Does anyone have a way to measure whether or not I have left thrust in my engine mounting? Im assembling my first airplane. I have a Midwest Areobat, and it is getting an OS 46FX engine put in. I want to make sure that the engine placement doesnt have any left hand thrust built in, but Im having trouble devising a way to measure this.

Thanks for your help!

J

The best way I've found is by using a "Roberts Incedence Meter". They are a little pricey if you're just starting to build up your tool collection. Try to borrow one. The directions are very good. You'll be able to measure within 1/4 deg.

Thanks for the input. Ill try searcing one of those out.

Are there any ways of checking to make sure the engine is either at 0 degrees or right thrust, without the use of one of those?

If you can accurately draw a center-line on the top of the fuse and continue that line out the front of the engine compartment, you can see if the engine shaft is along that line, or if it vears off to one side, or the other. To accurately measure the amount it is off would be difficult. I'd shoot for straight down the line, then add one washer to both the left hand side of the engine mount where it attaches to the firewall.

Get the centerline drawn on the aircraft as described above.

bubble-level the aircraft, on ts side with that line level.

you can now find the thrust offset with a 3rd grader's protractor and a foot of 1/32 music wire. pu a Z bend in the music wire... put the S bend in the pencil hole. Hold the protractor to the crankshaft and watch the wire swing to the angle mark. Turh the protractor around and do it again. (samd difference from 90? OK you got a good protractor. that difference from 90 is the angle.) If the wire is closer to the end of the protractor at the ngine.. the angle is up (whichever side of the plane is up)

The most practical method for a right/left offset is to put something on the prop shaft, such as a stick about 12" long... 6" per side.
Then from the center of the tail post, measure to each tip. Any mismatch will be the offset.
Or fix something like the stick to the firewall and meaure from the tailpost.
for an up-down offset, level the fuselage to the datum, which is probably a line front-to-rear.
Check the plans.
Use a bubble level.
Place the stick on the firewall vertically, and using the bubble, find any deviation from vertical.
If the motor mount is installed and the motor mounted, put a piece of brass tubing on the crankshaft, again with the fuselage level, and use the bubble to see any offset.
A long enough piece of tubing can be measured at each end from the table top, and the difference will be the offset..

You shouldn't need to worry about it. The Aerobat is an ARF. I don't think it even HAS any side thrust built into it. And if you're just looking to see if there is any (As opposed to "How many degrees"), just eyeball it, You'll be able to see if there is any.

BTW, if you haven't already seen it, there's a great review of the aerobat (with videos) here:

http://www.rcuniverse.com/magazine/a...article_id=164

I certainly hope you have arranged for help to fly this airplane, or you are not going to have to worry about left thrust for more than about 10-15 seconds.

Put your propeller horizontal and measure the length from the tail to the left side then from the tail to the right side. Find the difference between the two lengths and divide it by half the propeller length. THe arc sine of this number is the angle of thrust.

Thanks for all the replies. I ended up buying the robart incidence meter. I set the plane on its side, leveled it, then attached the meter to the engine shaft and measured the incidence. It ended up being around 0 to .5 degrees right.

Again, I really apprecite your help.

That's really funny! Ha! Ha! Arc sine, yet! This is U.S. Most can't calculate engine capacity given diameter and stroke!

I use a GP laser incidence meter. Place the plane on its side, zero the meter on the vert stab, then attach to engine shaft using the provided adapter.

I've been using a similar method since '63, and it works very well indeed if you're carefull. The triangle described isn't really a "right" triangle, but for angles smaller than 10 degrees, the error is small.
For those without an engineering/scientific calculator, you can use an online one such as at math.com: http://www.math.com/students/calcula...scientific.htm
FWIW, an arcsin is a one click calculation; enter the ratio of the side opposite the angle of interest to the hypotenuse, push the "arcsine" button and read the angle. Highschool sophomores do it all the time in trig class. Even in America.

I just loosely mount the engine in the firewall....put the plane on the floor,
stand over it and eyeball it looking down. You want right thrust, not left.

Dave.

Dave's got the right idea. Trig calculations (what do you do if your fuselage isn't PERFECTLY square? how much you wanna bet it ain't?) and incedence meters!! In our daddy's day do you think they were doing that stuff? And they were turning theirs loose for Free Flight! They risked it all, and managed quite well, thank you very much, by eyeballing. If you can't see a little right thrust, add some untill you do...It's like the thread about flat building boards. To listen to some folks, you just can't build on anything with .010" of bow or sag over a 4' length, can't design a proper aircraft model without CAD, and SURE can't use anything but some arcane gourd-rattling, wierd-science RIT dye method to tint your canopies... Keep it simple, and you'll get more out of it, I bet. I do.

Geez, is it really -that- simple?
Yes it IS!
But ya gotta worry about something...
And now he has "experience"...
.
(Eliminate the fuselage align-misalign by using the tail post as suggested.)

Doesn't matter if your plane isn't square. Since you are measureing from the rudder to the propeller. If your rudder is not straight, then your right thrust will have a little up or down thrust in it.

I like the trig solution a lot, it is as accurate, and as easy to do.

As for the preciseness and squareness of a model..Im all for being as precise as possible...to a fault even, but at the same time, there is no way on earth it is possible to build a perfectly square airplane, or any wood based product, and keep it perfectly square. The minute the temp raises and the humidity goes up, anywhere you have different wood types or material types, you'll get expansion and contraction at different rates...While Ill continue to build as close to exact as possible because Im neurotic that way, I think it is funny how the technology and tools of today somehow dictate that anything less wont work and work well, and it is possible that some make it much more complex than it really has to be. But thats all part of the fun.

Thanks for all your help, folks. This forum is the best thing Ive found in a long time and Im learning TONS from being here.

[quote]ORIGINAL: jwalsh1

I like the trig solution a lot, it is as accurate, and as easy to do.

As for the preciseness and squareness of a model..Im all for being as precise as possible...to a fault even, but at the same time, there is no way on earth it is possible to build a perfectly square airplane, or any wood based product, and keep it perfectly square. The minute the temp raises and the humidity goes up, anywhere you have different wood types or material types, you'll get expansion and contraction at different rates...While Ill continue to build as close to exact as possible because Im neurotic that way, I think it is funny how the technology and tools of today somehow dictate that anything less wont work and work well, and it is possible that some make it much more complex than it really has to be. But thats all part of the fun.

Thanks for all your help, folks. This forum is the best thing Ive found in a long time and Im learning TONS from being here.


Just to make sure you know where I'm coming from...

I'm a Journeyman Toolmaker. Precision is my living, and trig is second nature, as I use it every day. But getting too worked up about it as applies to sport R/C is sorta like killing a butterfly with a sledgehammer. There's a time and place for that kind of precision, but I feel that there's a fairly liberal point of diminishing returns. I think the important points of construction are light weight and good joinery. I don't own an incidence meter, and have no immediate plans to get one, and I've been just using eyeballs and experience to judge thrustlines. So far it has worked just fine. I'm one of those types who want to keep the whole R/C experience fun by keeping it simple. I realize there are those who truly enjoy sweating the details, and that's fine. But after a day of it at work, I'm not too hot to make too much work out of what's supposed to be relaxation. That's not to say I don't do any exacting work on the part of the hobby, either. MikeSell and I are developing some serious CNC bolt-on power pieces for the small Norvels, and are working on a radical Norvel radial.

Couldnt have said it better myself.

does an aerobatic plane need to have any engine side thrust built into it? Isn't the whole point of "incidence" to correct for engine torque?
singinrock
Mt Sterling ky

Measuring for thrust is not a problem. How much or if it is necessary is what I would like to know

The best way to tell how much is to fly the plane, go verticle at full power and hands off the rudder controls, if the plane pulls left you need more thrust to the right if it pulls right then you have too much, adjust it until it flys straight verticle without any input, the amount of thrust shown on plans is only a starting point that will get you close

Terry

BINGO!

If the plane pulls to the canopy, you need down thrust. If it pulls to the wheels, you need up thrust. You can spend forever setting the static thrust and, odds are, you're going to have to adjust it later.

Singinrock,

Incidence is for lift, getting the wing at the correct angle to the horizontal tail therefore correct for the angle of the air flowing over it.
The rotating prop produces torque and a swirling airflow over the plane. There are many opinions on the effects of these two, and the cures. In practice right thrust is the simplest and probably the most effective fix. Most models need 3 degrees, aerobatic models need more, the most I've ever seen is 7 degrees, and that looks weird but the plane flew straight!

happy landings.