ground loops
 

What are the factors that make an airplane ground loop. I have an AMR low wing stick 30 cc size that ground loops like crazy. The airplane flies very well once off of the ground. The landing gear has been replaced due to the ground loop. The ground loops are so violent when they occur that the landing gear is wrapped under the model.
thoughts please.

Jerry

I'm afraid this will become a bit of a heated conversation and you will need to weed through the responses. That being said, I will venture my take on this.

Being a Stick I have noticed that many variations don't have any right thrust. During your take off run there are forces that will pull the airplane to the left. 2-3 degrees of right thrust will greatly help this situation.

Most guys set their rudder throw in such a manner that allows them to taxi with a reasonably small turn radius. If a dual rate and/or expo is not used that throw is too sensitive for nice takeoffs. The pilot tends to over control and ground loops the airplane.

You mention that it is a 30cc sized Stick so I will assume a 30cc size gas engine eith a Walbro carb. The issue with a Walbro is the throttle curve is very non linear. The typical setup has the power coming on quite suddenly. The way to tame this is to fabricate a new throttle arm of about 1.5" on length. If there is an offset in angle you can correct the eith the new arm as well. Once the new arm is attached, go into your TX and adjust your throttle servo travel to 110-120% for both high and low. Once that is done mechanically set up your throttle so that you get idle at low stick and 85% carb opening at full stick. That last 15% opening does not add any additional power. You can fine tune these with the throttle end points but do not go below 100% throw on either high or low and keep the two as close to the same value as possible.


The end result of all this will be that your power will come on more gradually, it will not pull the nose to the left as much if at all and you will not be fighting the rudder. Trust me when I tell you that if you do these simple modifications your takeoffs will become virtually effortless.


Keep in mind that this assumes that both your wheels rotate freely and don't have any weird angles. Don't buy into the toe in or tie out stuff, it makes no difference on our models.

response curves
 

I have been experimenting with throttle and rudder response curves. So far no change. It does not ground loop every time but when it does it is a very sudden and violent event. The landing gear gets pressed up under the fuselage.

I'm not surprised as those two things alone without the physical changes I suggested just aren't going to make much difference. Any aircraft even a model is a complex devise. Usually fixing a poor handling charictaristic will require changing several things. If you follow the instructions in my first post the airplane will perform takeoffs much easier I promise.

Thanks speed.. Upon inspection the throttle arm is only about 5/8" in length. That is short.

EAA article suggests toe-out for the wheels. pages 3-5

Trust me, I tried it. Didn't make any difference except having the wheel pants angled messed with my knife edge trim.

1) Moving the main gear back some will help. Being too far forward exasperates the tendency to ground loop, plus it makes it more difficult to raise the tail on takeoffs.

2) Anticipating the ground loop will help. Think ahead of the plane.

3) Not over-controlling will help. Small, gentle movements of the rudder stick. And increasing expo on rudder will help some.

Try #2 and #3 first. If those don't fix the problem, then look into #1 (if moving the main gear back is feasible).

edited: For reference, here is another thread on this topic: Ground looping

I agree with the landing gear idea. I have a Das Liddle Stik converted to conventional gear. My first setup had the same problem. Just turned the landing gear around, so instead of angling forward it now angles back. It just barely stays on the nose when tipped forward. It is much more controllable on the ground, and will only ground loop with overcorrection, only on landing.

I would agree that the landing gear placement can help. Typically having the wheel axles in line with the wing leading edge works well. I still think that on a Walbro equipped gas engine, getting a nice flat torque curve by using the methods I suggested previously will prove to more beneficial.

Jerry, what's the runway surface like? Are we talking a smooth paved runway type surface or a rough grass surface?

runway surface
 

Good afternoon Franklin.

The runway surface is well manicured short grass. Most small electric's do ok with it.
As mentioned earlier the position of the axel with respect to the leading edge of the wing seems to have an effect on it. This landing gear was about an inch and half in front.. After reversing the landing gear it is about half inch in front. This has reduced the sensitivity bur it still dances going down the runway. Some T6 aluminum is on the order list to be used for a new landing gear to further address this problem.

Jerry

Jerry, one inch on a 30cc airplane is not going to make a noticeable difference. Have you adjusted your thrust angles and performed the carb/throttle modifications I suggested?

Throttle curve
 

The Xmitter has a non linear throttle curve that simulates changing the arm length. This change and moving the landing gear back were done at the same time. Three successful take offs were made. The airplane did a bit of a dance going down the run way but it was controllable. Also rudder expo was increased to further desensitize its ground characteristics. It is still not as stable as I would like. So the landing gear will be set back just a bit more.

It's really not the same thing. A throttle curve will still be required with the longer arm but you are at this point lacking resolution. The high resolution you get from the long throttle arm and only 85% throttle opening is a bigger part of the fix. Take the peak out of the power curve and things get much easier.

End point
 

Forgot to mention changing the end point. This improves the resolution. The full motion of the throttle stick is now used for about the first 80 percent of the carburetor movement. This airplane will break ground and fly at just over quarter throttle. It was ground looping with the throttle up abut three clicks, not quite fast enough to fly. I can not tell what made the most difference since the changes were all made at the same time except for the landing gear.

Have you worked with a DLE 35 RearExhaust? If so what did you use for a throttle arm? The only thing I have that will fit it is the one that came with it.

Jerry

I've run engines from 20cc to 200cc, all of them got the longer arm. Granted some installations make this a bit more difficult. I typically make the arms out of fiberglass (G-10) or carbon fiber. A combination of a screw through the existing one on the stock throttle arm and a little JB weld keeps it in place. As said earlier, it's a combination of things that make the biggest difference, it looks like you are figuring that out. Have you adjusted in any right thrust yet. Video is an example of what is possible when you have everything right. Note the very small amount of rudder used in spite of the crosswind I had that day. This is a DA 150cc with a 1.75" throttle arm and 3 degrees of right thrust.

When during the takeoff roll are they occurring most often? Tail down or tail up?
Does it ground loop more often in one direction or the other (nose left or nose right)?
What is the direction of crosswind in relation to direction of ground loop?

ground loop
 

It always goes to the right in a ground loop. This happens when the throttle is up about 3 clicks. Not even moving fast. This kit has built in right thrust. Could it be too much right thrust? Tail is still on the ground.

This thing goes around with so much force that it has rolled the tail wheel off of its rim. I have never had another tail dragger that does this. Tomorrow I will post a picture of the fuselage. This airplane has an overly long tail moment. Or at least it appears to. It is time to measure the thrust angle no need to guess at it.

If there is time I will look at the throttle arm issue tomorrow.

Thanks for the thoughts guys.
Jerry

You have to be a cautious consumer of information here. Did you notice how some were content to start making rather detailed recommendations before even asking which direction it’s ground looping? And that's important because, as you'll see below, the aircraft is doing something completely opposite to what we'd expect. Let’s do this methodically and see what SHOULD be happening based on physics (and aerodynamics) vs. what IS happening and see if that provides a clue.

In the interest of brevity I’m not going to state the bazillion caveats would be necessary if this were a formal engineering paper, but let’s capture the big ones. With the aircraft axes defined per note 1 below and the aircraft at rest: The main wheels and tail wheel are parallel to the longitudinal axis. The main wheels are ahead of the center of gravity, and the tail wheel is behind it. The wings and horizontal stabilizer are perpendicular to the longitudinal and vertical axes, but parallel with the transverse axis. The vertical stabilizer is parallel with the longitudinal and vertical axes, and perpendicular to the transverse axis. Additionally, all control surfaces are neutral with respect to their respective flight surface. And lastly, the aircraft is a tractor configuration, with the propeller on the nose of the aircraft, ahead of the main wheels (and CG), and the rotation is clockwise as viewed from the cockpit. The model is not equipped with brakes. Tail wheel is NOT free caster. The tail-wheel moves the same direction as the rudder with a magnitude proportional to rudder displacement. Assume friction-less wheel axles. I’m sure there are more, but these are the major basic ones.

What are the forces on the aircraft (aerodynamic forces still negligible):

• Due to asymmetric blade effect of the rotating propeller, there will be a nose left force
• Due to prop-wash effect there will be a nose left force on the tail
• Asymmetric blade nose left force increases with increased throttle
• Prop-wash nose left force increases with increased throttle
• If the nose left forces are offset by the tail-wheel force, the aircraft will roll straight
• If the nose left forces are not offset by the tail-wheel force, the aircraft will yaw left
• Gyroscopic precession will impart additional nose left force as nose pitches down
• Once tail-wheel breaks contact with the ground, the aircraft is direction ally unstable (note 2).

Why is all this important? Because what we see is that every force on the aircraft at this point would predict a nose LEFT yaw as the engine speed increases, the aircraft starts rolling, and the nose pitches down. However, your aircraft is demonstrating the opposite behavior. So where to start?

• Check the amount of right thrust built in?
• Under static conditions, is the tail wheel parallel to the aircraft longitudinal axis?
• Under static conditions, are the main wheels parallel to the aircraft longitudinal axis?
• Do these remain parallel under dynamic conditions? (i.e. no struts bend under loads as throttle comes up)
• Is the angle from the aircraft CG through the main wheel axles approximately 15 degrees forward of vertical axis per recommendation in note 3?
• Is tail-wheel direction parallel with the longitudinal axis when rudder is neutral?
• Does the tail wheel stay aligned with the rudder under load?
• Check control surface rigging. For example, is there some built in right yaw input (tailwheel or rudder)?
• Inadvertent control inputs as throttle increased?

My point in this is that under normal conditions, every force on the aircraft would predict a nose left ground-loop. Going the opposite direction entirely makes me look to things other than specialized throttle arms for the problem.

Note 1: https://aviationnuggets.com/blog/6/a...principal-axes
Note 2: Andy Lennon, R/C Model Aircraft Design, p75, figure 4
Note 3: Ibid, figure 6A

What it sounds to me like you are describing is a wheel problem. A violent ground loop just shouldn't be happening on an Ugly Stick with its wide gear and long enough tail moment. I'd check for the following:
1. Are both wheels set up perfectly straight compared to the fuselage centerline? You'd be amazed how much they can be off by and still look fine. Do some measuring.
2. Do both wheels spin freely? Also check if they are able to bind with some side load. If a little yaw on takeoff makes the wheel tilt so they bind against the gear, that's probably your culprit.
3. Is your landing gear actually strong enough? If it's flexing with every little bump, you will never have a good handling plane.
4. Assuming your rudder trim makes the plane fly straight (have someone watch you fly and verify this), does your gear setup also make the plane roll straight? If not, the transition from tailwheel control to rudder control of the yaw will cause a snappy ground loop.

I've had two planes that were a beast on the ground and fixed them both. The first was a second hand Cub whose axles weren't straight to the fuselage centerline. A couple of tweaks fixed it right up. The other was a very poorly built trainer that had right yaw trimmed in to compensate for the roll of a warped wing. As it sped up and lifted the nose wheel, it would take a hard right on the runway. Proper trimming and a readjustment of the nose steering fixed it.

https://cimg3.ibsrv.net/gimg/www.rcu...53edb2b73f.jpg
Let's not over complicate things here guys ( Franklin ). This really is simple and something I have seen many times with these gas engines. Most guys who go from glow power to gas set up the throttle the same way. Doing this is sure to have a spike in power at the spot in stick travel that the OP describes. He is either trying to predict this and over controlling the rudder or as the nose swings he is over controlling the rudder and creating a ground loop. He has mentioned that his throttle arm is only 5/8" long. We still don't know how long the servo arm is. To get to the recommended 2:1 ratio the servo arm would need to be 5/16. I suspect it is longer then that. The op has stated that he went halfway with my throttle recommendation and it helped right? Stands to reason that if he goes all the way with it he may have even more success. Perhaps Franklin can tell us how he sets up the throttle on his gas engines? In the meantime maybe some more evidence on how a nice smooth throttle curve can make takeoffs and landings much easier.

There's no need for anybody to get offended. Several people have taken the time to share detailed explanations of causes for ground loops, and I'll bet the OP will be able to check them out and fix his plane. The fact is the problem could be a combination of more than 1 factor. We don't know sitting behind our keyboards. I appreciate everyone who tries to offer their knowledge and experience to help solve a problem whether they get it right on the first try or not.

Need time to study this info
 

I am not ignoring you guys. Went to the field yesterday. Only had one ground loop that was to the left and I was a bit slow on the rudder. Three good departures breaking ground at about half throttle.

So far the changes were shifting the landing gear back and the throttle curve. The plan is to install a new landing gear with the axel at or behind the leading edge of the wing and measure the rpm at each click of the throttle to get an idea what is happening there.

Franklin, the angle idea needs to be looked at and understood. Looks like I will be in the shop at least three days next week due to weather.

Jerry

Attachment 2268553If you can't get your hands on the Andy Lennon book, here's another resource. Slide #10.
Attachment 2268553