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Ground Downthrust

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Ground Downthrust

Old 07-24-2005, 08:51 PM
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Spanky1
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Default Ground Downthrust

Good day.
If I understand some of the forums, the downthrust (on a trainer) is to help overcome the lift created when adding throttle and increasing the airspeed......right?

This leads to an observation that needs verification.

In putting a larger engine on a trainer (.40 to a .45 2s), the intent was to aid in carrying video equipment easier. BUT, when full throttle is applied for take off, the plane seems to stay on the ground the same distance as before! Is this because, on the ground, the bigger engine is pulling it into the ground because of the downthrust?
If thats the case maybe eliminating the downthrust with the same engine would help carry the extra weight..?

All hail RCU
Spanky
Old 07-24-2005, 09:41 PM
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Tall Paul
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Default RE: Ground Downthrust

If you mean you switched from a .40 to a .45, that's not much of a switch, power wise.
If you made the plane heavier as well, it's that weight that opposes the performance addition of the power.
I wouldn't be concerned, unless there were takeoff distance problems to consider.
Old 07-25-2005, 01:32 PM
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Bax
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Default RE: Ground Downthrust


ORIGINAL: Spanky1

Good day.
If I understand some of the forums, the downthrust (on a trainer) is to help overcome the lift created when adding throttle and increasing the airspeed......right?

Spanky
Actually, that's not quite right. When you add power, airspeed will initially tend to increase, and lift will increase. The model will the pitch up and climb. That's true of any airplane...ANY airplane.

Downthrust (upthrust, sidethrust, etc) is used to reduce the amount of pitch (yaw) change the airplane makes when the throttle is moved. The pitch change is dependent upon several different factors: thrustline location with respect to the aircraft's CG, wing location, tail location, wing incidence, tail incidence, and so on.

Properly speaking, if a model's in stable level flight at less than full throttle, adding throttle, and nothing else, should result in the model increasing altitude. The rate of climb would be dependend upon how much throttle was added. Reducing throttle would result in a descent. You can't avoid that. What the thrust angle will do is make the model's pitch change milder and more manageable. With out the thrust angle, the model may pitch hard nose-up when throttle's added, and vice-versa when the throttle's retarded. Of course, with too much angle, the effects would be reversed.

Old 07-27-2005, 08:49 PM
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Not24
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Default RE: Ground Downthrust

Basically on a trainer type of model, the down thrust in the engine is to make the plane fly nose level at any power setting. Beginning pilots can't worry about pitch changes with power changes, and this goes a long way to solve that problem. The trainer, when trimmed for level flight at half throttle, will climb slightly at full power and glide at a shallow angle with the engine at idle, and the nose doesn't pitch significantly enough for the student to have to make corrections in pitch.

There are several airplanes that don't pitch up when power is applied, and I'm talking full scale now. It all depends on where the thrust line is. A Lake Buccaneer, an amphibious seaplane, has the engine on top of the wing. It pitches down when you add power and pitches up when you reduce it. The Ercoupe, designed in 1946, has no rudder pedals. The engine in that plane is tilted down 7 degrees and right 3 1/2 degrees. It flies dead straight no matter where the throttle is set. That's just a couple off the top of my head.
Old 07-27-2005, 10:16 PM
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michpittsman
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Default RE: Ground Downthrust

It all depends on the airfoil; a flat-bottom wing will need downthrust to avoid large pitch changes with throttle, and hence speed, changes. The faster a flat-bottom wing goes, the more lift it produces; not so a semi- or full symmetrical where speed changes do not result in large trim changes.....Jim
Old 07-28-2005, 06:16 AM
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Spanky1
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Default RE: Ground Downthrust

Interesting, I'm learning. It sounds like downthrust is meant for flight.

My curiositys are on the take off.
Because you haven't developed lift, isn't the plane being pulled into the ground when full throttle is applied? So when you are in soft grass.... if you look at the mechanics. Maybe I'm thinking to much.

Spanky
Old 07-28-2005, 06:46 AM
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Default RE: Ground Downthrust

Downthrust doesn't have that much effect on the ground. The reason being that the elevator has more than enough authority to control the pitch of the plane even early in the takeoff run. In other words, you should be able to lift the nosewheel clear of the runway fairly soon in the run.
Old 07-28-2005, 10:51 AM
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Default RE: Ground Downthrust


ORIGINAL: Tall Paul

If you mean you switched from a .40 to a .45, that's not much of a switch, power wise.
This is not necessarily the case. For example, the power increase from an OS .40 LA (~.78 real hp) to an OS .46 AX (~1.5 real hp) is close to 100%.
Old 07-28-2005, 12:21 PM
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Default RE: Ground Downthrust

For a tail dragger, thrust effects on the ground depend on the relation of the motor to the wheel location on the ground.
On a typical WWI biplane, with the very short nose, tall landing gear close to the prop arc, steep ground angle, the overturning moment from the prop is huge!
You need up elevator before advancing the throttle to keep the tail on the ground.
Cubs are quite similar, but in the lateral axis. Landing gear too far forward, ground angle steep.
For a tricycle geared plane there's no problem, the nosewheel pushes the nose up.
Old 08-03-2005, 12:23 AM
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former spad
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Default RE: Ground Downthrust

There is no easy answer to your question because there are many factors that work together to force a plane to stay on the ground. Propeller selection/efficiency, engine torque/rpm, landing gear placement, wing and tail incidence, elevator trim. Your plane may simply be overweight.
Old 11-15-2005, 10:28 AM
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awleder
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Default RE: Ground Downthrust

While attempting to fly a Telemaster 40 for a friend on its maiden flight we found that we could not get it off the ground. It turned out the nose-wheel was set extremely low. So much that the tail was about 3 inches above the normal flat attitude. We installed a different nose gear to allow the airplane to set level when viewed from the side. It got off the ground fine after that. I believe that in the nose down configuration it never allowed the wing to attain a positive incidence and even application of full elevator would not allow it to break ground.
Any comments back are welcome.
Old 11-15-2005, 02:35 PM
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britbrat
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Default RE: Ground Downthrust

The elevator should easily overcome the nose-down configuration. It sounds like the main gear is set too far aft of the C-of-G to permit rotation from the nose-down attitude.
Old 11-19-2005, 12:16 AM
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multiflyer
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Default RE: Ground Downthrust

Awleder,

britbrat has it right. Check out this thread:

All Forums >> RC Airplanes >> Questions and Answers >> RE: taking off advice

Much written about landing gear.

Multiflyer

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