Here is my issue. I have laser cut short kit, and I need to drill the motor mounting holes. The plans call for 3 degree down angle (Which is built into the firewall mounting) and 2 degree of right thrust. At least that's what my memory thinks it is.

But for this conversation it doesn't matter.

A quick question first. Even with the right thrust, is the center of the prop still centered, left-to-right, in the fuse? Or is the mounting point of the engine centered and as a result of the right thrust the prop is not centered?

OK, assuming that the prop needs to be centered side to side even with side thrust built in, the engine mount holes will have to be off to the left side of center on the F1.

So what's the method for determining how far to shift the mount holes? And what's the method for shimming the motor mount to induce the correct amount of right thrust. I'm sure some trigonometry is involved. But I'm really rusty at trig.

I'm going electric on this particular plane, so the easiest way to induce right thrust is to shim the stand-offs on the left side.



TIA

Lyndon

There are two ways to do this. The first is mathematical. Take the distance from the prop washer to the fire wall, and multiply it by the sine of the offset angle (sine 2 degrees =.035)

The second is to draw a plan view and draw a line from the prop washer back at an angle of 2 degrees and measure the distance from where this line intersects the firewall to wher the centerline intersects.

The prop hub should be centred in the fuse. The mount offset on F1. The amount of offset depends on the distance from the firewall to the back of the prop. Some basic high school trigonometry will determine this.

Either method described above works perfectly to calculate your offset. Then to put it into place, use washers between the engine mount and firewall.

Dave

Don't mess with all that math: http://www.lcrcc.net/offset_calc.htm

I disagree. Math rocks. Seriously. Just think, the guys that make the big bucks are the one that WRITE http://www.lcrcc.net/offset_calc.htm. Even if you don't get that good, it is good exersize for figuring stuff out.

Of interest to me is why didn't the plans or the parts accomodate the thrust angles in to the design. Thus making it goof-proof.

Seems a bit of poor planning on soe one's part. Not yours.

Wm.

Why would you want to offset the thrust angle of the engine? Other than some Jets, can you name a full scale plane that does it? I know that some like it so they can more easily hold an up-line, or not have the plane wander when doing a loop, but is that not what the rudder is for? Same with down thrust, sounds like too much throttle or the wrong incidence to me. if you have a problem with the model balooning under throttle, why not back off the throttle a little? I stopped using thrust differential quite a while ago, and have become a better pilot for it. Just my 2 cents worth.

Bill, AMA 4720
WACO Brotherhood #1

Virtually all full scale planes with a radial engine have both right and down thrust. Go look at a T-28, for example. The old WW II fighters has big engines with lots of torque and they had right thrust.

Most of the older pattern planes from the 1970s had right thrust. We used old cross flow engines with more torque than horsepower and swung fairly large props. When tuned pipes came into vogue, we went the smaller props for a higher speed so we quit using right thrust. With the advent of large props, some use of right thrust came back. Nearly all 3D planes have, or need, right thrust. The slow speeds, long props make the plane want to pull left so right thrust is needed to compensate, if you want your plane to fly well. Certainly, you can hold rudder, but it is better to use the right thrust.

Ed,

I measured a WACO (don't remember the model) several years ago, and it had no right or down thrust built into the mount, or the firewall, for the 224 Jacobs radial. I guess that it all depends on what you are talking about. I do know that some of the commercial jets do have some down thrust built in. I have also measured several opposed 4 and 6 clyinder applications on general avaition aircraft, and found neither right, nor down thrust evident either, thus my initial statement. I know that for years, we have used it in our models, especially those used for pattern, and F-3a. But was that not used to keep the plane's heading more automatic?

Bill, AMA 4720
WACO Brotherhood #1

Hi, I have a question, I just got done mounting my motor on my plane. after its all said in done I am noticing now that the entire motor is mounted a little off the center line to the right. So i think it has a little right thrust plus the motor is a little off to the right. (Its not real bad but it does have me wondering). Is this going to be ok or do I need to rip it off and totally remount my motor?

thanks
David G.


I might just be fooling myself, its probably just the thrust angles Im looking at, but it might be combined with a slight off center.

Can somebody check my math???

I am using an electric motor. The motor mount is "X" shaped. I use stand-offs between the motor mount and the firewall to get the prop the correct distance away from the firewall.

The short distance between motor mount holes is 1-7/8" (47mm)
I want the base of the prop to be 4" (101mm) from the firewall.
I want 2 degrees of right thrust.

If my math is correct. I need to shift the engine a little more then 1/8" to the right (but less than 3/16"). I also need to add 3/16" of extra standoff to the left mount.

Is this correct??? I'm not sure about if I got the extra stand-off amount correct.


TIA

Lyndon