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Old Jul 10, 2002, 01:53 PM
jrb
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Up thrust; not down!

This is always good for some thought provoking discussion.

The Cargo “too much climb with full power” issue posted around here and as demonstrated in Herb’s video make this worthy of some more discussion.

My thought is that those folks should remove rather than add down thrust to reduce the climb issue under power on the Cargo; this assume 1st that power off (glide) flight characteristics are good.

Also note, that it was reported around here that Larry of SR had suggested this to a guy who had a climb under power issue with his re-built X250.


To start the discussion, what control makes an airplane go up? I bet I know what most people said/thought, and they’d be wrong of course.


Let’s look at an example that truly fits the Cargo but also applies to high or low wings planes with the motor mounted in the nose.

Envision the Cargo’s the wing section viewed in section through a nacelle such that the wing section is horizontal.

Now add down thrust to the motor/prop.

A bit of imagination needed here; but now rotate the image in you mind such that the prop is vertical.

Now when you look at the wing you can clearly see that it has a positive angle of attack (AoA) to the approaching airflow!

Much like a wing sections mounted in a wind tunnel with a positive AoA!

The airfoil climbs to a stall. Surprised, I’m not.

This is also similar in concept to “blown flaps” which can dramatically increase lift.


Back to the other question, you were right if you said throttle!

Two planes sitting at the end of the runway, one guy pushes forward on the throttle, the other pull like heck back on the yoke; who gets into to the air 1st?

Hope Sparky replies
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Old Jul 10, 2002, 03:14 PM
Motor Maniac
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I do believe you are wrong

If you look at this from a vector force point of view you will see what is happening. What makes the plane climb is really lift from the wing, which increases as you add power because you get more lift at higher speed. So you have a lift vector from the wing. Now add a force vector from the motor/prop. If the vector is up (as in up-thrust), this will add more force up, thus more tendency to climb. Downthrust on the motor will add a downward force vector, helping to cancel some of the lift vector from the wing.

Also think about the down force from the tail that helps keep the pitch correct. If you add upthrust on the motor this will tend to pitch the nose up even more at higher speed.

All in all it is a delicate balancing act and one setup for one plane may not be right for another plane.
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Old Jul 10, 2002, 03:25 PM
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Thanks for the respsone 10.

Though I titled it up no down, my text said to remove rather than add down thrust; which I have demonstrated in flight eliminates ballooning.
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Old Jul 10, 2002, 04:14 PM
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How's this work??

If down thrust pulls the nose down, then why is it designed into a plane in the 1st place? Shouldn't it be up thrust, to pull the nose up, climb?
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Old Jul 10, 2002, 04:27 PM
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The up-thrust will likely cause the plane to fly in a nose up position... the nose is pulled up. I experienced that with a bent motor mount, poor control resulted, as well as bad gyroscopic forces. But, this needs to be set right for the right type of plane, and the right flying style that will be incorporated. It's oftenly relative.
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Old Jul 10, 2002, 05:08 PM
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I think it has to do with the CG being too far forward (in relation to teh center of lift)
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Old Jul 10, 2002, 06:24 PM
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for a model with an engine in the nose, the extra flow overthe wing won't make a significant difference in the amount of lift. on the other hand, a couple degrees of down thrust would act like elevator trim proportional to the throttle. due to the distance from the CG, those couple of deg's would create a reasonable arm for the pitching moment to cancel the balooning effect.
however, on models with the motor mounted on the wings, up thrust might actually help, but it would be by reducing the lift and increasing drag..

Avi
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Old Jul 10, 2002, 06:48 PM
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At any single flight condition for trimmed (hands-off) level flight there will one power setting and elevator trim position that is correct for the gross weight and desired airspeed.
.
The only variables the pilot can alter are power and elevator position. (we will NOT consider special cases such as dumping fuel, etc.)
.
Change the power setting, and the airplane is going to change something in the flight path because the energy in the system has changed.
Adding power.. the elevator trim position is now incorrect (not enough down).. and the airplane will accelerate upwards and forward, until it achieves a new attitude and/or altitude for which the new power and unchanged trim position are correct... if that is possible within the constraints of the airplane performance.
Decrease power, the airplane is also now at an untrimmed configuration, and will descend/pitch down (due to the unchanged elevator position) again seeking a trimmed flight condition, which might not also be possible..
.
Power controls altitude.
Elevator controls airspeed.
.
"But, but... I pull up and the plane goes up..."
Yes. And it slows down as it does so. That's why looping from level flight can result in a stall at the top of the loop.
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(Except for TMs, which can't get to the top of a loop, and stall going vertically or fall off to the side. )
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Unless energy is -added- to the system by increasing power, the change in airspeed can result in loss of flying speed.
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Downthrust is a method of counteracting some undesired characteristic of the plane.. generally on a high-wing, it counters the drag of the wing as speeds increases. Up to a point. No single fixed control can be effective at all flight speeds.
On a Kadet for instance with its 4 degrees of downthrust, flying at full (over)power in level flight the elevator will be down about 10 degrees to keep the plane level. Suddenly reduce the power.. less thrust, the wing is still lifting/dragging because the speed hasn't had time to change, the plane's nose goes up. Until it slows down to an airspeed suitable for the new thrust level.
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Old Jul 10, 2002, 07:11 PM
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Yea, what Sparky said...

jrb,

I think Sparky is right on with his explanation. Gives the example of the Kadet with downthrust. Every plane is different, so my point was that the downthrust does have a downward force vector. This vector is there to balance the dynamic forces on the plane (lift from the wing, down force from the tail) under specific conditions (speed, AOA, elevator deflection, etc.).

May have made a bad point about upthrust helping to climb. As Sparky points out it is speed (=increased lift) that really results in climb (unless you have tons of thrust and the prop can pull the plan vertically). But I would think that upthrust would cause higher AOA on a plane with the motor in the nose, maybe less so with the motor(s) on the wing (as they would be closer to the center of lift and have less of an affect on AOA).

I'm certainly no aeronautical engineer. Just wanted to put my thoughts in.
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Old Jul 11, 2002, 08:01 AM
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Banjul
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I was told early in my flying career that throttle controls ascent/descent and elevator controls airspeed. Sounds backward, huh???

VP
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Old Jul 11, 2002, 08:28 AM
jrb
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As I expected Paul and I agree on all the points he listed, though it may initially appear that we differ about more.

Yes VP its sounds backward but its is true, both in the way you and Paul stated the action of pitch and power.

As Paul also listed our planes are at “full (over) power”(ed) by a long shot when compared the full scale. So full scale characteristics can’t totally used.

Our planes at their high/over powered throttle settings tend to fly on their thrust line (vector). Hold the vector horizontal and look at the AoA of the wing.

I’ve built a lot of kits and have yet to find a low wing model that has up thrust; most (virtually all) kits have down thrust independent of wing position. Like the Cargo mentioned above, motor c/l and wing are collinear. (1 point of difference)

As Paul wrote, in flight only pitch and power can be adjusted to affect climb. Back on the ground we often have the means (usually with washers) to re-adjust thrust. (a point not written about above).

I suggest that if you’ve added more down thrust and the climb condition doesn’t improve (really worsens with an open mind) give “removing” down thrust a try; you might just be surprised

PS: am an AE, MS from UTSI, worked in wind tunnels and propulsion test cells @ AEDC for 5 years, and 15 more designing and building aero test facilities and equipment.
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Old Jul 11, 2002, 09:25 AM
S55
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What is your advice for a high wing bi-motor with motors on the wing? Looking at various bi-motor planes my impression is the motors have the same angle of attack as the wing, so there is some up thrust there. Few degrees are hard to notice though.

Thank you,
S55
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Old Jul 11, 2002, 10:11 AM
jrb
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Sorry for the confusion, “up thrust” in the title was not meant to imply actually mounting with the thrust line pointed up; but to change the 1st reaction of adding more down thrust to “removing down thrust”.

The configuration matters little; but I assume the nacelles are mounted above the top wing, with the top wings above the fuselage and the bottom wing at the bottom of the fuselage.

I’ve never built a model or seen a set of plans with “up thrust”; even power pods use a bit of down by design, but in use sometimes rubber band on with a little up – doesn’t much matter when they’re used for climb only, not sustained powered flight.

A degree or two of down thrust is always the best place to start; add a bit if you want more climb at lower power settings; remove some if it climbs too much at your preferred power setting.

Up shot, its always best to have the ability of changing the thrust vector.
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Old Oct 03, 2002, 04:24 PM
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San Diego , Cal
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Howdy.
I'm new to the forum and really sorry I missed out on this thread when it started as I could use some help.
I have a Mini-Max that flew really great until an unfortunate incident with a steel storage container. It was rebuilt, however, with balsa. It is within grams of being the same weight ( a little heavy, due to reinforcement and epoxy). The front half of the fuse and the motor mount are all new. It flys beautifully, maybe better than before, but I have to hand- launch now and can't do ROG's.
I know the thrust line is different now but can't figure out which way to go. I think the motor actually may be to low on the fuse with respect to the leading edge and the prop- wash is all wrong.
Can anybody help?
Thanks,
Bud
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Old Oct 06, 2002, 07:08 PM
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Fascinating discussion. I believe adding downthrust causes an airplane to more readily climb. The thrust vector is normal to the propeller disc and along the motors axis. Let's say that 2 degrees of down thrust exisst and that the axial thrust is 3 pounds. The down vector is the tangent of two degrees times this force. Tangent 2 degrees = .035 so we multiply .035 by 3 pounds and find .1 pound or 1.6 ounces forcing the nose down. The angle of attack,relative wind to the wing airfoil, has increased by 2 degrees. If a Clark Y airfoil, the lift slope "a" is equal to .07 meaning the lift coefficient increases by .07 each degree so Cl is .14 greater.
Lift = qClS, so let's say the plane is traveling at 30 mph (44ft./sec). At sealevel the dynamic pressure, "q" is .00238 x 44^2 x wing area/2 =2.3 S pounds/ft.square. Lift becomes .14 x2.3S/16
or 5.16 S ounces squarefoot . So, 1.6 ounces downward force and 5.16 S pounds /square foot of wing upward force. If the wing area (S) is 288 squareinches (6 inch chord,48 inch span) then the up force is 288/144=2 foot square for rectangular wing and the up force is 10.32 ounces vs 1.6 ounces down. Climb like a homesick angel. If zoom, reduce the down thrust.
Dick Curtis , BS in AE, 1958.
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