Pitching moment due to thrust and drag.

[mbenkhort]

Hi everybody,
I would like to know if thrust and drag(parasite & induced) have pitching moment and intervene in longitudinal stability and in NP location.

Thank you very much.

 

[IRstuff]

Please refrain from double posting.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[moon161]

I expect so, unless centers of pressure, effort and mass all lie along the fore/aft centerline. A free body diagram should help make it clear.

 

[rb1957]

yes, the line of thrust typically produces a moment term ... hence power on and powre off stability are different

 

[mbenkhort]

Thank you all,
but let me tell you that I tried the experiment on Microsoft flight simulator where everything ( thrust, reference center and center of gravity) was along the fore/aft centerline and still change in thrust results in aircraft pitching up/down. At level flight, an increase in thrust induces a climb and an incease in aircraft pitch, and a thrust off (power off) results in a descent and decrease in aircraft pitch. It seems to me that manipulating thrust has lead to a change in overall aircraft pitching moment, but I don't see where the force responsible for the change in pitching moment come from, since thrust force is already on centerline and thus cannot produce any pitching moment?

 

[Kwan]

Here is a site with some info.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770025187_1977025187.pdf

moment = thrust * moment arm

There are probably other factors such as airflow from the thrust over the wing, etc.

MS flight simulator may put in a generic pitching moment but I'm not sure.

I flew in a lake air which has an engine mounted above the fuselage. Add power and the plane pitches down and vice versa.

 

[rb1957]

maybe MS flight simulator isn't flight reality ?

 

[YoungTurk]

Many other factors to consider in the aerodynamics and dynamics in general, but rb is probably right.

 

[rb1957]

"and still change in thrust results in aircraft pitching up/down. At level flight, an increase in thrust induces a climb and an incease in aircraft pitch" ... sounds like the line of action of the thrust, whilst on the CL, is below the aerodynamic centre (like on a cessna 172, high wing, fuselage mounted engine, below the wing)

 

[btrueblood]

Vote #3 for Rb.

Does MS FlightSim have the a/c roll with changes in power?

 

[KirbyWan]

The trust line is not exactly along the centerline of the aircraft, but that's besides the point. If you increase thrust, you increase speed which increased lift generated at the wing which will cause a climb. The increase in speed also increses down force on the horizontal stabilizer which will cause a pitching moment up and also cause a climb. This is why a pilot needs to retrim an aircraft after every change in power and attitude.

Since the trust from a single engine piston aircraft such as a Cessna 172 causes the airflow to corkscrew around the fuselage, a thrust change can effect a change on the roll and yaw stability.

MS Flight sim (which I have) has a fairly accurate flight model. It's based on performance tables instead of a pure physics model, but it doesn't go far astray.

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[Surestick]

" At level flight, an increase in thrust
induces a climb and an incease in aircraft pitch, and a thrust off (power
off) results in a descent and decrease in aircraft pitch."

Assuming the aircraft is trimmed for level flight (which really means trimmed for a certain angle of attack) this is normal. As you add power above what is needed for level flight at the trimmed-for AoA the A/C will climb. If you could measure the AoA accurately, no doubt you would see a slight change with power changes but for most aircraft it's probably not much by design.

For aircraft with engines mounted farther from the centreline than normal the change might be noticeable. I've heard the Republic Seabee had a nose up pitching moment with power reduction due to it's engine placement. Not exactly what you want at low speed and altitude on takeoff if the engine fails!