utomow

Hi all,

just wondering why do we need right and down trust on airplane engine? Any detail explanation is very appreciated.

Thanks.

Rgds,
W. Utomo

Campgems

W. Utomo, this is a laymans explanation, but one that I think is valid.

Down thrust. Most planes requiring a large amount of dow thrust are those who's thrust line, the axis of the crank shaft, is below the source of drag on the airframe. The wings are usualy that drag source. The photo is an example of extreeme drag and no down thrust. This is a Radio controlParafoil designed and built by a local college aerospace class. It is in level flight at the time the photo was taken. As you can see, the thrust line was way below the maximum drag. The results is for the thrust to arc upwards with the drag as it pivot. More down thrust on the engine would have put more stress on the lift of the wing, but would also have given more forward velocity, resulting in more lift. In this case, drag won over lift, photo 2

A later flight in which the "balance" of the engine cage was changed so the engine had more down thrust was a success, other than stearing issues. I look forward to recording the next flight.

I'll leave someone else the pleasure of describing how right thrust keeps the plane going straing on the ground.

Don

da Rock

Right thrust is just as simple as down thrust. "Simple" you say???? None of this is really simple, is it

It's used for other things, but lots of times it's to cure the tendency of a model to turn left when it's pitched up. Where do we see that most often? and with really bad effect? On takeoffs.

There's this thing called p-effect. It happens with props, thus the "p" in the word p-effect. When a prop that's buzzing along level is pitched, it's no longer heading into the air perpendicular to the flow of that air. And one side of it sees an increased angle of attack, while the other side sees a decreased angle. (That'd be the right side and left side if you pitched the nose up on takeoff.) And the side seeing the increased angle of attack would pull better than the other side. And the prop would try to turn the airplane while you were trying to get it to pitch.

You get yaw when you're wanting pitch.

Since we most often fly right side up, and we most often see bad things from p-effect when we're doing things that pitch the nose up, we've most often built in some right thrust to cure that problem.

Best solution overall? Nope, but it's what has evolved.

da Rock

Why doesn't p-effect give us as much of a fit in the other direction, on nose down moves? Sort of the same reason we've evolved into using right thrust to cure a problem that it's not perfect for. Our airplanes' "usual" layout. And the fact that there haven't ever been a lot of modelers doing a lot of outside loops and the such.

HighPlains

My Feedback: (1)

Since most of the vertical stabilizer is above the thrust line, you need right thrust due to the above-described effects. However when doing inside maneuvers, a bit of right rudder is also required to maintain your line. You also use a slight amount of left rudder to do outside maneuvers. Most airplanes (beyond the basic trainers) fly quite well doing outside maneuvers as well as inside. The usual deficiency is the pilot and trim set-up.

LouW

The use of right and down thrust is mostly a relic from the old free flight days. Without benefit of inflight control, a fairly predictable flight profile could result from angling the thrust line. Free flight models were launched with the engine screaming at full power and the idea was to gain the maximum altitude in the brief specified engine run. The airplane then needed to glide somewhere near its minimum sink.

With the airplane trimmed for its best glide, it would want to pitch up drastically at full power. Down thrust was used to control this tendency (along with a rearward cg location).

Since the purpose of the flight was to achieve maximum endurance and since straight line flight would quickly take it out of sight (and result in a very long walk to retrieve it), the airplane was trimmed to glide in a rather tight circle. A typical approach was to trim for a left turn when gliding and a right climbing turn under power. This required right thrust.

There were many variations on this adjustment but basically an angled thrust line was used to control the flight profile where no control input was possible after launch.

With a full range of inflight control and trim, thrust line offsets are relatively unimportant.

Nathan King

What you have described is called the spiraling slipstream effect. If there was as much rudder underneath the aircraft as above, the left yawing tendancy would be reduced; however, it would be difficult to land with those super long gear required to keep that big chunk of airplane from scraping the ground.

gboulton

My Feedback: (15)

The whole "right thrust, down thrust" thing is WAY overblown.

The reason...simple...the effect of it, pretty much BY DEFINITION, varies with the thrust being produced by the engine.

In other words, no matter WHERE the engine is pointed (within, of course, obvious reason...it can't be pointed straight up with relation to the fuse, for example) there is a THRUST at which the forces are nicely aligned. For any other point on the stick, the thrust is not quite enough, or a bit too much.

So...put the engine at 0/0...or 2/2...or 3/2...or wherever. All you're doing is offsetting (or not) P-factor, torque, etc...at one particular point on the curve. Everywhere else, you're STILL out of line.

Where's the "best"? Eh...personal opinion. There's a couple of multi-time world champions who fly at 0/0. A few others wouldn't touch a plane that way.

FWIW, MOST of the "hard core pattern guys" I know view it this way:

They fly for a bit to find the throttle setting that the MAJORITY of their routine will be flown at, and then tweak r/d thrust to as close as they can get to 'perfect' alignment at that throttle setting. That gives them the largest amount of time where the airplane is truly "hands off", they say. From there, they simply manually correct (or mix out, depending) any tendencies of the plane to leave straight and level flight at other throttle settings.

crasherboy

As to right thrust,it needs to be there to offset engine torque that at low air speeds where the wing is not at a good flying speed and can not do this on its own. So you put in right thrust. Have you seen a jet with built in right or left thrust? Unless you go jet, I don't think there is any one for all speed thrust angle. I know in full scale light planes ,you trim for level flight when you reach cruising speed,but if you cut the power down you start to go,then you have to correct your flight pass with your elevator. I suppose that one plane design can act different than another. All depends on how the aircraft is set up.

Rodney

I see several comments about right and downthrust not being very pertinent in the above ocmments. I tend to dissagree strongly. On all my planes, minor changes in thrustline have often greatly improved flying characteristics. On Bipes and high wing planes, it is very much a part of the trimming out process to get good flying characteristics. I like a model to go where you point it and not change directions with change in throttle settings. This will not happen unless the thrustline is properly tuned to the aircraft in question. It is almost as important as the relative incidence settings between wing(s) and stab.

KMR1

How about gyroscopic affect? Could adverse yaw be a result of this? I know the old Radials with rotating cylinders made life difficult flying. Would offset counter this?