Line of Thrust

Dazzler

Dazzler

Well-Known Member
"If an increase in power tends to make the nose of an airplane rise, this is the result of the line of thrust being below the center of gravity"

This statement appears in the Flight Instructor Knowledge Test question bank, but what does it mean? I can not find any references to "lines of thrust" in any source.
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Don't laugh too hard. I think the only place the phrase appears in an FAA publication is in two CFI knowledge test questions.

The questions are interesting because they're among the few knowledge test questions that actually require someone to correlate what they know to a different kind of question.

Dazzler, I'm =sure= you know the answer to your question. Go look at that "Aerodynamics 101" picture of the four forces acting on an airplane.
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Does that give you a hint?
 
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I think the only place the phrase appears in an FAA publication is in two CFI knowledge test questions.

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I tend to agree with you. I've looked in the Handbook of Aeronautical Knowledge, and the Airplane Flying Handbook (which supersedes the AC 61-21A Flight Training Handbook) and have not found the answer.

Getting back to the question, I know that the four forces intersect at the center of gravity. Now when power is applied, the nose will rise, but how is that related to the line of thrust?
 
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I think the only place the phrase appears in an FAA publication is in two CFI knowledge test questions.

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I tend to agree with you. I've looked in the Handbook of Aeronautical Knowledge, and the Airplane Flying Handbook (which supersedes the AC 61-21A Flight Training Handbook) and have not found the answer.

Getting back to the question, I know that the four forces intersect at the center of gravity. Now when power is applied, the nose will rise, but how is that related to the line of thrust?

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Because if the thrust is applied along an axis not in line with the CG, the resulting force will create a moment about the lateral axis and cause the nose to rise/fall. In the CRJ, since our engines are mounted above and rear of the CG, more thrust causes the nose to tip down ! Conversely, pulling thrust back in a flare causes the nose to come up , ala autoflare (well, sorta).
 
The CRJ (especially the 200) has an extremely marked tendency to pitch during increases and decreases of power...irregardless of changes in airspeed, you get to trim it back off.

The tendency is so great that at 50 feet when we pull off the power, it takes about two fingers to flare...all thanks to the engines being mounted about 3 feet above the center of gravity...thank you canadian engineering.
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"If an increase in power tends to make the nose of an airplane rise, this is the result of the line of thrust being below the center of gravity"

This statement appears in the Flight Instructor Knowledge Test question bank, but what does it mean? I can not find any references to "lines of thrust" in any source.

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There are several ways that the term 'line of thrust' is commonly used. The most common deals with twin engine airplanes. You have probably heard the term 'centerline thrust'. This refers to aircraft like the Cessna 337 Skymaster, Adam A500, or a single engine jet like an F-16 or the proposed Diamond Jet. Multi engined aircraft with centerline thrust have an advantage in an engine out scenario as there is no yawing moment due to the engine loss.

The FAA is using a much less common way of looking at things with their use of 'line of thrust' in this example. Instead of thinking about how thrust is distributed along the lateral axis, they are interested in the vertical axis.

I will use the Lake Amphibians as an extreme example. The engine is mounted above the aircraft on a long pylon. In this example the 'line of thrust' is very much above the CG. If you add power, there will be a large pitch down moment and if you remove power, the drag from a windwilling prop could theoretically cause a large pitch up moment. If you ever look at one of these airplanes, they have quite a large trim tab on the elevator.

You can see that if the airplanes line of thrust is below the CG, the situation would be reversed. Since the line of thrust and the CG are probably never going to be exactly equal, this will probably always have some effect on flight characteristics. How measurable this will be is debatable. In a conventional single, the elevator is located in the propwash and this could cause the nose of an aircraft to rise when power is applied just as easily as the line of thrust being below the CG.

Most of the information contained in FAA publications about aerodynamics is correct in a broad sense, but they tend to say the right thing the wrong way, sometimes to the extent that their statements are actually incorrect. Learn what you have to and pass the test, but I would rely more on Aerodynamics For Naval Aviators and the Illustrated Guide To Aerodynamics by Smith in the future.
 
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Because if the thrust is applied along an axis not in line with the CG, the resulting force will create a moment about the lateral axis and cause the nose to rise/fall. In the CRJ, since our engines are mounted above and rear of the CG, more thrust causes the nose to tip down ! Conversely, pulling thrust back in a flare causes the nose to come up , ala autoflare (well, sorta).

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I have not really looked at the CRJ, but I have noticed many of the DC-9/MD-80 series engines are installed so that the rear of the engine appears to point somewhat downward when the aircraft is on the ground. I was always curious why this was done. Currently my pet theory is that it was done to aid in low speed handling and stall recovery, but I am unsure of the real reason.

I do know that some jet aircraft with rear mounted engines sometimes have them mounted so that the tail pipes are canted slightly outward. This can be done to lower Vmcg and shorten the runway required for takeoff. (In a jet, V1 must be higher than Vmcg. If V1 is reduced, then the runway required for takeoff is also usually reduced.) It does cause a slight reduction of efficiency in cruise, but is sometimes deemed worth the penalty.
 
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[Getting back to the question, I know that the four forces intersect at the center of gravity. Now when power is applied, the nose will rise, but how is that related to the line of thrust?

[/ QUOTE ]Non-technical answer, and it will take a bit of visualization.

Go to the picture. Put the CG say, in the window where the wing meets the fuselage. Put a bar through the lateral axis so the airplane can pivot up and down around it. Pretty basic so far.

The thrust line represents the thrust vector and remember that thrust =pulls= the airplane. So turn it into a string

Three tests.

1. The string is attached to the nose of the airplane at a point exactly on the longitudinal extension of the CG. The airplane will not pivot around the CG.

2. The string is attached to the nose of the airplane at a point above the longitudinal extension of the CG. The airplane will pivot down.

3. The string is attached to the nose of the airplane at a point below the longitudinal extension of the CG. The airplane will pivot up.

Still don't see it? Try a seesaw perfectly balanced on it's fulcrum. Pull up on one end or the other. What does it do?

See-Saw%20Balance%20(large).jpg


Turn it vertical.
 
Now that we all understand this ...how do we apply it? I like to teach the value of knowing this while doing steep turns. If you reduce power on your roll out it will cause the aforementioned pitch down, this will counter the up trim that you put in while performing the steep turn and this will keep you level in the roll out. Obviously quicklly retrim and put back in your pre-maneuver power setting to maintain the desired airspeed and altitude.

Jim
 
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Now that we all understand this ...how do we apply it? I like to teach the value of knowing this while doing steep turns. If you reduce power on your roll out it will cause the aforementioned pitch down, this will counter the up trim that you put in while performing the steep turn and this will keep you level in the roll out. Obviously quicklly retrim and put back in your pre-maneuver power setting to maintain the desired airspeed and altitude.

[/ QUOTE ]The problem is that there are two completely different, but related things going on. Take straight and level flight. If you're trimmed for, say 90 knots and increase power, the nose will move to a climb attitude as the airplane begins to climb at 90 KTS. The opposite for a power reduction. I'm pretty sure this happens independent of the location of the line of thrust, and would also apply to your steep turn example.

But what about an example that isolates the "location of pull" effect?

annonoman, do you know enough about the Lake or some other top engine airplane to be able to say, for example, that an increase in power results in an initial nose down moment before the airplane moves into the expected climb?

There is something similar we are familiar with. In a CE 172, deploying flaps results in an initial nose up moment before the airplane moves to the nose down -steeper descent profile. There are other factors at work, but the location of drag above the CG would be one factor.
 
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There is something similar we are familiar with. In a CE 172, deploying flaps results in an initial nose up moment before the airplane moves to the nose down -steeper descent profile. There are other factors at work, but the location of drag above the CG would be one factor.

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<devil's advocate>true indeed, but the same thing happens in a PA28 of any flavor, with the possible exception of a T-tail arrow, which i've never flown.</devil's advocate>
 
In case of extending/retracting flaps, I would think you must factor in how the CP moves aft/forward. When the CP moves, you will once again get a pitching moment. I guess it will depend on the relative shifts of the CP, and on how much drag is increased.
 
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true indeed, but the same thing happens in a PA28 of any flavor, with the possible exception of a T-tail arrow, which I've never flown.

[/ QUOTE ]I think you have more experience with that series than I do, but I've tended top notice a pitch down tendency in the PA-28, at least with the first notch of flaps. And there are other things at work also, as Chris pointed out.
 
you're correct, it does, if we're talking about full extension of the flaps. if you go straight from 0* to 40* flaps in a PA28, the nose will drop immediately. if you put the flaps in incrementally, the first two settings give a slight pitch up, then the obligatory pitch back down. the cessnas....to be honest, i don't have enough time in them - i've only got about 10 hours and most of that was several years ago when i wasn't the pillar of knowledge i now am.
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just trying to add some fuel to the discussion...like i mentioned, i believe you're totally right with your explanation, and obviously you know plenty more about cessnas than i do.
 
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annonoman, do you know enough about the Lake or some other top engine airplane to be able to say, for example, that an increase in power results in an initial nose down moment before the airplane moves into the expected climb?

There is something similar we are familiar with. In a CE 172, deploying flaps results in an initial nose up moment before the airplane moves to the nose down -steeper descent profile. There are other factors at work, but the location of drag above the CG would be one factor.

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I have never flown the Lake. I just wondered about the location of the engine and it's effects on handling. The few that I have examined on the ground had a very large trim tab.

I have read that the nose up pitching moment in the C172 is caused partially by the changes of airflow over the tail when flaps are extended. In a high wing airplane the tail is affected by the downwash over the wings. When flaps are added this increases the downwash on the tail, increasing its angle of attack and causing a nose up pitching moment. As the aircraft slows, this would be reduced and the nose would pitch down.
 
anonaman,

I think you made a typo... an increase in the downwash caused by flap extension should create a local reduction in the AOA of the horizontal stabilizer, which should cause a decrease in the lift, which will lead to a nose-up picthching moment.
 
Flightstars have a high thrust line engine and there is a distinct nose down pitch change when full power is applied without adding back pressure.
 
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anonaman,

I think you made a typo... an increase in the downwash caused by flap extension should create a local reduction in the AOA of the horizontal stabilizer, which should cause a decrease in the lift, which will lead to a nose-up picthching moment.

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You have to remember that although the horizontal stabilizer is an airfoil, it does not create 'lift' in an upward direction like the wing. The stabilizer is used to create 'tail down force'. If the flaps are extended, causing an increase in the downwash over the tail (more airflow coming at an angle from above the tail) then this will increase the AoA of the tail, causing more 'downward lift' or 'tail down force' and raising the nose.

This is somewhat confusing, as when we think of AoA, we are used to thinking of the wing of an aircraft. If the nose goes up relative to the horizon then the AoA of the wing is increased.

With the horizontal stabilizer, everything is reversed compared to the wing. In normal operation the horizontal stabilizer operates like an upside down wing to create 'tail down force' in normal flight. For example we could say that the wing usually operates with a positive angle of attack to create lift and the tail usually operates with a negative angle of attack to create tail down force. Pulling back on the yoke causes the the AoA of the tail to become more negative, making more tail down force and causing the nose to rise. Since the tail usually operates in this negative AoA condition I think it is easier to just think of the AoA as increasing when the yoke is pulled back (increasing tail down force) or being reduced when the yoke is pushed foreward (reducing tail down force). Otherwise you put yourself in the position of saying that taildown force is increased as the AoA of the tail decreases, which is somewhat confusing.
 
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