Propeller Effects

[washedup]

(see Bod's post way above)
FYI the lomcevak (correct spelling?) has many varieties.  It is basically a spin around different axes of the plane.  It is a departure from controlled flight that is only in part due to gyroscopic effects, the maneuver is mostly due to aerodynamic effects...



[This message has been edited by washedup (edited 12-10-2000).]

[Jigster]

 

Originally posted by funked:
"but they don't show the reactions we are expecting when violently moving the throttle."

Huh?  Get in a Spit 9 with no throttle damping and do some violent throttle motions on the runway.  The plane jerks to the left pretty suddenly.  Not sure what more you guys want.  I don't fly the other planes much so maybe the Spit is the only one that does it?

[This message has been edited by funked (edited 12-10-2000).]

Right, such as the Typhoon which has an enormous prop and alot of horsepower. It only ever so slightly rolls to the right when going from right above stall speed to full throttle.

On other planes though, it's very evident...the G10, La5, P-51, (among others) have a strong jerk tendency when you punch it.

I doubt it is across the board but some planes seem to have very little reaction to throttle inputs.

[wells]

The lomcevak is a transition between an upright snap roll and an inverted snap roll.  The elevator control is reversed 1/2 way through the maneuver when the plane still has lots of energy, before the upright snap is completed.  The momemtum of the rotation combined with the opposite control input makes for a nose over tail kindof tumble before the plane settles into an inverted spin.

[bod]

Wells, what you are describing is definitely not a lomcevak. A lomcevak is a controlled maneuver (controlled by throttle. ie gyro effect). I found a good description on the net. I got the book they refer to, and it really is excellent reading (practiced a bit in a C-152 Aerobat, not the lomcevak though  . Another maneuver that needs gyroscopic forces to be done right and *controlled* is the flat spin by the way.

from http://www.charlesriverrc.org/articles/flying/donstackhouse_whatisalomcovak.htm

 

>Can anyone describe what a Lomchevak is. I have heard it several times and don't know what it is? <

That's easy! It's simply one of the most misunderstood families of maneuvers in the history of aerobatics. As solid evidence of this, I offer ALL of the previous posts to this thread!

If you REALLY want to understand Lomcovaks, find a copy of the book "Aerobatics" by the British champion aerobatics pilot Neil Williams, pub. by Airlife Publications, ISBN # 0 9504543 0 3 . Turn to page 189, the beginning of Chapter 22, "What is a lomcovak?" and begin reading. After you finish, go back to the beginning and read the entire book, you'll be glad you did. This is simply the best book on the general subject of aerobatics I've ever read!

A Lomcovak is NOT an inverted spin. It is NOT a non-precision maneuver. The pilot is NOT "just along for the ride". Recovery does NOT just occur at random without any control by the pilot. It is NOT a single maneuver, but rather an entire family of maneuvers, all of which are very precise and controlled from beginning to end IF they are properly done.

The key element in a Lomcovak is that the airplane's attitude and motion is controlled by four primary flight controls rather than the usual three. The fourth attitude control in this case is gyroscopic precession from the prop, controlled via the throttle. This is why it's extremely difficult (that's spelled i-m-p-o-s-s-i-b-l-e) to do a true Lomcovak with a model; in most cases our props simply aren't heavy enough to provide sufficent precession forces.

There are five main types of Lomcovaks. There are also variations within each type. All are performed under negative "G".

The one most folks are familiar with is the "Main" Lomcovak. This begins from an inverted climb. As airspeed decays to near zero, the pilot initiates something initially resembling an inverted snap roll, so that the aircraft has a rotation rate about the pitch, yaw and roll axes as it reaches zero speed. The aircraft performs three foward tumbles, each one at 45 degrees to the plane of the previous tumble. At the end of the third tumble, the aircraft recovers into a vertical dive.

The "Cap" Lomcovak begins like a hammerhead, but as the airplane rotates to the halfway point, with the fuselage horizontal and the wing vertical, a combination of gyroscopic precession (caused by the yawing rotation from the vertical entry) and down elevator cuases the airplane to perform a single forward tumble, with the plane of the tumble horizontal. As the plane returns to its initial position, throttle is closed and the aircraft yaws the rest of the way to a vertical downline for recovery. I haven't done a complete true Lomcovak myself, but I once did part of a positive "G" variation of this by accident in a DeHavilland Chipmunk, which is how I discovered that Chipmunks do not like to do hammerheads to the left! It's a really weird feeling the first time.

The "Positive Conic" and "Negative Conic" Lomcovaks describe a cone shape in the sky, with the airplane pointed upwards as it sweeps out the cone shape with its underside. The point of the cone is at the prop for the positive conic, and at the tail for the negative conic.

Finally, there is a version resembling the "Main" Lomcovak, but entered from knife edge rather than inverted flight. This one is particularly violent.

The pilot is near the center of rotation for most Lomcovaks. These maneuvers are very disorienting, but not generally too stressful in terms of "G" forces on the pilot. However, since Lomcovaks use the precession forces from the prop as one of the flight controls, as you might imagine, the forces on the prop, crankshaft, engine mounts and engine are extremely severe. The centrifugal forces on things like wing panels can also be surprisingly high, and usually totally different from what the engineers were thinking when the airframe was designed. It is prohibited in a number of aircraft, and results in severe life limits on the rotating components in a number of other aircraft.

Shortly after the maneuvers were invented by the Czechs, some of the top Russian pilots started trying them in their Yak 18's. Shortly after that, there were a series of prop, crankshaft and engine fractures on Yak 18's, including one where the entire engine was yanked off of the firewall by its roots! Right after that, the word went out from "upstairs" to the members of the Russian aerobatic team that anyone caught doing Lomcovaks in a Yak could expect his address to be changed to a gulag in Siberia IMMEDIATELY.

There are a number of other manuevers that also impose severe stress on props and engines. Snap rolls and flat spins are some of the worst. However, nothing can break a crankshaft quite so well as a good Lomcovak. One project I was involved in during my previous career (before I quit to go into the R/C model business full-time) was assisting development of a Kevlar-bladed prop for aerobatic aircraft that would have the strength to tolerate this sort of abuse, and low enough inertia to protect the engine as well. It was well received by the aerobatics enthusiasts, a number of whom had already been through the harrowing experience of landing an already sensitive and tailheavy aerobatic aircraft deadstick, with a few score pounds of aluminum missing from the nose.

[Lephturn]

Interesting stuff.

Nobody has responded to my post about engine controls in AH.

When we "firewall the throttle" in Aces High, aren't we just changing the manifold pressure?  If so, why would there be any extra torque?  I wouldn't expect much of a reaction unless we quickly changed the RPM of the engine itself.  I don't think we do this when we advance the throttle in AH.  Somebody please correct me if I'm wrong.  Also does this vary from plane to plane?  I don't know.

------------------
Lephturn - Chief Trainer
A member of The Flying Pigs  http://www.flyingpigs.com
 
"A pig is a jolly companion, Boar, sow, barrow, or gilt --
A pig is a pal, who'll boost your morale, Though mountains may topple and tilt.
When they've blackballed, bamboozled, and burned you, When they've turned on you, Tory and Whig,
Though you may be thrown over by Tabby and Rover, You'll never go wrong with a pig, a pig,
You'll never go wrong with a pig!" -- Thomas Pynchon, "Gravity's Rainbow"

[flakbait]

From a logical stand point:

You can not fix engine RPMs and vary prop RPMs with a large change in speed. EG: the aircraft idles at 900 engine RPMs and stays there. If you could do this, you'd have aircraft that burned 5 GPH and ran over everything at speeds near 400mph. So, in order to increase speed you MUST increase engine RPM. Manifold pressure is one indication of changing engine speed.

Leph, every aircraft type will be different. Each engine can only pull so many inches of MAP, and props rotate at an individual speed. This means each aircraft will have it's own vices. Stomping the throttle in a 109 should flip you over. Do the same thing in a Jug and you'll probably do a good wing wobble. Weight, design, construction materials, engine speed, prop speed; each is different in every type of aircraft.

I don't know how or what HTC did to FUBAR prop RPMs. Dialing a Hog back to 35" MAP and 2,150 prop RPM should drastically increase range. It doesn't. Funked has proved fuel flow is too high at cruise settings in another post. Prop RPM is like an old Star Trek set joke; GNDN. Meaning it Goes Nowhere, Does Nothing. Torque from the engine, vortex effects etc... should literally give you whiplash if you stomp the gas in anything near stall. It doesn't.

Regardless what HT, Pyro, and GOD HIMSELF says, these effects have either: A) been SNAFUed or B) has a glitch that needs fixing. Either way, it's not correct right now. F4U takeoffs are a sinch, the 109 doesn't bite your bellybutton when you floor it, and the N1K2 simply screams down the runway like a jet. There is a problem, and it will get fixed.

Only question is, how long it'll take.


If I seem a bit peeved, it's because I miss the 1.03 effects. Something nice about watch some wanna-be hot rod slam into the tower on takeoff.

-----------------------
Flakbait
Delta 6's Flight School
"For yay did the sky darken, and split open and spew forth fire, and
through the smoke rode the Four Wurgers of the Apocalypse.
And on their canopies was tattooed the number of the Beast, and the
number was 190." Jedi, Verse Five, Capter Two, The Book of Dweeb
   

[This message has been edited by flakbait (edited 12-11-2000).]

[Lephturn]

Hehehe, I understand your motivation flakbait.  

All I am pointing out is that we have very simplified engine controls.  When you talk about "stomping the throttle" in real life, this likely meant revving the RPM's up quickly.  In AH, I don't think it works that way.  I'll have to check tonight at home, but it seems to me our throttle pretty much only works the MAP, not RPM.  All I'm saying is that you basically can't "slam the throttle" in AH in the same way, so we can't compare the two.

The thing is, do we ask Pyro to look at the forces themselves, or at how the engine controls are modelled?  They are two very different things.  I'll do some checking tonight at home and see what I find, but I suspect the control has something to do with it.

------------------
Lephturn - Chief Trainer
A member of The Flying Pigs  http://www.flyingpigs.com
 
"A pig is a jolly companion, Boar, sow, barrow, or gilt --
A pig is a pal, who'll boost your morale, Though mountains may topple and tilt.
When they've blackballed, bamboozled, and burned you, When they've turned on you, Tory and Whig,
Though you may be thrown over by Tabby and Rover, You'll never go wrong with a pig, a pig,
You'll never go wrong with a pig!" -- Thomas Pynchon, "Gravity's Rainbow"

[bod]

 

When we "firewall the throttle" in Aces High, aren't we just changing the manifold pressure? If so, why would there be any extra torque?

All the AH planes have constant speed propellers. This means that an automatic  controller in the hub (in most cases) adjust the aoa, or pitch of the blades so that the RPM are constant regardless of engine power. If you increase throttle, the RPM wants to encrease as well, but when the controller feels this initial increase the pitch is adjusted so that the propeller grabs more air - the aoa on each blade is increased. With increased aoa the drag (torque) and lift (thrust) on each blade is increased so that the RPM is helt constant. When decreasing the throttle the controller will automatically decrease pitch. Very much like an automatic transmission in a car, but with infinitely many shifts.

Basicly this means that with the pitch lever, the pilot sets the RPM that the controller shall maintain, and with the throttle he sets the HP (thrust and torque). With a set RPM any change in throttle will change the torque.

With a set throttle a decrease in RPM will increase torque, but also decrease HP, so the net effect is slightly more torque and less thrust. Increae in RPM will do the opposite.

IMO the engine management is modelled correctly, with exceptions of the late war LW planes who all had Kommandogerät (everything is fully automatic with only one main lever to adjust, i think). The gyroscopic forces seems to be way too low IMO, but then i have never flown a real warbird.

[This message has been edited by bod (edited 12-11-2000).]

[wells]

Governors can only compensate so fast (about 2-3 degrees/sec or about 200 rpm/sec)  If you cob the throttle faster than the prop can compensate, you should see an increase in rpms as well, before they settle back down.

[MiG Eater]

Not only can you hear this in a constant speed airplane (re: Wells' comment above), you can feel it with a noticable acceleration or deceleration.

The one thing that AH constant speed prop systems definatly does not do is increase manifold pressure when RPM's are reduced.  You must reduce throttle before reducing RPM's with a real constant speed prop driven aircraft.  Conversely, RPM's are increased before increasing the throttle setting.  Reducing RPM's while keeping the throttle full forward can reduce engine life to a matter of seconds as internal engine pressures exceed the metal's ability to contain them.  (I saw the results when this happened to a P-51 in 1991.  Blew two holes through the engine casting when the connecting rods let loose.  The pilot landed dead stick as he was over the runway when it happened.)  

MiG

[Jigster]

As stated before, it's much like a transmission...say you start out in low gear with an automatic...

What will happen when you punch it? If it's got enough power the wheels are going to squal because there is more torque then is needed to set the car in motion. The transmission can't respond to engine speed changes instantly, and it does have limits on how much load it can put on the engine.

If it could instantly respond to engine RPM it would shift into second or third to supply enough load on the engine to keep the RPM's down and remove the excess torque, but that doesn't happen because of mechanical linkage. In contstant speed props the either the oil pressure or the electrical type hubs create a noticable delay between input of power untill the actual pitch change occurs (because the prop has to change speed first)

thus the delay in compensation, if I remember right. One has to change before the other.



[This message has been edited by Jigster (edited 12-11-2000).]

[bod]

MSCFS has a very good engine-model with seemingly correct RPM-manifold pressure-thrust relations which also take into account the dynamic behavior wells mentioned. Even the Komandogerät in the 190 is modelled.