I'm getting to the point in construction where I am getting ready to mount the 2.1 Moki. The plane will later have a 3 1/2" aluminum Spinner added.

The plane is an 18 lb, 84" Big Bingo. I am told it is a close relative of the smaller Sig Star 60 in performance. The plans show zero right thrust, and neither is an option about adding right thrust mentioned in the instructions.

I love building planes but know little about flying them and how they act up there. So I need you to tell me if I should add right thrust, or was this plane purposely built without it for good reason? I'm always suspicious when a designer leaves something out that I know is usually a good thing to have.

right thrust is simply a band aid for poor power setups - easy fix - add right thrust .
on your setup - a little won't bother you
On my really high power to weight setups - I use none .
simply not needed
Technical answer:
if the speed of the airplane is constant - you don't have to add trim to correct (for flying at a different angle of attack).

Dick,
Naturally, I'd rather carve and shape the nose of this plane to an engine sitting straight inside of it, instead of crooked. It just naturally looks better too.
But I just know when I'm done with the plane, one or two guys at the field will tell me "Ya know, you shoulda added 3-4 degrees right thrust to it. It woulda flown better"

Randall:

I just made a post about thrust anges in "Scratch building."

Go here:

http://www.rcuniverse.com/forum/Note...1855431/tm.htm

for a discussion.

Bill.

If you use right thrust, do you really want left thrust when you fly inverted?

JF:

No. The left thrust when inverted is there if you have right thrust when upright, though. Just as the upright down thrust becomes up thrust when inverted.

That's why I set the engines dead away on anything I expect to be aerobatic. Remember when you turn the airplane 180 degrees in roll the thrust lines are also rotated that same 180 degrees.

For a purely sport plane down and side thrust can make it easier to fly, but otherwise I'll make the engine 0-0 every time.

Bill.

Bill,

That's what I like to hear. Not only that I should or shouldn't do something, but a simple and clear explanation for your reasoning that I can understand and then draw my own conclusions by. It makes sense what you've described, and I will follow it. I wish the manufacturers would provide a simple explanation on such building essentials with their instruction sheets. For example, Sig tells you with their Seniorita kit instructions that you can apply right thrust or leave it out. They explain that it simply helps to offset the thrust of the engine rotation and so then is preferable. But being that the Seniorita was a high wing trainer (not an earobatic plane), they don't proceed with explaning to you that right thrust is NOT desirable with an aerobatic plane. So I was forever under the eroneous impression since building that Sig Seniorita kit many years ago, that right thrust is always a good thing to incorporate in ANY PLANE!
I was further mislead to thinking right thrust is preferable in all planes after seeing 3 degrees was added in my Giant Top Flite P-47 too. (Yea, because I now know that the P-47 isn't designed as an aerobatic plane either!)

When I completed my scratch 89" Sig Astro Hog In January, the original 71" plans showed zero right thrust as well. I didn't like this, because I know my 4 trainers always had right thrust in them. So I added about 2 1/2 dgrees of right thrust to the engine on the advice of the "experienced" guy at the hobby shop. But now, after understanding how right thrust can affect aerobatics by you, I'm glad I didn't go all the way up to the 5 degrees right thrust he said I could go. No harm done, as I know that the scratch Astro Hog, 21 lb plane powered by a Moki 2.1 isn't going to fly like a Big Bingo anyway. (Even though I cut the wing dyhedral on the scratch Astro Hog down in half because I wanted it to be more aerobatic, I know 2 1/2 degrees of right thrust isn't going to be a big drag on its performance anyway. But I'm glad I didn't push it with going 5 degrees right thrust!).

Thanks,
Randall

Randall:

Thanks for the kind words.

Go here:

http://www.rcuniverse.com/forum/Wher...1063414/tm.htm

and start with post number three.

I started the thread to be a source listing for hard to find things, but then when I wanted a place to keep the stability discussions I stuck them in there too.

Surface incidences, c/g, thrust lines, so forth, all (hopefully) explained clearly, and in ways that don't need any math to use.

Bill.

Right thrust is used only for correcting the spiral slipstream effect, caused by having an asymmetrical vertical tail. When you roll inverted, the tail also goes inverted, so you still need right thrust (now left thrust). If you're sport flying it doesn't really matter. For precision aerobatics, you want a hands-off vertical line achieved with right thrust.

Baron

Bill, I checked out that thread. Plenty of info there for future reference. Why do I have the feeling that you are somehow related with aeronautics professionaly? Do I observe keen knowledge here not normally associated with someone who repairs Maytag Wash Machines for a living.

You know.....on that topic....it reminds me of the time I got involved on the old RCONLINE with some guy from Holland. Man, did this guy shoot me a bunch of mathematical equations to summarize the current condition of aerodynamics with my plane. I had no clue how the answer was derived from all of the EMC2 he illustrated back to me, but appreciated the answer. (Which in laymens terms, amounted to a "Yes, Randall it will fly") I'll never forget when I told him that I was impressed with his mathematical brilliance, and surelly he must be an engineer or professeur, or something like that. He posted back to tell me "No Randall, actually I operate an Elevator here in Amsterdam! I learned the math in college.)

Gosh, what a country America is Huh! We can't speak Dutch or a second language (or our own language properly), add or subtract without the help of a calculator, but still don't have to stoop so low as to operate an elevator.

Thank you too Baron for your input. Sounds like this would be something to take into serious consideration when building and flying pattern.

Baron:

I should have responded to you also. You have a valid point, but I always try to have enough sub fin or low rudder area that the effect of the toroid flow is minimized. And full scale designers often do it too. Look at the rudder of a Cap - the ugly shape is just for that reason.

And many GA planes (full scale) have the leading edge of the vertical fin offset to the left, again for the same reason.

But when I'm scratching a design I automatically center all empennage areas as much as I can, and I usually forget the spiral flow of the slip stream when I talk about thrust lines.

So thanks for pulling me up short, I needed it.

Randall:

No professional association since I got out of the USAF. Since then I've been building, flying, crashing, and hopefully, learning. Both books and practical, models and full scale.

Bill.

Bill,

I do have to address your statement in the thread linked to above...

"When you add power it takes torque to spin the prop, the reaction to the torque tends to make the plane roll to the left. This is what right thrust is for. (Forget "P-factor," that only affects the plane at a high angle of attack,or take-off with a tail dragger.) Unlike the ballooning there is no sudden yaw when you cut the throttle, the torque and the reaction disappear together, unlike the added air speed which causes the sudden climb."

Torque is a roll force, and a change in thrust line only slightly moves the direction in which that roll force acts, and creates a yaw force, not a countering roll force. The real cause of the need for right thrust is the spiral slipstream hitting the vertical tail from the right (when viewed from the front), and not hitting much or any fuselage side area below the centerline from the other direction, causing the plane to yaw left. This remains constant whether you're inverted, upright, vertical, etc. A perfect aiframe would have a perfectly symmetrical vertical tail, just like the horizontal tail. Then there would be no need for right thrust. Spiral slipstream is the only one of the 5 'evil' propeller forces which can be countered with trimming procedures.

EDIT: You got to the 'Reply' button before I did! Your above explanation explains why you can ignore spiral slipstream in certain situations. Like you said, many full scale planes hav a skewed vertical fin for this reason, and a lot of pattern designs are going to larger and larger sub fins. However, every scale aerobatic plane I've flown (Including Caps) has needed right thrust (with the fin neutral of course).

Baron

Baron -do you add right thrust to your little foamies?
we have not found that these need it-of course they are profiles and the whole fuselage is one big fin--also I found that by having power to accelerate going up -I just don't need any corrections - Yes I know about spiral flow but just can't find this problem on the really high power stuff.
I can trim for hands off then point em straight up - do lots of multiple rolls and the track is still spot on -
On my "std " IMAC stuff - if they won't hold full speed going up -they do drift right -but not roll
On pattern planes we tried 45 degree downlines which showed that under power - --if roll trim was correct- the plane would change roll by shutting to low idle .
I never saw a good fix for this --
so anymore -I just add some if needed --no fixed rule -

Hey Dick,

Yes, I have electrics with right thrust (achieved sometimes by offsetting motor to the left on profiles). Like you said, some need it, others don't. My PIGI 3D has a set of washers in the motormount for right thrust, which was badly needed. My old Radical Edge and the Exhila have the motors offset to the left. My FunPlanes.com SX2's have none, as they don't seem to require much, if any. The SX2 does have significant profile area below the centerline. On all of these I pretty much set right thrust for a hover, though, as I'm not concerned with drawing precision lines.

You mention pattern planes having roll problems. Sounds like a torque issue to me. I don't think there is any good way to correct for torque (except counter-rotating props), as the scenario has almost endless possibilities. For instance, a plane trimmed for cruise level with no roll, still torque rolls when vertical at zero airspeed (even if the throttle setting is the same), because the aileron trim has a decreased effectiveness.

Now back to spiral slipstream and right thrust. From your description, I wonder if your 'high powered' scenarios are going fast enough vertically to actually straighten the airflow significantly before it reaches the tail... in this case, is zero thrust offset also suitable for a hover at zero airspeed? I doubt it, but no setup will be perfect. If you travel slow and pull gently into a vertical line, then gun it at low airspeed vertical, do you see a yaw? My philosophy would be to setup a plane for it's intended purpose. If all you want to do is 3D and hover, set the thrust so it hovers purely (however much - if any - is required). If you're flying precision aerobatics, set it up so a normal vertical line is as yaw-free as possible at whatever your typical throttle setting and entry speed will be. I guess I've never flown an IMAC plane with your lightness and power, as all I've ever tried have needed right thrust to hold vertical at full throttle. Most also do deviate slightly after some extended time in a hands-off vertical, but can be setup close enough that most maneuvers are within the 'pure' region of the line. Then when you throw in roll and snap elements and wind, you're all over the rudder all the time anyways, so absolute perfection on the setup is not needed usually, but then again, every little bit helps.

Like you said, I think the bottom line is use right thrust if you need it for your purpose, and if not, don't worry about it. Just remember, right thrust only cures yaw from spiral slipstream. It does not cure yaw from P-factor, as it is AoA dependent. It does not cure torque because it's a roll force. It does not cure yaw from gyroscopic precession because it's dependent on the rate at which you're yawing or pitching. Etc. etc.

Baron
P.S. I bet it'd be fun to get my hands on one of your little rocketships

Yeh -i just wish I had the ambition to polish up my flying skills tho -
My new IMAC plane will be a Cassut racer - reaaly low aspect ratio - should work - the fuselage is almost a profile diamond shape .
If it is no good - well, what the heck -
so far power wise here are three we setup - one just was destroyed due to a lousy overly large rudder counterbalance and a servo which went wild - then lockout then ka boom -
It was done for a friend - a 2.6 composite with a ZDZ160 -he wasn't in the throttle when the rudder went bonkers .
another is a 35% Giles - ZDZ160 ( just right) and my little goofy 1280 sq in ZDZ40 model at 11 lbs -
all very zippy!

Right thrust is very dependant on how you want to plane to fly. I set mine up at zero because I have become used to the fact that planes always pull left when you throttle up and when you pull verticle so I compensate with the right amount of right rudder. For a beginner, 3 or 4 degrees of right thrust will help the plane fly straight all on it's own taking away some of the unexpected habits of the plane. So it's all up to you. If the plans say zero right thrust, make it so.

I heard a general rule of thumb a while back; Add more right thrust for slower RPM. A speed plane turning at 15,000+ shouldn't need any. Whereas a slow park flyer turning 4,000 might need 6-7 degrees.