Engine trustline offset

[Arvid K. Gromhol]

Is there a way to calculate the slipstream effect on vertical fin, so that an engine offset may be exactly calculated?

[uav51]

Calculated exactly, no. Estimated well for a specific flight condition considering all the factors involved, yes, but it can be a very involved. Rule of thumb guestimates usually work well enough for a model, and can be adjusted from flight experience.

[Tom Harper]

Once the model is in the air there is no rotational vector to the slipstream.

[vintage1]

I do not think that is correct.

There are 4 basic effects of a prop turning, all of which vary.

- basic torque tending to roll the model left, which is to some extent counteracted by..
- the spiral slipstream hitting the model, and tending to roll it right
- the spiral slipstream hitting the tail, and tending to yaw it left if the tail is above the CL and right, if below,
- the gyroscopic effect of the rotating mass with tends to induce a yaw on application of elevator etc. (P effect?)

Any sidethrust you may or may not use to compensate for these is at best a compromise...never an exact figure..I have models that will track straight at one speed but not at others.

In a glide the prop - unless folded - tends to induce a turn as well..

[Arvid K. Gromhol]

Once the model is in the air there is no rotational vector to the slipstream.

I plan to use i high torque RCV .60 engine (6000 RPM) and is worried about the yaw at take-off.

[fhhuber506771]

Just advance the throttle a bit slower and make a scale appearing take-off.

90%+ of scale power:weight ratios would drive RC modelers nuts. 0.3:1 is typical for light planes. (RC typical is 0.75:1 or higher... most 3Ders have 1.5:1 or more.)

[Tom Harper]

Arvid,

Until you are in the air all of the effects stated by vintage cause problems. Actually there are a couple more but they all go away with airspeed. I have found engine thrust offset to be a very undesirable force in RC models (It's great for rubber powered free flight). You are better off to compensate for any initial effects by using the controls and following fhhuber's procedure. Rotational effects of prop wash are temporary. Once you put in offset you are stuck with it.

On my foam and paper planes the rudder flexes dramatically on the ground due to rotational propwash. In the air they trim exactly zero rudder for hands off flight. There is no evidence of rotation in the slipstream.

[fhhuber506771]

I am constantly (and consistently) amazed at people having problems taking off thier pretty scale P-51's and Cubs. Its really simple:

If you slap the throttle from idle to full.. expect a 90 deg "torque turn" If you advance the throttle smoothly, the model accelerates smoothly and you have time to properly compensate with rudder until the model has airspeed.. then touch up elevator and you get a smooth take-off.

One of my favorite airplanes is the Piper Cub. With the wheels properly aligned.. I never have a problem taking off. I also like the P-51 and have no problems with them.

Every "Big Bird" event I have gone to.. I have seen at least one P-51 "torque turn" off the runway, collapse the main gear and rip itself up. :rolleyes: and you could hear the throttle being slammed from idle to the firewall in each case.

[noodle]

Once the model is in the air there is no rotational vector to the slipstream.

I don't think so. Have you got more info on this?

noodle

[John Pavlick]

Guys,
This is an interesting subject. As someone mentioned, any amount of thrust offset is a compromise. The slipstream effect diminishes at higher airspeed and also at lower throttle settings. The faster you fly, the less thrust offset you need because the oncoming air overpowers the slipstream effect. As the plane slows down, the fin / rudder sees less oncoming air and more slipsteam from the prop. Obviously, if you reduce the throttle to idle, the slipstream effect will also be less pronounced. Keeping this in mind, here is one way to get things in the ballpark. I'm sure there are other ways of doing this and someone may even prove me wrong but this is what I do. First, trim a low throttle (idle) down-line for no yaw using rudder trim. Next, pull an up-line and watch what happens. Ideally you would like to use full throttle but if you have a lot of power this may not be possible (the plane will climb forever). Use as much throttle as possible, but it must allow the plane to slow down / stop at the top of the up-line. The plane should track straight all the way up. If it tracks straight at first, then drifts off to the right near the very top - you have too much right thrust offset. Conversely if it drifts to the left, you don't have enough. If the entire up-line is off to one side then either your rudder trim isn't correct or you simply pulled the line with your wings not level. The first 20 feet must be straight up or the test is meaningless.
Some guys have been using throttle -> rudder mixes to address this "problem". I try to avoid that and rather just fly the plane. I don't see how you can accomodate the speed component with a simple mix. You can do an acceptable job with just rudder trim and thrust offset alone, at least in my opinion. Basically what you're looking for is straight flight at cruise (rudder trim) and a reasonably long straight up-line with no rudder input along the way. Normally, this will require a little bit of right rudder trim. Since the rudder trim dominates on outside maneuvers, you do need a little input when pulling or pushing corners. A tight pull will require some right rudder on entry and a push will require a little left (right rudder trim causes adverse yaw condition at this point). Like I said, this is a compromise. There is no ideal thrust angle / rudder trim setting. No matter what you do, it won't work under EVERY condition. FWIW - most modern pattern ships are using about 2-3 degrees of right thrust AND a little right rudder trim. Your mileage may vary. If you add too much right thrust the plane will actually track sideways and you'll start to see assymetrical control response. You don't want to go there. Generally less is better. I'm sure some very short coupled airplanes will behave differently than pattern planes (very long tail moment) but the basic rules still apply. Keep in mind you're not finished trimming the airplane until it hits the ground...

John Pavlick