B
Blucor Basher
Right thrust
If you want to have your brain scrambled, Google "Why does my model airplane need right thrust?" and read for about 10 minutes. Then resolve never to get into a technical argument with people on the internet. You'll live longer and be happier!
We 3D pilots, living in a world of big power and big propellers on our aircraft, have to deal with the effects that big props have on our airplanes. Our situation is somewhat like that faced by pilots of the last generations of piston-engined military aircraft and a lot like what modern full-scale aerobatic pilots deal with. They also have big props driven by big engines hanging out in front of their planes, and they feel first-hand the forces those props impart to the airframe, although any plane with a propeller is subject to these forces.
A 3D propeller looks simple, it's just two little wings spinning around a hub. The little wings present to the air at a positive angle of attack and therefore produce a force along the axis of rotation, thrust, going forward. Goose the throttle and this force increases dramatically, and the airplane accelerates and we start having fun. Forward thrust, however, is not the only force imparted to the airframe by the prop. Not by a long shot. The spinning of the prop creates all sorts of forces.
The prop is affected by gyroscopic force and P-factor. There is also a torque reaction that rolls the airplane when you accelerate the prop. Air being accelerated back toward the airplane by the prop forms a spiral slipstream that impacts the aircraft as it passes along the fuselage. Lots of complex things are happening. If our plane was powered by a rocket motor with no rotating parts, we would have none of these forces to deal with, but until the rocket-powered 3D plane is perfected, we have to deal with them.
You can (and probably should if you want to really progress as a pilot) read a lot about each of these individual propeller effects. One great resource is Scott Stoops' book "The Pilot's Guide to Mastering Radio Controlled Flight", but now that it is out of print, prices are rising rapidly (http://www.amazon.com/Pilots-Mastering-Controlled-Flight-Edition/dp/0976711400). For me, it is definitely worth the 50 or 60 bucks a used copy is commanding now. However, in this article, I'm only going to discuss one particular propeller effect as an example and then go on to the practical ramifications of propeller effects.
Gyroscopic force is one of these forces that is really mystifying and confusing. Perhaps you had a science teacher in grade school who demonstrated this or perhaps you just played with a loose bicycle wheel and discovered it that way. Being a 70's kid, one of my favorite toys was the "Wizzer", an enclosed gyroscope-top thing that did all sort of tricks via gyroscopic force.
There's even an old Wizzer commerical on YouTube -
[video=youtube;_KRbCfXi1Nw]http://www.youtube.com/watch?v=_KRbCfXi1Nw[/video]
Excuse me for a moment, I need to go look for a Wizzer on ebay. Perfect. Scored one new-in-box for $10.
Anyway, however you first discovered gyroscopic force, it probably fascinated you (since you are also the kind of person who likes RC airplanes) and you remember a little bit about it. Our propellers are perfect gyroscopes and so they also demonstrate this behavior. To understand a little about the effect it has on the airframe, watch this excellent non-technical video on the Veritasium YouTube channel.
[video=youtube;ty9QSiVC2g0]http://www.youtube.com/watch?v=ty9QSiVC2g0[/video]
Luckily for us, in the demonstration, he even happens to spin the wheel in the same direction as our props spin when viewed from the front. You can readily see that precession will act to rotate the airframe in yaw, to the left. Gyroscopic force is just one prop effect, although it can be a strong one on any airplane with a very large prop, like our 3D planes. Although the various propeller effects will in some cases counteract each other, we know that in most flight modes, the net effect of all of these forces will be to cause the nose of the airplane to turn to the left. At high prop RPM's, gyroscopic effects are a large part of this.
So, what counteracts these forces to keep an airplane going straight instead of turning left? Mostly, we depend upon the tail surfaces of the airplane to keep an airplane pointing straight rather than choosing an unexpected path. However, tail surfaces work by airflow over them, and this means that their effectiveness increases with airspeed. This also means that tail surfaces are not very effective at low airspeeds. Gyroscopic effects for a given prop, however, increase with the RPM of the prop. So we can see that in a situation where airspeed is low (ineffective tail) and RPM is high (big prop effects) we cannot expect the tail to keep the airplane pointing in the direction we want. Since prop effects usually combine together to make the nose of the plane swing to the left, we can expect that in any situation where we are using a lot of throttle at very low airspeed, the airplane is going to want to turn left. On conventional full-scale prop powered aircraft, this mainly happens at takeoff:
[video=youtube;gzxt2SzOsAY]http://www.youtube.com/watch?v=gzxt2SzOsAY[/video]
I like that video especially for making "dipsy-doodle" a technical aviation term. Now that you know what to look for, checkout the bootfull of right rudder Kirby Chambliss uses during this maximum performance takeoff. At 0:05 I'll bet he has 20-25 degrees in:
[video=youtube;n5vfKDAWHZs]http://www.youtube.com/watch?v=n5vfKDAWHZs[/video]
So (to establish the basics) we can see that in high-power, low-airspeed situations like takeoff, the combined prop effects tend to swing the nose of the aircraft left. We can also see that this causes a pilot a lot of work to keep the aircraft tracking straight.
In model aviation, we can see this same effect upon takeoff. I think anyone who has ever spend much time at an RC flying field has seen this go down:
[video=youtube;VeXh71bVK1s]http://www.youtube.com/watch?v=VeXh71bVK1s[/video]
In 3D flying, however, we have other common circumstances that include high power and low airspeed, particularly hovering. All of these high-power, low-airspeed situations and the attendant prop effects cause more work for the pilot, because the pilot has to use the rudder actively to cancel out the left-turning tendency. So, what can an aircraft designer do to assist the pilot and take some work off of his thumbs?
Most RC pilots already know that the typical aircraft design includes right-thrust, that is, the motor or engine and propeller point slightly to the right of the center line of the aircraft. On 3D aircraft, this angle is usually set by the shape of the motor mounting box. Typically, on most 3D aircraft, the motor or engine points 2.5-3 degrees to the right, and this dimension is called "Right thrust angle". This directs approximately 5% of the total thrust to the right to counteract the left-turning tendency, and reduces the amount of right rudder needed in high-power/low-airflow situations. Because it is an angle applied to the power system, the amount of right thrust increases as power and RPM increases and decreases as power decreases.
All good and fine, right? A genius solution, and now we know and can all go back to flying. Not quite. We really need to know two more aspects of right thrust to use it to our advantage. Firstly, (and most importantly) we need to know that the right thrust angle on any quality ARF aircraft or set of professionally-drawn plans is going to be sufficient for the aircraft to fly reasonably well. You can fly a 3D aircraft, as a matter of fact, with zero degrees of right thrust. The effect is subtle, and except for takeoff and hovering and certain maneuvers, you might not even notice for a while. I say that this is important to know because right thrust angle is one of those things which is often blamed for poor aircraft behavior when in fact it is almost never the culprit.
The scenario is very common at every flying field - A pilot is having a problem with his aerobatic airplane. It is most likely caused by a center of gravity issue or control surface throws like 95% of tuning problems, but during the course of diagnosis it is inevitable that someone will suggest right thrust as the culprit. Nope. (BTW, the problem is also not lateral balance. It never is, but someone always suggests it too). If the pilot starts changing his right thrust around to try to fix some problem, he will waste time, get confused, and possibly make things worse.
For this reason, I recommend that new 3D aerobatic pilots ignore right thrust angle as a setup variable on their aircraft. New 3D pilots should be buying well-known quality aircraft anyway so that they can concentrate on learning 3D flight instead of re-engineering bad planes - leave that to other people. As such, new guys should put right thrust out of their minds and if someone suggests they should change the right thrust on their plane, say "Thanks very much" and completely ignore the suggestion. Concentrate on CG location, choosing the right prop, correct control throws, and practice, practice, practice!
Secondly, for more advanced pilots, it is important to know that right thrust angle is a compromise. There is no "correct" right thrust angle, only an approximation based upon a variety of factors. Once in a while, we get a call or email from a pilot who took one of our aircraft and added a ton of right thrust (sometimes it looks like 9 or 10 degrees) and finds out that the airplane is easier to hover and tells us that our right thrust angle is "wrong". This pilot has not yet learned that right thrust is a compromise. The consensus angle of 2.5-3 degrees represents a setting that takes some work off of the pilot at low speed and helps to keep the airplane from turning left at moderate speed but still allows the airplane to track straight with one rudder trim setting at a variety of speeds. The pilot who puts in a ton of right thrust to make his plane easier to hover will find that his plane no longer tracks straight in high speed flight and that the rudder trim he needs is speed-dependent.
So, when would someone maybe want to change their right thrust? As an example, if you have an airplane that can use a wide variety of prop sizes (like for instance our 48" Vyper, I fly it on 11x8, 12x6, 13x6.5, and 14x7 props, a diameter difference range of 27%) then it probably has a compromise right thrust angle intended for the middle of the size range. A very large diameter prop will likely benefit from a slight increase in the right thrust angle, although the effect will be subtle, but again this is for advanced pilots.
The standard, classic test prescribed for testing your amount of right thrust is to first fully trim the airplane (this is the first step of every trimming test - get the plane flying straight hands-off) and then to fly the airplane at full throttle away from yourself and pull up to a vertical climb and watch the airplane during the vertical climb. As the aircraft slows, watch to see how long until the aircraft starts to veer to the left from prop effects. If you are tuning for a pattern or IMAC sequence, then you use the altitude of your uplines you will fly in competition as the standard for this test. If you want it to go farther up the upline before veering left, add more right thrust. Now, can anyone see any problem with using this test on a 3D airplane? Yep, for starters, a lot of our planes don't really slow down on a vertical climb. Secondly, since we're not flying a prescribed pattern routine, we don't have a set distance of vertical climb to work with as a standard. So, this test can be less than completely useful for us. However, we can see the principle contained within the method of this test - the amount of right thrust you need is determined by what you are doing with the aircraft. If literally all you did all day with your aircraft was to hover and you didn't care how wacky it flew at higher speed, then perhaps adding a bunch of right thrust is the ticket, however, most of us use the full speed envelope of our planes and so we will be forced to make this compromise.
So, if we are very familiar with our aircraft and we want to play with right thrust value to see if we can extract more performance for our particular flying style, we can certainly change our right thrust. Sometimes at the field, you will see people with their cowl off, adding washers to their motor mount to try different right thrust angles in flight. Usually, pilots are disappointed by the small effect this has (especially in proportion to the work required to take off the cowl and dismount the motor) but sometimes they make a positive change to their plane for themselves. Here's a tip - if you want a quick and easy way to simulate what your plane would be like with a little more right thrust angle, instead of taking it apart, use your transmitter. Program in a 2% right rudder to full throttle mix. While not exact, this is a very close approximation of the performance of the plane with a slight addition of right thrust. If you want to try less right thrust, use a 2% left rudder to full throttle mix. In this way, you can experiment without taking the plane apart. If you find you like the change, you can always actually change the thrust angle on the plane, just remember to then remove the transmitter mix!
So, to sum up:
- Props are thrust producers, but they also impart a lot of other forces into our airplanes, like gyroscopic force and many others.
- These forces are strongest when the prop RPM (engine power) is high.
- At high airspeeds, the tail of the plane counteracts these prop forces effectively, but not at low speeds.
- Therefore, at high power at low airspeed (like takeoff and hovering) we feel prop forces most.
- The net effect of these forces is usually to make our aircraft turn left.
- To take some work off of our left thumb, we mount our power system and prop at an angle pointing right so that at high power it balances the left turning force with a right turning force.
- This angle is a compromise based upon the need for the airplane to handle well at a variety of airspeeds.
- Any decent quality aerobatic ARF already has an adequate right thrust angle, changing it is only for advanced pilots.
- Applying the classic test to check right thrust can be difficult on a modern 3D airplane since our planes power right past the limits used in the classic test.
- So, we might need to experiment to find perfection, and this is easiest done by experimenting first with rudder to throttle mixes before taking your plane apart.
I hope this little discussion of right thrust has been helpful. For a lot of pilots, especially newer ones, it remains mysterious (as evidenced by the calls and emails we get) but it is only a detail meant to make our job as pilots a bit easier. We should know about it, but not be obsessed about it, since any decent aircraft designer will basically take care of it for you.
Ben Fisher
www.3DHobbyShop.com
