Applied about 50% rudder to fly knife edge, applied remaining 50% and no noticable increase in angle. Is this my plane, wrong prop choice, etc? Rudder servo is a Tower TS-67 (?) with around 90oz/in at 6.0v. Plane is a Sig 1/4 scale Cap. Is there some 'trick' to getting it to go correctly? The only reason I'm not immediately suspecting the rudder is that I had a smaller cap that I let my instructor preflight a while ago and he did a knife edge loop right off the bat and I was never able to do so. Perhaps I'm not getting the sticks right, the entry, etc? Any help appreciated.

Hi Joe
How many kilogram pull is your servo?
your servo may not be strong enough.

what size cap is it a 23%?

i have a YT International Cap 21 which is 23% ruder servo is futaba S9404 which is more than enough power!!!for knife edge looping.email me [email protected]

Dean here is a pic of my cap on a knife edge

lol Dean you made that pic too small.


Enter on knife edge, add enough rudder to begin the climb, but take it easy.... to recover (ie once past the top) add full rudder, and you might need a fair bit of power too. Correct KE with aileron and elevator all the way around. You'll probably find you haven't got man enough engine, CG too far forward, rudder throw too little or model just too heavy.


Maybe soemone else could add more, i'm not that familiar with the maneuver, I've not yet owned a model that'll do it.... sure I have an overpowered Cg ultimate but I don't have a good rudder servo, so again, out of the question. My CAP 232 used to manage half of one, and it's just about to get a YS 63 so hopefully it'll complete one too!

I suspect something about the setup of your plane may not be right, but since I haven't flown that particular kit, can't say for sure.

This weekend, I went out with a Great Planes "Venus .40" ARF, assembled by the book, using only S3004 servos, and did knife edge loops... Never had to use full rudder. (with an OS .46FX and 11-7 APC prop)

I reviewed the "Venus .40" kit on my web site at http://www.nextcraft.com/gp_venus40arf01.html Very impressive for the price.

Good luck!

The servo is a TS-65, it is 77oz/in at 4.8v and Tower doesn't list the 6.0v rating but I remember it was right around 100oz/in which I think SHOULD be enough. The plane is right at 11lbs, has a Saito 1.80, CG is slightly ahead of aft recomendation (will change as I get used to plane) and rudder throw is right at 3". I am running a Master Airscrew "classic" 16x10 prop which I think may be a power robbing dog but I can't confirm as of yet. I'll get some different props to play with this weekend hopefully.

The reason I didn't immediately suspect the plane is that on my old plane, a .46 powered Kyosho Cap w/standard servos, my instructor did one right off the bat and I was never able to duplicate. Unfortunately I no longer live near that field and can't ask for more assistance from him but I'm guessing there's a 'trick'. Maybe I'll try moving CG aft a pinch at a time.

- Joe

Sounds like you don't have enough servo to handle the remaining 50%. If you can note the rudder angle on the ground and then in the air, you should see if the servo is strong enough to hold it.

I'm no expert, but it sounds like you need much more in the way of a rudder servo. And the knife edge is more of a power manuever. Get a different prop and try running 30% fuel through that Saito. The engine will love it.

Put an apc 17 X 8 prop on that saito 180. You'll be very happy that you did. Jim

1. I am using 140 oz in servo for rudder on that plane. I learned the hard way after a 90 oz in servo went bad on me. I am also using 6 volts.

2. That propeller is a dog. The APC will give you lots of RPM/thrust which is critical here too.

3. Your CG sounds O.K. I've tried going farther aft but precision aerobatics get very hairy on windy days.

Left or right rudder may have different authority because of engine and prop factors. Once you get it try doing a half roll at the top.

I found that thrust has a lot to do with knife edge loop. I had H9 Cap w/ OS 1.60. It does knife edge loop the best using 17x6 prop @ 10,000. When I use 17x8 @ 9300, it's not as good. So I'd maximize your thrust. Many people has good luck using 17x6 APC on Saito 180 swinging at about 9700 rpm. If you don't mind the aircraft's top speed being 54 mph, 17x6 would be your best bet.

I had a Sig 25% CAP and it would KE loop just fine. I used a ~80oz @ 4.8V servo on the rudder and it too worked just fine. My rudder throw was about 45 degrees. I am now using 140+ Oz servos but it would KE loop on the 80 oz servos. I dislike and do not use the throw in inches measurement because it makes little sense. But I ran down to the garage quick to measure my rudder throw that way and it is over six inches measured at the bottom of the rudder... a lot more than your three.

So, there have been some good suggestions such as moving the CG aft and reproping. I'd try those too. But I think you just don't have enough throw yet.

2 words

MO POWER


On an 11lb plane 80-100 inoz servo should be enough so that you can at least start climbing into the loop


A Saito 180 on 11lbs isnt exactly ballistic power, and a 16x10 wont help any on vertical pulling power either.

To do Knife Edge loops, and most other 3-D type stunts you need great vertical pull, not top end speed. You also need Prop wash over the Rudder, not airspeed. Get an APC 17x6 and be amazed at the improvement. My YS 120 FZ with an APC 17x6 at 9300 rpm develops over 17 lbs thrust (accurately measured).
Also, a Rudder requires much more force to be deflected in a 70 MPH air stream than it does in a 40 MPH prop wash. Slow that plane down and you can throw the tail around much better.

I think Aerosplat & Can 773 has it right. You need prop blast & lots of it. Fly close to stall speed

Some hints... Fly left to right (when you add power an airplane it should yaw to the left) Enter slowly at less than 1/4 power, as soon as the plane is KE...full rudder & full power. That should get you vertical. With enough power you will be on the up line. When you get to the top you will need opposite rudder to keep the loop round. About the 45-degree down line hammer the full rudder and full throttle again. That's the point you will find out if you plane will really KE loop

I have (had, I sold it) the exact same plane in question. I used a YS140L for power running 30% nitro and a 16x12 prop. I had unlimited on that prop and more than enough power for a KE loop. I too was in the 11 lbs range. I also flew with a 17x10 with similar results, maybe a little bit better pull at lower speeds. I think the Saito 180 has comparable power to a YS140L as long as high nitro is used (~30%) and the right prop. Ditch that master airscrew prop, but don't throw it away, you can use it to stir paint. Use an APC on the plane.

I still think you are way short on rudder throw. Your 3" is only about 20 degrees throw. That might be enough to start the loop but the pull out in the last quarter of the loop might have a pucker factor of 9 with only 20 degrees of throw.

Thanks for the input, I will definately add some more rudder. Unfortunatley my radio does NOT have dual rates on rudder and NO exponential either so I'll gradually try to bring it to 45° deflection. Hopefully I'll be able to handle it on the ground with such deflection. Thanks.

- Joe

It will handle better on the ground.

Update: Increased throws and yet it will now loop. After 4 good flights I noticed my rudder servo was stripped, guess I need something a little beefier At least it didn't crash in flight.


- Joe

When it comes to rudder pull-pull, I am comfortable only when I use metal geared servo w/ 2 ball bearings. When the pull-pull is tight, it's very demanding on the output shaft and thus the gear immediately next to the shaft. For your Cap, I'd go w/ Hitec 5945 for great torque and speed. When I flick rudder back and forth using this servo, it seems like I can slap someone w/ the rudder.