thexfiles

My Feedback: (7)

i'm in the process of building my second 4*60.
i am making some modifications to the plane because i was not happy with some of the 4*60's flight characteristics.
first, i increased the size of the tail feathers and plan to shorten the wings by one panel on each wing.
i am also using a foam wing that will be covered in 1/16 balsa.
my question has to do with dihedral.
some of my club members have suggested removing the dihedral.
first, it's easier to build the foam wing without dihedral and aerobatic flight is much improved.
rolls are tighter and inverted flight etc is much improved.
i am not an expert flier, nor am i a beginner.
what change in performance can i expect if i make the wing straight?
thanks for your help.

shonny

Listen to your fellow club members and give the dihedral a miss. Theoretically it won't be so stable, - in real terms: it won't be very different, just that inverted flight is pretty well identical to upright.
Go for it man!

shawnh

I took the dihedral out of my trainers wing (Modeltech Right flyer 40T) and its alot more fun to fly. Like you said rolls are much tighter. I bet it will do at least 6 rolls in the time it used to take to just do one roll. When I do a dive towards the ground it tends to go stright down and not try to correct and pull out on its own. I have only put about 3 flights on it since the change in the wing but I have it with me today so I can fly after work and explore some more of its new potential.

Ed_Moorman

My Feedback: (1)

DO NOT TAKE THE DIHEDRAL OUT.

A low wing plane needs dihedral or sweep back to cancel out opposite direction roll when rudder is applied as in knife edge, point rolls, etc. If you take out the dihedral, every time you put in rudder, it will roll in the opposite direction. I speak from experience, having sawed a few wings apart to "fix" the dihedral for better knife edge.

What you are looking for is neutral roll stability. When you put in rudder you get pure yaw. If you have any low wing planes in your club that don't have dihedral or sweep, ask the owner to take it up, roll into knife edge and release the aileron-elevator stick. You'll see pretty fast what the plane is doing. Many people will tell you the plane knife edges great, but they are holding aileron and elevator.

I have had 2 4-Star 60s and still have one. It is bashed into a WWII Kawasaki Tony, but with shortened wings and larger tail surfaces. It has stock dihedral and will hold knife edge with rudder only as far as you can see. I do have a Magnum .91XLS in it.

High wing planes, on the other hand, need anhedral or sweep forward to cancel out same direction roll when rudder is applied.

Mid wing planes shouldn't need anything.

cabanestrut2002

ed is right. the dihedral is there to stop the model rolling when rudder is applied.

my pattern model has a small amount of dihedral, not for stability, just to get perfect yaw-and it does.

JimTrainor

Bruce Tharpe recommends that the dihedral not be taken out of his Venture 60, it is a 4*60 clone: http://www.btemodels.com/vt_faq.html .... he says it will spoil the knife edge characteristics.

My understanding is the Bruce Tharpe designed the 4*40 and 120.

majortom-RCU

My Feedback: (40)

Good advice from Ed Moorman. I flew a Kangke CAP232 Sport this summer, had lots of fun with it, but when I needed lots of rudder, the thing would roll off in the wrong direction and head for the ground. My first reaction was to give it more rudder yet, which would roll it inverted and heading straight down. Irked the heck out of me, 'cause I'd never had this characteristic in any of my previous planes. It took a lot of time mixing rudder to ailerons to straighten this out, and it still wasn't quite right by the time I hit the top of a pine tree I thought I was clear of. I have re-kitted this otherwise good flyer, but this time I will have the mix in it from the outset. When my pattern coach saw this behavior, he put his finger right on it--low wing, no dihedral, rolls opposite the rudder. (Some folks, when they saw this model the first time, thought it was a 4*.)

Ed_Moorman

My Feedback: (1)

If you have another plane that rolls backwards, try cutting the wing tips at a bevel. You know how some trainers have the plastic tips that are beveled upwards 45 degrees? These add dihedral effect, so if your plane rolls backwards and needs dihedral, try cutting a bevel in the tips. You might want to start with a steep 60 degrees and work from there to try to correct the roll. Beats sawing the wing apart.

BMatthews

Excellent advice from all here.

Suggestion. It may be advantagous to REDUCE the dihedral without taking it all out. Since you have a 4-60 do some more flying and based on how much rudder induced roll it has try to make an educated guess on how much you can remove to achieve neutral roll coupling.

Also about reducing the wing span and dihedral in conjunction with enlarging the "tail surfaces". Reducing the span and dihedral is already reducing the spiral stability. Enlarging the fin and or rudder for more total area will also reduce the spiral stability. You may be creating a model that wants to fall into a spiral in a simple turn. If you find it does that cut away the vertical area until it stops.

Also since you are reducing the wing area you are in effect enlarging the stab as well as the % of the wing area is what counts.

Now if these two things are all about MOVING the hinge line to make the control surfaces larger than forget all this. But be ready for cutting down the vertical if you need it.

thexfiles

My Feedback: (7)

thanks for the super response to my question.
to be honest, i had not taken the roll factors into account.
here's what i plan to do:
i'm going to leave the wing span "stock" length. i can always cut them down later.
and i will halve the recommended dihedral.
my frist 4*60 wanted to float when landing and i overshot the runway a couple of times as a result.
also, when i tried to roll the plane it did barrell rolls. the roll rate was awful and no fun at all.
i don't know if this is related but i crashed my original one coming out a loop. i forgot i was on low rates and the plane did not respond to full up elevator coming out of the dive.
that is another reason i increased the tail feather surfaces.
the 4* is a great plane to fly as a next step from a trainer but my goldberg chipmunk is a lot more fun to fly.
thanks again for all your advice.

BMatthews

Well if you were on low rates you can't really blame the model can you....

Seriously, I really don't think you need to increase the tail surfaces OR change the wing other than the diheral.

If it was me I'd go for a fatter and more symetrical airfoil, halve the dihedral and plan on moving the balance back a bit to where the model shows close to neutral pitch stability but will still pull out of a dive very slowly by itself. Tune the elevator travel at tlat point to obtain a snappy but not violent elevator response and forget about the rate swithces. Then it'll loop and snap roll with the best of them. Moving the balance back beyond what was intended for this semi trainer will sharpen up the elevator response much more than you think.

Learn to use very small stick movements when the airspeed, and thus the control sensitivity, is high and stir them around as needed when the airspeed is low. This often changes from one extreme to the other in a single manuever so you really don't have time to bother with dual rate switches. And the fatter airfoil should help slow the model down and produce more drag in the landing approach.

Ed_Moorman

My Feedback: (1)

The reason your 4*60 floated so much and rolled poorly is the wing is too long. The thing has a wing like a trainer. If you are a total novice flier, just out of a trainer, that's OK, but if you can fly and are doing acro, the extra wing will make aerobatics harder. I cut off 2 ribs and added back rounded wing tips. I also moved the gear to the wing. Does it track straight on the ground.

As for spiral instability, been there, too. This is caused by having the vertical area too large. Too small and you get yaw instability. You see a slight yaw instability on many scale planes, especially warbirds. Watch the take off and first turn. If the plane tends to slide sideways before the nose tracks around, the vertical tail is too small. Scale effect.

Spiral instability, on the other hand, occurs when the tail is too big. After you bank into a turn and start pulling up elevator, the plane will tend to increase the bank. It was called spiral instability because in a full scale, if you got into a spiral and pulled back, the spiral tightened up. I have had this a few times from adding more rudder area trying for knife edge loops. With a trainer or other plane with very little elevator authority, you could spiral in with full up elevator. Most of us would roll out and pull and the roll out will cancel out the increasing bank. It is also not violent and you will notice it in turns.

As for the 4*s, they all have fairly small tail surfaces. The rule of thumb for RC design is a horizontal area of 20%-25% of the wing area. I like the larger size since it gives me more lattitude in CG placement. The smaller surface means you need to balance more nose heavy. Vertical area is usually between 1/3 to 1/2 of the horizontal area.

Look at the rudder on any 4*; it is very narrow. The 4*40 easily handles a 3/4-1" increase in rudder width. The 4*60 could use 1-1 1/2 inches. I added height to mine, about 2 inches.

I also put the servos under the stab for balance.

Ben Lanterman

Reply to shawnh,

The airplane dihedral isn't related to the tendency to pull out of a dive and has only a small effect on the roll rate.

Staying in a dive or not is a function of CG location, angular incidence between wing and tail, etc. Not dihedral.

Dihedral determines the roll stability in ungided flight and gives a roll response due to rudder deflection.

Roll rate due to ailerons is primarily a function of the aileron size and deflection. With a typical trainer with typical dihedral when yaw is built up during the roll then dihedral can add or subtract to the roll rate depending on the direction and magnitude of the yaw, however, is a small effect compared to the ailerons.

There are conditions at high angle of attack where the yaw can be substantial to the extent that roll rate can even be reversed. But just flying around normally will not encounter these kinds of flightconditions.

You won't get the effects you are writing about with just dihedral removal. Look at other things you might have changed.

Ben

shawnh

Ben Lanterman. I have found from personal experiance that taking the dihedral out of the wing of my flat bottom trainer now allows the plane to go stright down much easier than before with the dihedral. The plane does not try to self correct nearly as much as when it was a stock trainer. Nothing else on the plane was changed other than recovering it CG stayed the same. The plane now rolls better because taking the dihedral out of the wings made the ailerons more responsive. I never changed the size of the alerons.

Ben Lanterman

Shawn,

I have been an Aero. Eng. for 38 years (13 of those on the F-15 aero design team), a modeller for 52 years and I do have a reasonable handle on what can effect what. The equations determining the flight of an airplane are fairly well developed and do indicate what can effect what.

Also I have noticed that when I am flying I can take a break, drink a cold Coke and the next flight will feel much better with great roll rates, etc. I can polish my car and it seems to drive better. On a beautiful day the airplanes fly much better than on a dreary overcast day. When I am flying my arthritic knee doesn't hurt near as much as when I have to clean the garage. The placebo effect is alive and well in us. So in an engineering context your observations arn't worth much.

Not trying to be mean, etc. it is just that the placebo effect is just as effective in model airplanes as it is in everything else. It is the just the way humans are wired. If you make a change to the model and expect it to change everything then it will.

If I changed the dihedral angle on a model (and I did on a small rudder-elevator glow powered model about a month ago to try to get the roll rates up) and didn't expect to see any change in pitch, I wouldn't. My model was a rudder-elevator airplane and I did see the expected difference in roll rate due to rudder input (it went from a dog to something controllable). But nothing in pitch.

One of the problems with your reporting of what you have observed is the time elasped between flying with the straight wing and dihedral wing. If you can fly the airplane back to back (the same flying session) with and without dihedral wing and straight wing it might help you understand.

The only effect of dihedral when the airplane is in a dive is the ability to keep the ailplane roll trimmed better. It will not change the pitch characteristics of the diving airplane because of the very low angle of attack involved. At a high angle of attack there are some inertial pitch roll yaw coupling effects that can be encountered but you said in a dive.

As I mentioned the dihedral effect does get into the roll response, if the ailerons are causing an adverse yaw to occur, so that is reasonable but not to the magnitude you first quoted, many times the roll rate.