Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Stoney on April 12, 2010, 08:52:11 PM
-
Is there a method to test lift due to thrust in-game?
-
Do you mean like manually trim for level flight at normal power and then go to military power and note the climb rate?
-
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.
-
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.
-
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?
-
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.
-
Forgive the rough sketch, but to illustrate:
(http://i125.photobucket.com/albums/p61/stonewall74/roughsketch.png)
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.
-
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.
-
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.
-
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.
-
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
-
Forgive the rough sketch, but to illustrate:
Lets be clear! That is not a "rough sketch". That is a "technical Illustration".
-
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+
-
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
-
"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+
-
Now I understand your thinking charge, but the effect in this topic would not really be any consideration in prop design.
So now a few basic prop principles.
On all, possibly almost all, WWII fighter had constant speeds prop. This works simply by increasing or decreasing blade angle to maintain the same RPM at almost all flying speeds until the prop is mechanically unable to increase or decrease pitch. In speeds from stalls to well above max level speed the prop will maintain the same RPM.
2nd it is always preferred to make a longer prop then a shorter one for efficiencies sake. But mechanical limits come into play very quickly. Tip speed is normally just compensated for in the gear box of the prop. Most WWII gear ratio's were 1.5 - 2.0. A 109G10 is 1.6 a P51D is slightly over 2.0.
In almost all cases prop length is simply determined simply by ground clearance. After which other methods must be used to increase thrust.
At slower speeds the pitch will continue to increase until the RPM setting is reached. This has the effect of turning the prop past stall AOA of the prop, so it is wasting lots of power in drag to keep the rpm down. As you increase speed the planes speed with lower the AOA on the prop, and it will gain efficiency as it's AOA starts coming out of the stall regime.
Now once you have made the prop as long as possible with out striking the ground, you need to do something to absorb the power. The choices are add more blades, or add more area to the blades. Making the blades wider changes there aspect ratio and increases induced drag just like it does on a wing.
Adding Blades also decreases efficiency because of disruptive air flow from the previous blades. These effects decrease efficiency at high speeds, but increase the efficiency in the climb speed ranges.
But none of these choices and trade offs have an effect on how much thrust is produced in the lift direction, that is simple trigonometry. Also when doing lift calcs you must also take climb angle into account for required lift. If a plane is traveling straight up or straight down zero lift is required. So the total lift required (including thrust from the prop in the lift dirction) for non turning flight is the cos(ClimbAngle) * Weight.
HiTech
-
On all, possibly almost all, WWII fighter had constant speeds prop. This works simply by increasing or decreasing blade angle to maintain the same RPM at almost all flying speeds until the prop is mechanically unable to increase or decrease pitch. In speeds from stalls to well above max level speed the prop will maintain the same RPM.
HiTech,
I'm guessing this means at lower velocities, the prop must have its prop pitch at less of an angle to keep the RPMs up, thus its 'bite' is less. This would imply that at slower speeds the prop isn't pushing as much air as at higher speeds, thus there is an optimal speed at which you will get the most thrust out of your engine. Is this assumption correct?
Thanks
-
I'm not sure. Shouldn't it be the other way around?
Lower RPM = sharper "bite" to slow down the prop with more work/friction?
Higher RPM = shallower "bite" to make less work and thus the prop spins faster with less resistance?
Again, I'm not sure.
-
I'm not sure. Shouldn't it be the other way around?
Lower RPM = sharper "bite" to slow down the prop with more work/friction?
Higher RPM = shallower "bite" to make less work and thus the prop spins faster with less resistance?
Again, I'm not sure.
When I meant speed, I was talking about the velocity of the airplane, because as the aircraft flys faster, the sharper the bite the prop can be while maintaining the same RPM.
-
I'm not sure. Shouldn't it be the other way around?
Lower RPM = sharper "bite" to slow down the prop with more work/friction?
Higher RPM = shallower "bite" to make less work and thus the prop spins faster with less resistance?
Again, I'm not sure.
Yes and no crusty, depends on your definition of bite.
As the plane moves faster the angle relative to the prop hub increases, but relative to the air flow (AOA) it decreases.
Ardy see above, I am assuming byte means AOA, hence slower is more .
HiTech
-
Ardy see above, I am assuming byte means AOA, hence slower is more .
HiTech
'Slower' = Slower RPM I'm assuming, so when wep is hit and the prop is at max RPM, the prop is also at max AoA, hence why you can't change the RPMs with wep on?
Also, because as you increase the AoA, the prop spins slower, more 'bite' doesn't necessarily mean more thrust correct? I'm guessing that the relationship between prop speed and prop AoA in regards to thrust is non-linear. Is that correct?
-
No ardy slower = the planes speed. I was not referring to the effect of RPM settings. Ardy you never directly control the pitch of the prop. You only set the speed of the governor.
For any given RPM setting as the plane speeds up or slows down the prop rpm stays the same.
As the plane flys faster the prop angle increases, but the prop AOA decreases.
HiTech
-
I think this paragraph explains it clearly in relation to climbing and diving;
http://www.pilotoutlook.com/airplane_flying/constant_speed_propeller (http://www.pilotoutlook.com/airplane_flying/constant_speed_propeller)
"When an airplane is nosed up into a climb from level flight, the engine will tend to slow down. Since the governor is sensitive to small changes in engine r.p.m., it will decrease the blade angle just enough to keep the engine speed from falling off. If the airplane is nosed down into a dive, the governor will increase the blade angle enough to prevent the engine from overspeeding. This allows the engine to maintain a constant r.p.m., and thus maintain the power output. Changes in airspeed and power can be obtained by changing r.p.m. at a constant manifold pressure; by changing the manifold pressure at a constant r.p.m.; or by changing both r.p.m. and manifold pressure. Thus the constant-speed propeller makes it possible to obtain an infinite number of power settings."
-
I think this paragraph explains it clearly in relation to climbing and diving;
http://www.pilotoutlook.com/airplane_flying/constant_speed_propeller (http://www.pilotoutlook.com/airplane_flying/constant_speed_propeller)
"When an airplane is nosed up into a climb from level flight, the engine will tend to slow down. Since the governor is sensitive to small changes in engine r.p.m., it will decrease the blade angle just enough to keep the engine speed from falling off. If the airplane is nosed down into a dive, the governor will increase the blade angle enough to prevent the engine from overspeeding. This allows the engine to maintain a constant r.p.m., and thus maintain the power output. Changes in airspeed and power can be obtained by changing r.p.m. at a constant manifold pressure; by changing the manifold pressure at a constant r.p.m.; or by changing both r.p.m. and manifold pressure. Thus the constant-speed propeller makes it possible to obtain an infinite number of power settings."
So the Governor also controls the throttle if the selected RPM cannot be reached by merely adjusting the prop pitch?
-
Nope. It's a very simple device that only controls prop pitch via oil pressure or centrifugal weights. One possible exception is the Fw 190 which had some sort of mechanical computer that controlled engine management.
-
Nope. It's a very simple device that only controls prop pitch.
I'm assuming the governor does not have a full 90 degrees rotation on the prop blade. If thats the case you could easily bust your engine by setting the RPM to low, or diving to fast with a high RPM setting and overspeed your engine.
-
I would think the pitch limits varies from design to design. Most modern props can feather the blades and so could many WWII designs on multi-engine planes.
(http://webzoom.freewebs.com/458bg/ALC_LastCardLouis.jpg)
-
The governor only controls engine RPM. So once you get to the limit of propeller pitch, then the RPM will change due to the flight condition.
-
Yup. You see that in AH on some fighters if you dive them too fast the engine RPM increases.
-
Yup. You see that in AH on some fighters if you dive them too fast the engine RPM increases.
but no damage happens...nor if you set the RPM too low. Is that then just a FM/modeling issue?
-
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.
Hi Stoney,
Just to give you an idea of the difference it makes in a maximum sustained turn, I have calculated the sustained turn rate and radius under two sets of conditions. Firstly, in order to ignore the contribution that thrust makes to lift I assume that the thrust vector points in the direction the aircraft is traveling, so that all of the thrust is available to pull the aircraft around the circle, and none of it contributes to lift. Then I calculate the sustained turn rate and radius again, only this time correctly allowing for the components of thrust. That means some of the thrust assists the turn, but that less is available to pull the aircraft around the circle. The two effects work to cancel each out. Let's run the numbers for the following fictitious aircraft.
Weight: 8000lbs
Wing Area: 200ft^2
Engine: 1600HP
Clmax: 1.4
That's not all the data needed, but it give you an idea of the aircraft configuration.
In the first case, the sustained turn rate = 20dps and the radius = 781ft
In the second case, at the maximum AoA of 15 degrees the contribution of thrust to lift is 623lbs.
In that case, the sustained turn rate = 19.9dps and the radius = 763ft
So, the sustained turn rate actually gets worse by about 0.6% and the radius improves by about 2%.
That makes sense because that 623lbs increases the G slightly which reduces the radius, but the fact that there is now less thrust pulling the aircraft around the circle reduces the sustained turn velocity, so the sustained turn rate drops slightly. But those differences are small, and I suggest not tactically significant.
Hope that helps...
Badboy
-
Thanks Badboy, good to know. :cheers:
-
Hi Stoney,
Just to give you an idea of the difference it makes in a maximum sustained turn, I have calculated the sustained turn rate and radius under two sets of conditions. Firstly, in order to ignore the contribution that thrust makes to lift I assume that the thrust vector points in the direction the aircraft is traveling, so that all of the thrust is available to pull the aircraft around the circle, and none of it contributes to lift. Then I calculate the sustained turn rate and radius again, only this time correctly allowing for the components of thrust. That means some of the thrust assists the turn, but that less is available to pull the aircraft around the circle. The two effects work to cancel each out. Let's run the numbers for the following fictitious aircraft.
Weight: 8000lbs
Wing Area: 200ft^2
Engine: 1600HP
Clmax: 1.4
That's not all the data needed, but it give you an idea of the aircraft configuration.
In the first case, the sustained turn rate = 20dps and the radius = 781ft
In the second case, at the maximum AoA of 15 degrees the contribution of thrust to lift is 623lbs.
In that case, the sustained turn rate = 19.9dps and the radius = 763ft
So, the sustained turn rate actually gets worse by about 0.6% and the radius improves by about 2%.
That makes sense because that 623lbs increases the G slightly which reduces the radius, but the fact that there is now less thrust pulling the aircraft around the circle reduces the sustained turn velocity, so the sustained turn rate drops slightly. But those differences are small, and I suggest not tactically significant.
Hope that helps...
Badboy
Excellent, thanks BB. I suppose its something that should be considered, but not significant for comparison, unless the rough comparison shows the two aircraft almost even. I'll just make sure to caveat any comparison going forward with "ignoring lift due to thrust". That'll have to accompany the "ignoring CG affects on lift" from Hitech's post in the other thread.
-
I would suggest googling 'propeller efficiency' for prop questions. There is a lot of physics and some fluid dynamics going on at one time.
-
I would suggest googling 'propeller efficiency' for prop questions. There is a lot of physics and some fluid dynamics going on at one time.
Thanks, but we're discussing lift due to thrust, not thrust itself. Propeller efficiency helps us know how much thrust is being generated. My question is how to determine how much lift is due to thrust.
-
It is fun discussing this with you stone, it shows all effects involved when wanting precision, but for plane comparison, most can be ignored unless the planes both fall out very very close.
HiTech
-
I'll just make sure to caveat any comparison going forward with "ignoring lift due to thrust". That'll have to accompany the "ignoring CG affects on lift" from Hitech's post in the other thread.
Just curious, it sounds as though you are doing this research for a project of some kind?
Badboy
-
Just curious, it sounds as though you are doing this research for a project of some kind?
Badboy
Well, my foray into serious aerodynamic studies began when I decided to start drawing up a concept Formula 1 racer for Reno. As that progressed, it dovetailed nicely into some of the stuff I was reading on the forums. The more I've learned about the game and aerodynamics, I find myself curious about an aspect of performance, and then start digging through my books and then testing stuff in-game to see how they compare or as a method to test certain theories I have. Right now, I'm basically putting together a table of performance characteristics of the aircraft in-game. For example, I'm assembling all of the aerodynamic properties of the planes--the wing spans, wing areas, aspect ratios, "e" approximations, K approximations, Clmaxs, Clis, etc. One of the biggest reasons I got interested in the Ps, EM diagrams, and so forth was to be able, at least on paper, to do some modeling and trade studies on how to optimize my F1 design. Luckily, I can use those same tables and formulas and apply them to the planes in the game as well.
-
Stoney wrote :Luckily, I can use those same tables and formulas and apply them to the planes in the game as well.
Wow , physics sometimes work with force and distance.
HiTech
-
My question is how to determine how much lift is due to thrust.
isnt all lift due to thrust.
thrust producing wind, wind producing lift, lift producing flight?
-
Kenne do you read the posts or just the titles?
-
Kenne do you read the posts or just the titles?
is that a yes or a no?
-
No, unless you consider thrust vectoring aircraft such as the AV8 or some other jets, thrust typically makes up very little lift. Almost all of the lift is created by the wing.