (thats needed to make them efficient at high mach numbers) and the dihedral (that's required to keep the engines clear of the ground). Even with the huge vertical stab some form of yaw damper is usually (always?) fitted to airliners.

[Gary Warner]

HugePanic, if it glides just fine then it's the prop blast twisting that's causing the problem. The equipment in front of the fin will reduce the effectiveness, but to what degree, who knows as it will change with AOA and airspeed.

[vintage1]

I have a sneaking suspicion that whatis happening is psychological as much as anything. Rudiders on a delta type wing don;t do much. They certainly wont make it turn hardly at all.

You fly on the elevons..rudder is there mainly to sideslip the model for either knife edging or crosswind landings..

It's not clear as to what you really meant by 'uncontrolled' either. If the plane is fairtly fast, and hand launched, you always get a pretty nasty moment after launch when the airspeed is too low for a proper control authority on the elevons, and the wing is half stalled..power is all you can hope for to accelerate ouit of that condition BEFORE the ground comes and smacks it.

[HugePanic]

.....power is all you can hope for to accelerate ouit of that condition BEFORE the ground comes and smacks it.


nope, this is not problem!

as i said earlier, there was no vertical-stabilizer "function". and it got stable WITHOUT the motor running. the plane was "flying" like a flat plate of plywood with motor. NO controlls at any axis....

i now increased the vert. stabilizers surfaces and moved them outwards. (out of the prop wash)
i just (20min ago) had the next testflight, but i moved the COG back to far.
so i ripped the propeller off at the first attempt... :D

the replacement propeller is now waiting for next flight..
at least i could dial the cog now... so next time it should fly.. *lol*
maybee later today or tomorrow...

[flypaper 2]

Only way I could make them work, and not very well, was on a Zagi. They only turned outward and acted more like a drag brake. To turn left, only the left rudder turned outward. The right one stayed straight. I remember reading about the first test flight on the, I believe it was the 707, where on landing it had a Dutch roll at low speed. The test pilot recommended more fin area. For such a big change, the higher ups decided to put a placard on the dash to come in at a higher airspeed. After a few prangs in Europe, when they looked into it they realized they couldn't read the placard that was written in English. They put a bigger fin on it. The, what is it, the B1 or B2. flying wing bomber, uses clamshell ailerons to control yaw. You might give that a try for an experiment.

[Thomas B]

The curious thing:
Lots of aircraft have laaaaarge vetical fins due to interference with the wing at hing AOA (A380....), but i never heard of the effect that the propwash is disabling the vertical fin... :o

Actually, the A380 and other large transports, particularly those with 4 turbofans have a large vertical tail to help keep the aircraft flying under control when all the engines fail on one side of the airplane.

An A380 does not fly at any significant high angle of attack.

Just to help keep things clear, the rudder is the moving part of the vertical tail/vertical stabilizer. When you say rudder, people will assume that you are speaking of the functional control surface. Your model has vertical tails, but no rudder function.

[HugePanic]

some things to the A380 i know:

high AOA manoevers were at least one flight case that was relevant for dimensioning of the vertical stabilizer. (a flight test eng. told me that)
The wing would be blocking the stabilizer, so it had to be increased. keep in mind that there are larger versions (-900 or-1000) coming...
Longer fuselages worsen the stabilizer-blocking problem

Both wing, and empenage are dimensiond to fit for all a380 versions, so currently it's over-sized for the a380-800.

you are right too, i think the worst flight case might be a landing with one outer-enginge shut-down, with high cross winds.

here is a video:
http://www.airlinetv.net/view_video.php?viewkey=2936a96d3c8bd1f8f2ff

and some facts: