Lift due to Thrust

[Stoney]

Is there a method to test lift due to thrust in-game?

[FLS]

Do you mean like manually trim for level flight at normal power and then go to military power and note the climb rate?

[Stoney]

No, its basically the lift provided when the thrust line is above the climb angle/flight path, due to angle of attack.  From what I understand, its a condition more often associated with jets, especially vectored thrust aircraft, but given the massive amounts of power the engines of WWII fighters could produce, I figured it might be a consideration. 

For example, to be as precise as possible when discussing maximum lift coefficient, you would need to say "...ignoring any lift due to thrust, the maximum lift coefficient is..."

If its negligible, I'll press without considering it.

[bozon]

How about this for a test. A quick idea:

1. Trim the plane for level flight with engine on. Lets the speed stabilize
2. Note the airspeed.
3. Freeze the trim (switch to manual)
4. Cut the engine, keep the wings level and let the plane go into a glide.
5. Note the speed it will settle to.

The plane is trimmed to produce 1G lift for the given speed with engine on. When you cut the engine it will descent with the rate of descent offsetting the drag, and airspeed to produce 1G at constant rate of descent. If the engine produced lift, the new speed will be higher to compensate for the lost lift.

Another variant (perhaps better?):
1. trim for level flight with engine on.
2. Note the airspeed
3. Fix the trim
4. accelerate to a higher speed (may have to climb, dive and level out) and cut the engine. Hold the wings level, but dont touch elevation.
5. The plane will climb, reach an apex and then settle into a glide.
6. Note the speed at the apex (when rate of climb hits zero - momentary "level" flight).

Again, if the engine produced no lift speed #2 and #6 should be equal and if it does, #6 should be higher than #2.
In both cases you have Von and Voff for the engine on/off cases. Since the lift coefficient is constant (same trim = same AoA) and the air density assumed to be the same (dont change alt too much or simply use IAS), the only lift variables are the airspeed and the engine constribution: 0.5*Cl*ro*Von^2 + Lengine = 0.5*Cl*ro*Voff^2
The fraction of lift from the engine out of the total lift will be:
frac = 1-(Von/Voff)^2

There are probably a million things that are not accurate with these tests, but at least it will give some idea of the engine lift fraction is of any significance.

[FLS]

No, its basically the lift provided when the thrust line is above the climb angle/flight path, due to angle of attack.  From what I understand, its a condition more often associated with jets, especially vectored thrust aircraft, but given the massive amounts of power the engines of WWII fighters could produce, I figured it might be a consideration. 

For example, to be as precise as possible when discussing maximum lift coefficient, you would need to say "...ignoring any lift due to thrust, the maximum lift coefficient is..."

If its negligible, I'll press without considering it.

So there would be a thrust to weight ratio where it can be significant depending on airspeed and AOA and the maximum effect would be when "hanging on the prop" in the mush regime where you have max AOA but not max lift?


Bozon in your tests you don't have the thrust line above the flight line?

[colmbo]

Trim sets the airplane to an airspeed, not a g load.  Once stabilized it will be at 1G.

In your test it should be very close to the same airspeed, the only difference would be any trim change due to not having the prop wash over the tail.

[Stoney]

Forgive the rough sketch, but to illustrate:



You can see that at high AoA, and assuming high enough thrust, there will be a component of the total lift force that is due to thrust alone.  Obviously, during a ballistic zoom climb, practically all of the lift would be due to thrust.  Anyway, just curious as to whether or not there is a measurable amount for these planes.

[FLS]

The extreme example would be an RC plane hovering vertically like a helicopter. I can't imagine how you would test this for the less extreme examples. It seems like there would always be a lift component from thrust when the nose is above the flight path and the greatest fraction would be at max AOA where lift was minimized prior to departure.

[Stoney]

The extreme example would be an RC plane hovering vertically like a helicopter. I can't imagine how you would test this for the less extreme examples. It seems like there would always be a lift component from thrust when the nose is above the flight path and the greatest fraction would at max AOA where lift was minimized prior to departure.

Exactly.  My question is whether or not it the affects are large enough to consider, and if so, a method of testing them in-game.

[bozon]

Trim sets the airplane to an airspeed, not a g load.  Once stabilized it will be at 1G.

In your test it should be very close to the same airspeed, the only difference would be any trim change due to not having the prop wash over the tail.

Trim set the plane to AoA not airspeed, but these are closely related. It is actually the difference between the two that I suggested as a measure for the thrust-lift. However you do have a point that the trim will be affected by the (or lack of) prop wash on the tail. Perhaps this would be less significant in planes line 110 or mossie that do not have the elevators directly behind the props. edit: trim will also be affected by thrust that is not aligned with the center of mass, so I guess overall this testing method will be too biased to work.

Stony, if you just want ballpark numbers just use thrust*sin(angle) where angle is the angle between thrust and airflow. Since AoA is in the range of about 0-15 degrees and the engine will not be angled by more than a few degrees (there's also incident angle between the wings and the fuselage that factors in, but still a couple of degrees) you can just take angle=20 as an upper limit and see how significant it is. It basically means about 30% of the thrust pulls perpendicular to the flight direction as an upper limit.

[hitech]

Exactly.  My question is whether or not it the affects are large enough to consider, and if so, a method of testing them in-game.

Stoney: to give you a magnitude. just do sin(MaxAOA - Incidence + ClimbAngle) * Thrust.

Then things change also change with slip stream effects over both wing and tail. But the above equation gives you a since of the scale as compared to lift.


HiTech

[Yeager]

Forgive the rough sketch, but to illustrate:

Lets be clear!  That is not a "rough sketch".  That is a "technical Illustration".

[Charge]

I have often wondered why German planes had short and broad propeller blades and how those propellers delivered the engine torque to airstream compared to longer and thinner blades in many allied planes. What I'm talking about is the leverage arm of the propeller blade from maximum point of pressure to center of engine axle.

-C+

[hitech]

I have often wondered why German planes had short and broad propeller blades and how those propellers delivered the engine torque to airstream compared to longer and thinner blades in many allied planes. What I'm talking about is the leverage arm of the propeller blade from maximum point of pressure to center of engine axle.

-C+

Charger, I am missing something, why you believe the prop diameter has anything to do with this discussion?

HiTech

[Charge]

"Charger, I am missing something, why you believe the prop diameter has anything to do with this discussion?"

Because somehow I have a feeling that e.g. in a climb when the aircraft starts to slow down also the RPM wants to drop and that would eventually cause the plane to stall if there is too much load on propeller and the engine does not have enough torque to rotate the propeller at the best RPM for the engine power. If that is not the case IRL then the rest of my pondering is nonsense.

What I mean is that for some reason the German planes all had a small diameter large bladed prop and if you compare those to e.g. that in P51 or P47 they are quite different. What the heck were the Germans thinking of, or what where the Americans thinking of? Why are they so different? Optimizing for different speeds? If so then is there a benefit for either one in climb or in "extreme" climb?

If the propeller is large and has long blades the tips may of course easily over speed or the leverage momentum is bigger against the engine and it may cause it to lose engine power needlessly in a climb. But then again if the best flow area for a short paddle shaped prop is quite close to fuselage doesn't part of the flow get negated by turbulent flow near fuselage, as in FW190?

I was just thinkin that Stoney is wondering if the thrust is enough to cause lift by itself and I was thinkin about the strange lift off vector of Flugwerk FW190 and wondering too how well can these planes energize the flow around main plane and if there is any explanation or anecdotes of 190s somehow "hanging in the air" or P51s beating Bf109s in zoom climb -and they both have very different props. FW has more power but shorter prop and P51 has less power and longer prop, why not the other way around?

Thus I thought of bringing the propeller design into this discussion. Maybe its OT, dunno.

-C+