Down- and sidethrust on a twin?

[Ben_E]

I'm working on a twin project - will I have to build in some down thrust? If that is the case how much?

I recon that side thrust not will be needed - or am I wrong?

Ben_E

[DaveW]

Hi,

I have built several, with electric power, we do not need to worry about engine out problem.

Mine were not high power models, so I just set everything up with no side thrust or down thrust

One plane is still flying 10yrs later, now owned by friend.

I would guess that with a high powered twin, at little downthrust in the right motor would be beneficial, but thats just opinion. I haven't done it.

With a glo powered twin ( many years ago ) I very carefully built in down and right thrust on the right motor, and set the left one to a little left thrust. Didn't help a bit when one engine quit at high speed and low altitude. It rolled violently and went into the ground.

Dave

[tim hooper]

Ben,

I'm no expert, but I have had a couple of twins.

Sidethrust is used on the Twinstar, but not on the Sportwin.

As for downthrust, I'd guess the same rules apply as for a single-motored model. It depends on the thrustline of the motors; the Twinstar has the motors on its high wing, so there's a fair amount of upthrust built in. Conversely, the Sportwin has its nacelles slung below its low wing, and so has downthrust built in.

tim

[Gary Warner]

Good question, considering my next project is a twin. I think I know what to do, but I don't want to sound like an idiot. I do think that the "correct amounts" will be hard to nail down for any given plane and for any given flight attitude. Though I do think there some generalization rules on this.

Maybe Olie will chime in when he wakes up and gets a fresh cup of coffee. :D

[DanSavage]

Originally posted by Ben_E
I'm working on a twin project - will I have to build in some down thrust? If that is the case how much?

I recon that side thrust not will be needed - or am I wrong?

Down thrust will be required or not depending upon how far the propellers are above or below the centerline of the model.

Glow-powered twins usually incorporate outboard thrust on each engine so that the model will be able to be more easily flown in an engine-out situation. Fortunately, electric-powered twins don't need this because of their built-in reliability.

But P-factor and torque do get stronger with each additional prop/motor added. You can use asymetrical thrust in place of right thrust to counter both P-factor and torque.

Check the rpm of each motor with a tach and put the the stronger of the two on the left engine and the weaker on the right. I've did this with a four-engined model and it worked great.

Dan

[RCAV8R13]

Originally posted by DanSavage
Down thrust will be required or not depending upon how far the propellers are above or below the centerline of the model.

Glow-powered twins usually incorporate outboard thrust on each engine so that the model will be able to be more easily flown in an engine-out situation. Fortunately, electric-powered twins don't need this because of their built-in reliability.

But P-factor and torque do get stronger with each additional prop/motor added. You can use asymetrical thrust in place of right thrust to counter both P-factor and torque.

Check the rpm of each motor with a tach and put the the stronger of the two on the left engine and the weaker on the right. I've did this with a four-engined model and it worked great.

Dan
I'm sorry I can't leave this one alone.
P-Factor and torque are no factor at all on any model airplane. And P-Factor has no effect even on full scale aircraft.
First let's look at torque. Engine torque is aligned with the roll axis not the yaw axis. Secondly, model engines do not have enough torque to produce any noticeable effect on the flight path of any model airplane. Let's see what a little math can show us. An O.S Max 40FX produces 1.36hp @ 16,000 rpms. I did the math, that's 85.2 oz/in of torque! No that's not a typo that's 85.2 oz/in. I have servos with more torque than that!
Now let's attack P-Factor. If you calculate the off set in the center of thrust based on the difference in pitch of the blades you get this: 10x6 prop @ 14deg AOA (virtually every wing stalls at more than 14deg.) the off set will be 1.4mm! Yup, 1.4mm. Even if you calculate the difference in airspeed (one blade is going upwind the other downwind) you get 2.1mm. This is clearly not enough moment arm to cause any noticeable effect. Even if the thrust where half way out the blade (which it can't be) it would only have a moment arm of 2.5in!
O.K., so what is it? Why do we need right thrust? Simple, spiral prop wash. For 100 years now we (well not me personally) have been building airplanes with half the vertical stabilizer missing. Why? Because we don't want to have to climb up a two story ladder to get in to our Cessna 150! Landing gear would be impractically tall. As the propeller wash curls down the fuse it beats against the left side of the vertical stab. With no corresponding vertical area below the fuse to counter act the force, the airplane yaws to the left. This effect has a huge moment arm (from vertical stab. to C.G.) and therefore only a small difference in air pressure on one side of the stab will have a very noticeable effect on the flight path of the airplane.
In a twin engine aircraft it is still spiral prop wash that determines which engine is "critical engine". If the airplane is allowed to yaw too far, the spiral prop wash from the right engine will grab the vertical stab and yaw the aircraft even further to the left. If the left engine prop wash contacts the vertical it will also cause a yaw to the left, but in this case it will help.
RCAV8R13

[DanSavage]

Originally posted by RCAV8R13
In a twin engine aircraft it is still spiral prop wash that determines which engine is "critical engine". If the airplane is allowed to yaw too far, the spiral prop wash from the right engine will grab the vertical stab and yaw the aircraft even further to the left. If the left engine prop wash contacts the vertical it will also cause a yaw to the left, but in this case it will help.

So, shall we argue causes or discuss effects and their solutions as it relates to model airplanes?

Regardless of the cause, the effect is still the same and their solutions on a model airplane are limited.

How would you correct this tendency on a scale model of a twin engine aircraft? Or a multi-engine?

On which axis does gyroscopic precession operate in an airplane? Roll or yaw?

Dan

[DanSavage]

Originally posted by RCAV8R13
I'm sorry I can't leave this one alone.
P-Factor and torque are no factor at all on any model airplane. And P-Factor has no effect even on full scale aircraft.
First let's look at torque. Engine torque is aligned with the roll axis not the yaw axis. Secondly, model engines do not have enough torque to produce any noticeable effect on the flight path of any model airplane. Let's see what a little math can show us. An O.S Max 40FX produces 1.36hp @ 16,000 rpms. I did the math, that's 85.2 oz/in of torque! No that's not a typo that's 85.2 oz/in. I have servos with more torque than that!
Now let's attack P-Factor. If you calculate the off set in the center of thrust based on the difference in pitch of the blades you get this: 10x6 prop @ 14deg AOA (virtually every wing stalls at more than 14deg.) the off set will be 1.4mm! Yup, 1.4mm. Even if you calculate the difference in airspeed (one blade is going upwind the other downwind) you get 2.1mm. This is clearly not enough moment arm to cause any noticeable effect. Even if the thrust where half way out the blade (which it can't be) it would only have a moment arm of 2.5in!
O.K., so what is it? Why do we need right thrust? Simple, spiral prop wash. For 100 years now we (well not me personally) have been building airplanes with half the vertical stabilizer missing. Why? Because we don't want to have to climb up a two story ladder to get in to our Cessna 150! Landing gear would be impractically tall. As the propeller wash curls down the fuse it beats against the left side of the vertical stab. With no corresponding vertical area below the fuse to counter act the force, the airplane yaws to the left. This effect has a huge moment arm (from vertical stab. to C.G.) and therefore only a small difference in air pressure on one side of the stab will have a very noticeable effect on the flight path of the airplane.
In a twin engine aircraft it is still spiral prop wash that determines which engine is "critical engine". If the airplane is allowed to yaw too far, the spiral prop wash from the right engine will grab the vertical stab and yaw the aircraft even further to the left. If the left engine prop wash contacts the vertical it will also cause a yaw to the left, but in this case it will help.
RCAV8R13

Originally posted by RCAV8R13
This is a work in progress. It has half a dozen flights on it. Hovers well, but with both props turning the same direction it torque rolls a lot. I am designing motor mounts with adjustable up and down thrust to counter the torque. I'll put down thrust in the right motor and up thrust in the left. It is powered by two IPS A units with 10x4.7 props. Weighs 7oz and has plenty of power for strong vertical take offs. After I get the protoype working well I am going to build one using my "puzzing" technique to make the black and white color scheme of the M21-D-21 variant of the SR-71 Blackbird. The M21 was the one that carried the super sonic R/C drone! It needs only two servos as "rudder" is acomplished using two speed controlers and differential throttles. This makes for a very light set up.
Damn this hobbies fun!
RCAV8R13

Propwash? Torque? Propwash? Torque?

[RCAV8R13]

Originally posted by DanSavage
So, shall we argue causes or discuss effects and their solutions as it relates to model airplanes?

Regardless of the cause, the effect is still the same and their solutions on a model airplane are limited.

How would you correct this tendency on a scale model of a twin engine aircraft? Or a multi-engine?

On which axis does gyroscopic precession operate in an airplane? Roll or yaw?

Dan
I agree, it's more productive to discuss solutions than to point fingers at the culprit. Just thought you might want to know what's going on. Didn't mean to ramble.
The solutions are quite limited, but effective.
One solution is to design the problem out. By understanding the true reason for the effect we can more effectivly solve the problem by building the vertical stabilizing area to be symetrical just as the wing and horizontal stabilizers are. This isn't as hard as it sounds. You don't have to increase the area more than normal, it just needs be symetrical and bisected by the thrust line. O.K., it would look weard, but only because it is different from what we are used to.
The other solution is easier, and by far more popular, and that is to add some right thrust. This is a pretty good solution be cause the problem is dynamic with power. With the engine off the problem goes away. It's only a problem when you have power on. By applying the solution at the source of the problem the solution becomes as dynamic as the cause. It is customary on twins and multis to add a little more right thrust the right engine(s) because the problem is potentialy more severe when the left engine quits and you have to fly on the right (critical) engine.
The one solution that I think you should never do, is angle the vertical stab. This solution is not dynamic and does not go away when you throttle back.
On an airplane, gyroscopic precession acts upon the yaw and/or pitch axes. On a helicopter it acts on the pitch and/ or roll axes.
But model airplane propellers are much too small and much too light (and in the case of geared electric power systems, turning much too slowly) to have much, if any, noticable effect. In full scale aircraft the propellers are very large (almost allways geared) and very heavy. Full scale props have a lot of mass that has a long moment arm and threfore can have a large effect. G. P. has been known to break off crankshafts on full size aerobatic aircraft. Almost all have gone to light weight carbonfiber props to solve the problem.
Darn, and I wasn't going to ramble tonight. Oh well,
RCA

[DanSavage]

Originally posted by RCAV8R13
The other solution is easier, and by far more popular, and that is to add some right thrust. This is a pretty good solution be cause the problem is dynamic with power. With the engine off the problem goes away. It's only a problem when you have power on. By applying the solution at the source of the problem the solution becomes as dynamic as the cause. It is customary on twins and multis to add a little more right thrust the right engine(s) because the problem is potentialy more severe when the left engine quits and you have to fly on the right (critical) engine.
The one solution that I think you should never do, is angle the vertical stab. This solution is not dynamic and does not go away when you throttle back.
On an airplane, gyroscopic precession acts upon the yaw and/or pitch axes. On a helicopter it acts on the pitch and/ or roll axes.
But model airplane propellers are much too small and much too light (and in the case of geared electric power systems, turning much too slowly) to have much, if any, noticable effect. In full scale aircraft the propellers are very large (almost allways geared) and very heavy.

Right thrust is one possible solution as is canting the fin. A more elegant solution would be to use asymetrical thrust. In the case of a twin, tach both motors and put the strongest on the left wing and the weaker on the right.

When I built my model of the Super Guppy, I put the strongest on #1, the next strongest on #4, third strongest on #2 and the weakest on #3. This gave the suggested right thrust without having to resort to physically angling the motors to the right.

Engine out situations usually only occur with IC. Electric twins and multis rarely, if ever, encounter "engine out" situations, so discussions about it, on this forum, at least, are moot.

Not too sure which full-size aircraft you're discussing, but to my knowledge only one full-size airplane (Cessna 175 (http://users.chariot.net.au/~theburfs/cessna175page.html)) uses a geared propeller. Our family's Grumman Tiger certainly doesn't use a geared propeller. It is bolted directly to the crankshaft as are all of the propellers on all of airplanes tied down around it.

Dan

[Thomas B]

I must beg to differ when someone says that P-factor is not a factor on any model airplane. Many model aircraft do have negligible effects from this, but not all do.

Try flying a small SR-71 model with a single pusher prop. At low airspeed, such as right after an hand launch, P-factor WILL cause it to steer in a reactive way to the force of the lower half of the prop disk. The lower part of the prop disk is working in less disturbed air and this also seems to affect things.

This works out to very noticable left nose movement when the prop is turning as a pusher.

Some experiments with 10" props on a 18" span SR-71 had the model doing a nearly uncontrollable left turn at launch until airpseed was up to cruise, when the effect went away.

Spiral prop wash is not the culprit on an SR-71....;)

However, back when I used to fly a glow model with a parasol wing and a small vertical tail, it showed enormous spriral propwash effects. Believe it or not, it needed some left thrust to fly straight! Just for fun, I taped a piece of cardboard between the wing and the fuse to close up the open bay there. Now that the sprial propwash had to find another path back to affect the vertical tail, it suddenly needed right thrust!