I'm flying a 46 size pattern plane with generous amount of side and rudder area. When I roll to knife edge, with about half rudder input, it can't hold the altitude and start to drop. But when I put more top rudder, it drop even faster.
Please let me know what can I do to trim this plane. Thank you.


Alan Pang from Malaysia

Is it going fast enough? Maybe the rudder is stalling at higher deflection?
--Rick

My Ultrasport does that...

Try going slow, then roll 90 and hold full rudder then add power till the plane holds altitude..thats what works on my plane..even its not really going to help you if your doing a pattern or something.

Where is your cg? Do you have to add a lot of down when inverted? If the plane is nose heavy it will act that way.

Thank you for the info, I'll try to slow down and apply full rudder then power. BTW, My CG is about 3.5 inch from leading edge which is recommanded. I only need a little down pressure during inverted.

The key to good knife edge is airspeed. Try it at a higher throttle setting.

Jet

Do you have enough thrust or airspeed. You need to have enough airflow over the rudder. You can do it with forward speed or propeller air.

all of these posts are correct. from what you described in inverted
flight i would presume to say that your cg is not the problem. if you were nose heavy you would need excessive dn elevator during inverted. what size engine and prop are you using?
what type of rpm's are you turning? contrary to popular belief,
knifeedge flight is dependent on torque more heavily than airspeed. on your prop, length represents airspeed and pitch
represents torque (this is a loose analogy). if you are propped
for all out speed, you will have very sluggish vertical performance
and hince, no knifeedge. if you're rpms are high in the engines
suggested range then you are probally under pitched. if you're
using an 8 or 10 pitch, then try a 10 to 12 pitch (respectively)
also keep in mind that lugging the engine with a prop that is too
large will cause these same symptoms and also cause heat damage to your engine. also remember that some planes although designed with pattern or aerobatics in mind, just dont
like knifeedge flight. good luck and please e-mail me and let me know if this is any help to you.

planecajun,
I agree with what you are saying except that he needs less pitch and more diameter to get a good knife efge. I have seen unlimited pilots do a knife edge at a nice jogging pace. Thrust does this for you and you get thrust from diameter not pitch.

i may be mistaken, and have this backwards but i dont think so.
unlimited racers use large diameter props with very little pitch for speed, is this not correct? im confusing myself now!

To make a correct comparison you need to hold either Pitch or Diameter constant. Neither one in itself will equate to higher speed, or higher torque.
Here is the correct relationship.
Consider a 16 inch diameter prop, and assume a prop speed of 9,000 RPM. At that RPM, a higher pitch will result in higher airspeed for the airplane. A lower pitch will result in more torque (vertical pulling power).

General rule, for torque use a larger diameter, low pitch prop.
For speed, use a smaller diameter, high pitch prop. In either case, prop the engine to peak out in its recommended RPM range.

When you are flying Knife Edge, you are flying mostly on the prop. The fuselage side provides minimal lift, and the wings of course none. You want a prop that will provide more torque, and the nose will need to point up enough to keep the plane at altitude. A fuselage with a smaller side area will have to be flown with the nose higher in knife edge to maintain altitude. The more side area the fuselage has, the less nose up the plane needs to be. Also, the ratio of Vertical fin to Rudder is important. Too much vertical fin will blanket the rudder in knife edge and reduce its effectiveness, and increased airspeed will aggravate this problem.

planecajun, Think about it....as a racer in the past let me correct you on one point.....pitch = speed. A fast giant scale race plane will be a bit slow getting off the ground, due to a high pitch prop which is more efficient at speed. These props don't really work well untill some time after the first lap when the air speed of the plane has increased. Our racer ran a 20x22, more pitch than diameter, for SPEED. Now my free style plane uses a 12x4 for pulling power, torque!! It is confusing, so you just have to get out there and try a bunch of things till it all comes together. But only try one thing at a time, and keep track of the changes, so you don't end up chaseing your tail.

20x22!!!! Holy S&*@!

I am curious.
With a 22" pitch, what RPM are you turning?
At 10,000 RPM, you are moving 208 MPH!

I can't see that fast!

Geist,

Can you tell me how you come up with that speed figure? Is there a table that you use, or do you calculate it?

Jet

No problem:
20x22 propeller will move 20 inchs of air, 22 inches in one revolution. (or move the propeller 22 inches forward)

With this said, 22inch/turn times 10,000 turns/minute is 220,000 inches / minute or 18333 feet/minute or 1,100,000 feet per hour or 208 miles/hour

Let me know if this helps

Thank you all for the reply.

I'm using a OS 50 with 11x7 turning about 13700 rpm. SIG 15% fuel.

The plane fly quite fast and have good vertical pull although not unlimited. I've try to enter with half throttle, apply full rudder and throttle up.....dosen't change anything, the plane still drop. Is this conclude that my plane just wouldn't knife edge?

ALan Pang

My Somethin' Extra's got an Irvine 53 with a 13x6. Knife edge's fine. Like it says in the previous posts go to a larger prop.

Mike

Geist,

I had a feeling that that is how you came up with that number. However, I understand that the pitch spec on a prop is a theroetical number expressing how far forward the prop would move if traveling through a solid. Since air can be compressed, and drag holds us back, the props on our airplanes become less than perfectly efficient. There has to be a correction factor that takes in to account the efficeincy of the prop shape, air pressure, humidity, air temperature, altitude, overall drag of the airplane etc.

Jet

Are you sure that your rudder linkage is up to the task and not deflecting under load during flight? I would forget about props etc until you are 100% sure that your servo and/or linkages are able to deflect the rudder fully while in flight. You aint never gonna knife edge if your rudder aint movin'

Quite true, you will never 100% efficient and it will get worse as the prop approaches sonic velocity at the tips. Get a radar gun it will be more accurate

Besides rudder deflection, also make sure that your servo is strong enough to reach that deflection under load.

dont forget one of the most important factors also
Servo torque... I have had similar results because the servo would stall out and the nose would drop. Stronger servo and wah lah, it would knife edge and even climb on edge if I wanted it to. A stalled servo can become a problem also by drawing way to much current and or destroying the servo.