Skymaster F-15 Steering Stability
 

iAll,

I just finished my Skymaster F-15 ARF Plus and was taxi testing it today with hopes of a maiden this weekend. Everything seems to be working properly except for steering. At anything above about 5MPH, the plane becomes almost impossible to steer and control. While rolling down the runway, any attempt to correct for a slight drift left or right sets up a side to side wobble that continues to amplify until either a wing tip scrapes or the brakes are engaged and it comes to a stop. Any attempt to correct this wobble just causes it to become worse. By not touching the steering on the take off roll I could probably take off but I would be very concerned that any steering correction needed following landing would start this wobble and would continue to compound into a big problem before the model came to a stop.

I have tried taxing with and without the aid of a Gyro. I would say without the Gyro was slightly better.

I have the steering mixed to the rudder channel and I'm using high and low rate settings. Both lead to the wobble problem. Obviously High rates gets you there much faster. Just for reference, on high rates I could turn the model around on a 20' wide auxiliary taxi way. On low rates, it wouldn’t come close to making a u-turn in that same width.

The CG is slightly forward of the recommended 150mm - 160mm range since I have not located the 2S LiPo for the ECU in the aircraft yet to obtain a 155mm CG. Moving this battery aft slightly brought the model to the needed CG but I needed to extend the battery leads by 5"...which I have not done yet. Since I wasn't planning to fly, I just placed the battery in the nose so I could run the turbine.

The main gear is pointed straight...there is no toe in or out.

Main gear tire pressure is 40psi

The main gear springs seemed to allow the suspension to sag quite a bit (almost bottoming) so I disassembled the struts and added a little o-ring grease and stretched (elongated) the spring a little to offer a little preload. After doing this, each main gear strut had a 1/4 inch of travel remaining before bottoming.

I also noticed that my Kevlar Pull-Pull steering set up was a little loose. So I changed this over to a steel cable setup at the field that had much less play.

With all these changes/improvements, the model was better than when I first attempted to taxi but still way too unstable above 5 to 10 MPH for comfort.


Any help would be much appreciated.

Mike

RE: Skymaster F-15 Steering Stabilty
 

Might be your expo? What is your steering expo set at? I use -75 for my low rates. And -4 to -45 for high. (high rate is used when I turn around on the runway) Then I switch to low rates. The steering should barely move when taking off! What radio are you using? The expo is I believe, switched on JR and Futaba. I use Futaba and to get LESS sensitivity, I use - On JR I believe its + for less sensitivity. Hope that helps.
Mario

RE: Skymaster F-15 Steering Stabilty
 

try towing main in a little

RE: Skymaster F-15 Steering Stabilty
 

That's correct.

Futaba, less sensitivity is (-)

I dont know JR.....Just watch the servo curve on the screen. Should be atenuated close to the center and more steep at the end.

RE: Skymaster F-15 Steering Stabilty
 

Mario-
I'm using a JR 9303 radio. I have the steering servo mixed with the rudder channel. Rudder is set at 100% travel +30% Expo on High Rates and 70% travel at 30% Expo on Low.
The Mix 1 program for Aux 4 (steering) is:
Point-0 -100
Point-1 INH
Point-2 INH
Point-3 INH
Point-4 INH
Point-5 INH
Point-6 +100

Steering cables are attached to the steering servo horn at the second hole from the inner-most hole.

Thanks,
Mike

RE: Skymaster F-15 Steering Stabilty
 

It might be possible that your gyro is reversed and not correcting the steering properly. Also try turning the gain of the gyro up more.

RE: Skymaster F-15 Steering Stability
 

just dial down the throw, chances are you and the gyro are over correcting. with the narrow stance and short coupling you need minimal steering on the takeoff roll.

RE: Skymaster F-15 Steering Stability
 

Remember to have a few degrees of toe-in on the main wheels.

Tor

RE: Skymaster F-15 Steering Stability
 

is there any play between your steering leg and the retract unit maybe? combine with round profile front tire this play will turn the jet (like motor bikes turn)

sorry for the bad english

RE: Skymaster F-15 Steering Stability
 

cut the movement down on the nose wheel. if to much steering it will do what you are saying. less throw on the nose wheel is better handling on the ground

RE: Skymaster F-15 Steering Stability
 

Here is what I did to prove to a customer that the struts were too soft. Get it rolling straight with your current set up. Then, wire one strut as far up as you can. Block the other down as far as you can. Then roll it again. Does it still roll straight? If not, you need more spring.
If you were able to stretch the springs out, they sound mariginal. If you can stretch them out, they will pound back down just as easy.
Also, watch out for too much expo. If you are a little behind on your steering inputs, you have a tendency to grab more stick and get into the big throw part of the expo and over compensate, then you overcompensate the other direction and get into a pilot induced occillation condition.
I hope some of this helps. It is one of the more frustrating things to watch your plane trying to drive its self off of the runway.

Doug

RE: Skymaster F-15 Steering Stability
 

Thanks everyone for some great comments/suggestions. In exploring it some more I found and modified a couple things. First, I noticed that the subtle movements to the rudder did not change the position of the nose wheel. Even though I thought the pull-pull system was tight, there still was enough inherent slop that the servo horn would move but the nose wheel would not...it requires larger stick movement to get the nose wheel to move. Which I would think plays right into what RCDOUG was saying about Pilot induced oscillations. So I tightened the pull-pull system even more.

I also reduced the travel and increased the expo...especially on the low rate setting.

DELGATOGRANDE - As a former motorcycle (CBR1000RR) racer, you bring up a good point and I checked for play between the strut and retract. While there does not appear to be any play there the strut does have a slight side to side movement between the strut and the steering arm when operating the steering. I don't think this is enough to cause a counter steer effect however.

Those that mentioned Toe-in.... I understand toe-in and its stabilizing effects in the automotive world but wouldn't it be better to have a slight toe-OUT in this case?
I am thinking that as the plane starts to lean over and drag a wing tip, a toe-in condition would further assist this by trying to further steer into the turn rather than out. If the wheel was set to a toe-out setting when it came up on one side, the wheel still on the ground would try to force the plane out of the lean rather than into it.

I'll try it out tomorrow and let you know.

Regards,
Mike

RE: Skymaster F-15 Steering Stability
 

Guys-

Success!!
I made a couple of changes. First, I found out why the steering cables were loosing up. The arms that the steering cables were attached to at the nose gear strut were actually bending towards the servo and creating slack in the steering cables. I simply lock-wired those arms together on the back side of the strut to oppose the force created by the steering cables. I'm assuming the force on these pins was created when the gear slammed to the extended position. Next, I set the main gear to a slight toe-OUT as I figured a toe-out position would try to counter act further tipping if the plane tipped onto one or the other main gear. Finally, I reduced the low-rate steering travel to almost a 1/3 of where it was when I initially started taxi testing.
With all these changes the taxing and maiden flight was a success! No issues on the take off roll or landing.

Thanks for all the help.
Regards,
Mike