Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Straiga on January 01, 2005, 07:34:47 PM
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You all know I have mentioned somethings I feal is wrong with the way the planes fly in AHII. I like to have your opions from real pilots, the ones with certificates in your pockets. Also give a brief discription of your flying experience and total time and what certificates you hold.
What happens when your tail gets shot off.
In AHII you will notice that you will pitch up when your tail gets shot off.
My opion this is wrong, Why! Well first I was A captian on a DHC-6-300 twin otter with 19 aboard and 250 Lbs in the nose and 500Lbs in the rear. The CG was well with limits. This one flight to the ditch (Grand Cayon) we Iced this thing bad. We were 30 miles out from the final approach fix and we had about five inch of ice on the gear struts. Torgue was at red line just to mantain altitude (11K) and thats when the tail stalled and slamed us forward into a 50 degree nose down attitude, the flight controls were slamed forward. both myself and FO grabbed the yoke and begain pulling back. After a lose 2,000 ft we regained control of the plane we also melted have the ice off. We landed safely and while unloaded the PAX we watched the rest of the ice fall off the plane.
In this situation the tail ice up and stalled pitching the nose over in a bad way. Now how about when you shoot of the tail and lose all the weight from the tail.
We all know that the horizontal stab creates lift downward to balance the plane in flight. So when you lose this balance the nose will pitch down, not up. CG does not matter, why because you lose the weight of the tail your CG moves way forward. So how the plane was loaded does'nt matter.
So whats your opion?
Later Straiga
CFII Airplane Single & Multi- Engine, Rotorcraft-Helicopter, Instrument
ATP Multi-Engine, Rotorcraft-Helicopter
Flight Engineer Jet
Over 15,000 hrs
More opions to come.
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without the tail, the airplane can pitch either way.
The CG can be either ahead or behind the neutral point of the wing, it doesn't really matter as long as it is ahead of the neutral point of the wing/horizontal tail system.
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So how come when you stall the horizontal stabilizer on any airplane the nose pitchs down this is a fact in all conventional aircraft.
When airplanes increase in airspeed you need more nose down elevator trim. Why because the tail down force increases with airspeed pitching the nose up. Also the main wing is creating more lift. So you need to trim nose down to counter this pitching force. If you lost the tail the nose would pitch down.
Normal conventional airplanes have the CG forward of the center of pressure. As airspeed increases the center of pressure (lift) moves aft. The tail section provides the balance to the airframe in a tail down force. During a normal stall the center of pressure of the main wing moves forward into the burble of the stall.
How much weight do you think a average WWII fighter tail section would weight? Probable about a 1000 Lbs its just a guess. Now the tail section is part of the CG of the airframe is it not. So when you lose this weight because it got shot off what happens to the CG? It moves forward even more. This Big CG change pitch the nose forward. Aerodynamics at this point has gone bye bye.
I just renewed My CFII and there were about 150 flight instructors there doing the same on this weekend. I ask several questions, about which all came from AHII aerodynamics. This question though about the tail section being shot off, came back with several answers. All the instructors agreed that the nose would pitch down not up.
Can you see my point
Later Straiga
http://www.lerc.nasa.gov/www/k-12/airplane/trim.html
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Constant speed props. In AHII if you set the prop RPM to set rpm setting say 2400 and dived to increase speed from 100 to 300 mph, the prop rpm will speed up. This should not happen. it should stay at 2400 RPMhttp://www.allstar.fiu.edu/aero/flight63.htm (http://www.allstar.fiu.edu/aero/flight63.htm)
I also have been talking about countering torque roll with rudder and airplane rigging. Read this.
http://www.allstar.fiu.edu/aero/flight13.htm
Straiga
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In order to counter torque roll with control surfaces wouldn't you have to monitor the torque being produced at all times? I'd have to do some research but doesn't torque change not only from throttle position but also from your current altitude?
It would end up being an auto trim function in the end. Rudimentary control linkage between the throttles and the rudder assembly or a computerized monitor that will adjust a electrical actuator. If infact your torque output changes with altitude then you'd have to calculate in those changes in some way. To do this purely by a mechanical connection isn't feasible.
Quote from another aeronautical site.
The atmospheric pressure, temperature and humidity all affect the density of the air. On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the amount of oxygen available for combustion and therefore reduces the engine horsepower and torque. For tweaking the fuel/air mixture, the air density is the most important consideration
With this in mind in order to have an efficient "auto trim" you have to take these into consideration. In the end though all you'll be doing is creating a semi-control augmentation system (CAS) much like modern fighters but directed solely at engine torque issues.
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Originally posted by Straiga
So how come when you stall the horizontal stabilizer on any airplane the nose pitchs down this is a fact in all conventional aircraft.
Later Straiga
http://www.lerc.nasa.gov/www/k-12/airplane/trim.html
lol you stall the wing not the horizontal stabilizer. wing not producing any lift => nose falls down.
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Originally posted by Cobra412
With this in mind in order to have an efficient "auto trim" you have to take these into consideration. In the end though all you'll be doing is creating a semi-control augmentation system (CAS) much like modern fighters but directed solely at engine torque issues.
This may not be true, but wouldn't you only have to take into account just the net pressure? Since the pressure on a low/cold day would equal the pressure on a high/hot day (in theory), wouldn't the torque be affected the same by the pressure?
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Hawker you could do it purely off of pressure. You still have to have to compute this input and make your trim system output accordingly.
I think you would also have to calculate in your airspeed. Since it takes more deflection of a surface at higher altitudes to get the same input as at a lower altitude. As your speed increases though it takes less input to get an output.
This is how our flight control system works on the F-15. It monitors both static and impact pressures to ensure proper output at any speed or altitude. The flight control input to output are constantly being ratio controlled by not only a computerized system but also a mechanical system. We can put a TTU 205 test set (pressure tester) on the airfame, sea level output for altitude and increase its speed output and watch as the flight controls drive to compensate for the higher speeds. We also ratio control our input to output on the rudders also.
The system that Straiga is wanting to design would have to calculate in torque output depending on current altitude. It would also have calculate in surface deflection needed depending on altitude and airspeed. Mix these two ratios to ensure the proper amount of surface deflection is given depending on torque, altitude and airspeed. You would counter the torque issue but in the end your still going to have less maneuvering performance because you've used your control surfaces as the primary counter to the torque.
This isn't a bad idea but I don't see the market jumping on the idea because of the cost in comparison to a basic electrical motor trim system. It wouldn't be a slap in device and every airframe would have a different database that the system would have to go off of due to each airframes design and powerplant.
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OK. My 2 cents. I believe Straiga is correct in his analysis. Loss of the horizontal tailplane would result in a nose down pitching moment.
All conventional aircraft are designed to fly with the CG in front of the CL. Moving the CG to the rear results in increased efficiency but ultimately degrades longitudinal stablility and introduces problems with stall-spin recovery.
It is quite possible for the horizontal stabilizer to stall. All you have to do is exceed the critical angle of attack for the airfoil. This is sometimes seen in icing situations. Take a look at a Cessna Cardinal RG the next time you are at the airport. Several year models have slots on the stabilator that are installed upside down (that is oriented to augment aoa in the down force direction) because the stabilator was stalling in the flare and pranging the nose gear during original flight test.
Apart from the CG conditions, all cambered (asymetrical) airfoils generate a nose down moment in addition to producing lift.
If you examine real life accidents where the tail has come off the airplane at cruise speed or better, the normal sequence is a rapid pitch down followed by wing failure in a downward direction.
As far as the AH flight model goes, I don't think its a big deal. These guys have done a super job and having a computer airplane performa EXACTLY as a real airplane in all flight conditions is just not that important to me.
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Hmmm, interesting.
I wonder really how it should go. I belive this has actually been mentioned before.
Ok, if you were moving downwards and lose your tail, you should pitch on your nose very quickly right?
And if you were heading upwards,you might pitch upwards?
In AH you have the engines running full speed. So a contra-rot twin would theoretically leave you hanging like a helicopter.
Maybe HT will pop in and inform?
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Originally posted by Straiga
We all know that the horizontal stab creates lift downward to balance the plane in flight.
That may be true for airliners and cargo planes, not necessarily true for fighters. Most fighters would fly with a negative lift on the stab to trim out a slightly-forward CG while carrying ordnance or fuel.
My guess would be that if u shot off the tail of a plane it could go either way, no way of telling which way really without computer calculations of lab conditions (= there's no way to tell). It'd be like throwing a slice of bread up in the air u wouldn't know which side it'd land on.
As far as I'm concerned it doesn't matter how the plane handle without a tail section, because it sure as hell won't fly anyway. I'd be more concerned about my chute at that point. :D
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33Vortex
Certified nutcase
Hundreds of hours flying R/C planes (not logged), building and flying own experimental designs, some with rockets/bombs etc.
One national title 'Fun Fly' 1994 (only time I ever competed).
Will get PPL when I get around to it, watch yer six!
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Originally posted by Straiga
What happens when your tail gets shot off.
Here is what happens when the tail goes away…
There you are flying along, minding your own business, in level flight at constant speed. At this point all the forces on your aircraft are balanced. The prop’ thrust is balanced with drag, the lift with weight and so on. All the moments are balanced too, the pitching yawing and rolling moments are all in equilibrium. The nose down pitching moment caused by the weight and lift couple is balanced at the tail, and so on.
Now, since the question only concerns what direction the nose points when the tail goes away, let’s just think about those pitching moments. Most folk think like this… During flight the weight and lift couple was trying to rotate the nose downwards, and the tail was preventing this from happening. So, if we remove the tail, there will be nothing to prevent that rotation, and the nose will drop… but that’s not what happens. The flaw in that reasoning is that it overlooks the fact that the nose down pitching moment that existed during controlled flight, also goes away with the tail. You see, the tail wasn’t only responsible for the balancing moment, it was also indirectly responsible for the lift that was produced the nose down pitching moment in the first place. But the wings are only producing lift when they are forced to do so by the control surfaces at the tail. Those surfaces (using a small force but long lever arm) rotate the wings against the airflow, forcing the wings to fly at an angle to the free air stream, thereby causing downwash, and thus lift. When the elevators go away with the tail, the wings will initially begin to move upwards, due to the lift already there, but they won’t go far because as they move the lift decays rapidly until the wings weather vane, and no longer produce any lift. That all happens in just a few degrees, so when the tail goes away, the nose might move down slightly initially, but only momentarily, because now there is only an engine, with the wings and forward/mid fuselage acting as little more than dead weight that simply wants to fall downwards, with a propeller attached to it that is still producing thrust.
Now, all you really need to consider at this point is how a heavy lump of metal with a propeller attached to it would fall. I think most people can see intuitively, that the heavy lump would fall first, dragging the propeller behind it. An admittedly weak analogy would be the stable condition that arises with a man hanging beneath a parachute. The aircraft falls, dragging the prop behind it, and falls more slowly because the prop is producing thrust and slowing it down.
That's exactly what happens in Aces High.
Badboy
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Badboy thats the problem AHII doesnt fly like a real airplane.
If you go to the bottom and read THE TAIL STALL or any of the others it talks about tail stall which is the same problem when you lose the tail. Except when you lose the tail you lose a lot of weight, also which dramatically alters you CG. With the CG forward of the center of pressure and the resultant downwash from behind the wings pushes the nose down. Once the nose points down its staying down.
When lowering flaps the nose pitches down, because the center of pressure moves further aft.
Like I said also I posed this question to over 150 flight instructors
who all agree with this. Plus Nasa has done extensive research and has open a branch know as Icebox to study more on tail plane Icing. When you stall the horizontal stabilator on any conventional airplane the nose pitches down drastically. To the point were the flight controls get pull out of yours hands its that drastic. As airspeed and power are increased the the aircraft will stall sooner.
If you have bombs hanging CG is forward of the center of pressure. The nose will pitch down not up.
That may be true for airliners and cargo planes, not necessarily true for fighters. Most fighters would fly with a negative lift on the stab to trim out a slightly-forward CG while carrying ordnance or fuel.
Airliners fly the same way WWII fighters fly it the same princyple.
Main wing produces lift upward and the horizontal stab produces lift downward to balance forces in the airplane.
So why do we trim nose down as speed increase?
Casca, you got it right.
Sorry spitfiremkv keep reading you will see the light.
Cobra412, What Im talking about in cruise flight aircraft designers have rigged the airplane to fly true in equalibrium in unexcelerated flight. With little or no trim required. This is know as a bias in the airframe. The different airplanes I have flown with the airlines will have a bias as how the airframe flys and how to set the trim in cruise to fly true. Im not talking about the control surfaces, Im talking about rigging the airframe ie. vertical stab, taller vertical stabs, horizontal stab, main wing incidences, motor mounts, clip one wing shorter then the other, differencial radiators mounted on the wing and so on. Remember No computers involved.
http://www.av8n.com/how/htm/roll.html
Torque Effects
http://www.onlineaviation.org/my%20documents/aeropage.htm
Some inventions rigged to counter prop torque
http://aerodyn.org/Propulsion/propeller.html
And you have got to read all of this
http://www.avweb.com/news/columns/182097-1.html
This is good reading to.
http://twinbeech18.com/servicenotes/sn_elevator_droop.htm
Spitfiremkv this ones for you. (THE TAIL STALL) And dont listen to any documentary film makers they are clueless too!
http://www.ipilot.com/forum/message.aspx?pid=22431
Straiga
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Straiga when you say "rigging" it means something completely different to me or so it seems. The F-15 needs little to no trim also. Granted I'm assuming your speaking of propellor driven opposed to turbine driven.
Rigging to me is actually turning out a locknut and turning out the actual control rod itself for the control surfaces. From what I'm getting now your talking about actually building the airframe itself so it opposes yawing when in unaccelerated flight.
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Cobra 412,
BINGO you got! to counter Torque, roll, pitch, and yaw. Built into the airframe.
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Originally posted by Straiga
Airliners fly the same way WWII fighters fly it the same princyple.
Main wing produces lift upward and the horizontal stab produces lift downward to balance forces in the airplane.
I know that, but think I misinterpreted the term 'lift downward', the correct term would be 'negative lift'. If you look at the design of some bombers, and heavy lifters, they have a tailplane that produce lift to carry the weight of the tail. That is not the case with fast, maneuverable fighters.
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My understanding is that if you lose the tail, you're going to lose the downward lift ("negative lift" is an oxymoron) and the nose will drop. Past that, my "theory" would be a torque roll. Given enough altitude you'd exceeed Vne, where parts of the plane might begin to depart the airframe.
My understanding of "rigging" was the same as Cobra412s. I've only heard it in reference to tweaking bolts, cables, etc. before now.
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Vortex33,
Since I have been flying airplanes civialian and military, and I have even heard Nasa discribe horzontal tail plane lift. The horizontal tail plane is an inverted wing. The main wing or horizontal stabiliator dosent know its upside down or not, they produces lift do they not.
As for heavy airplanes the tail produces lift downward as the same for fighters, an airplane is an airplane. The DC -10-30 I flew with a 500,000 Lbs takeoff weight, created lift downward on the horizontal stab it has an inverted wing(horzontal stab) like all other airplanes. The C-5B is the same way as in a B-747, B-52, B-1B, P-47, Me-109, B-17.
If you live close to an air force base or major airport and and see a big airplane look close at the horizontal stab you can see the leading edge of the tail plane pointing down for trim takeoff. Even at the gate you can see the tail plane leading edge pointing down, a positive inverted angle of attack at 0 trim.
Negative lift or negative angle of attack means decreasing lift or decreasing angle on attack.
If I pick up a chair I would be lifting it. If I was hanging from the cieling and pick up the chair I would be lifting it.
Straiga
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Trimmable Stabilizer
For example, without the moving stabilizer, and with an elevator that moves 10 degrees up and down, if the pilot must hold five degrees of elevator position for level flight, then there is only five more degrees of elevator available in that direction. With the movable stabilizer, the elevator should always be faired when properly trimmed, so full travel is available in either direction.
http://www.avweb.com/news/columns/182097-1.html
Great thread. I think your absolutely right Straiga.
Did any other planes in WWII have a trimmable stabilizer?
Crumpp
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Originally posted by Crumpp
http://www.avweb.com/news/columns/182097-1.html
Great thread. I think your absolutely right Straiga.
Did any other planes in WWII have a trimmable stabilizer?
Crumpp
I'm no WW II airplane expert but the L-4A did (military variant of the J3 Cub) and it doesn't get much simpler than that. Doubt we will see it in AH. It would be a long flight with ten of them to take a base.
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Originally posted by Straiga
Vortex33,
Since I have been flying airplanes civialian and military, and I have even heard Nasa discribe horzontal tail plane lift. The horizontal tail plane is an inverted wing. The main wing or horizontal stabiliator dosent know its upside down or not, they produces lift do they not.
As for heavy airplanes the tail produces lift downward as the same for fighters, an airplane is an airplane. The DC -10-30 I flew with a 500,000 Lbs takeoff weight, created lift downward on the horizontal stab it has an inverted wing(horzontal stab) like all other airplanes. The C-5B is the same way as in a B-747, B-52, B-1B, P-47, Me-109, B-17.
If you live close to an air force base or major airport and and see a big airplane look close at the horizontal stab you can see the leading edge of the tail plane pointing down for trim takeoff. Even at the gate you can see the tail plane leading edge pointing down, a positive inverted angle of attack at 0 trim.
Negative lift or negative angle of attack means decreasing lift or decreasing angle on attack.
If I pick up a chair I would be lifting it. If I was hanging from the cieling and pick up the chair I would be lifting it.
Straiga
Doh, I'm not arguing with you, was just confused by the way you worded it. Lift is lift, negative or positive, usually referred to as lift or negative lift. You worded it differently and I interpreted it wrong. I don't blame you for a typo but if you look at some of the old bombers, the A-20 for example, the stab is built with the same, or close to the same, AoA as the main wing. Modern planes don't have that design. Like you say the trimmable tailplane is a common feature today.
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Originally posted by 33Vortex
... the A-20 for example, the stab is built with the same, or close to the same, AoA as the main wing. Modern planes don't have that design.
Now you've got me confused. Are you talking about the Angle of Incidence or Angle of Attack?
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Originally posted by Straiga
Constant speed props. In AHII if you set the prop RPM to set rpm setting say 2400 and dived to increase speed from 100 to 300 mph, the prop rpm will speed up. This should not happen. it should stay at 2400 RPMhttp://www.allstar.fiu.edu/aero/flight63.htm (http://www.allstar.fiu.edu/aero/flight63.htm)
I also have been talking about countering torque roll with rudder and airplane rigging. Read this.
http://www.allstar.fiu.edu/aero/flight13.htm
Straiga
Although I agree with you about the pitch-down vs pitch-up issue, I have experienced prop overspeed. It was in a Citabria that had been modified with a constant speed prop. Nosing over with the prop set at low RPM would, after a while, result in increasing prop RPM. I believe that it was caused by the engine being "driven" by the prop at maximum pitch. Once you hit the wall, where the prop can't adjust any more, the engine will unload and speed up.
I think.
shubie
<300 hours, all in C-172, C-182, C-177, t-6, and Citabrias, all VFR, all fun.
Right seat time in a King Air doesn't count, does it?
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Originally posted by jigsaw
Now you've got me confused. Are you talking about the Angle of Incidence or Angle of Attack?
Sorry, angle of incidence, relative the fuselage centerline.
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Originally posted by Cobra412
Straiga when you say "rigging" it means something completely different to me or so it seems. The F-15 needs little to no trim also. Granted I'm assuming your speaking of propellor driven opposed to turbine driven.
Rigging to me is actually turning out a locknut and turning out the actual control rod itself for the control surfaces. From what I'm getting now your talking about actually building the airframe itself so it opposes yawing when in unaccelerated flight.
Oh yes indeed they do! the vertical stabilizer on a prop single usually has an angle built into it to offset yaw at cruising speed. Only a couple of degrees, iirc, but it is there.
Twins don't have it, nor do turbojets or turbofans.
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There are a lot of questions in aviation that will probably never be completely resolved among pilots. One example is the downwind turn (my opinion is that it exists but is not a factor during normal operations). Another is whether it is better to wheel land or three point a conventional gear aircraft in a cross wind (my opinion: wheel land but it depends on the airplane). Threads on topics like these could probably yield a huge amount of eloquent and reasoned posts arguing the issue from both sides while a unanamous consensus remained elusive.
The subject of this thread is not one of these questions. It is settled theoretically and empirically. When the horizontal stabilizer departs a normally loaded conventional aircraft in the normal flight envelope it pitches down violently for reasons that have been stated previous posts. It is not even debatable. That is what happens. If the speed at which this occurs is appreciably above Va (manuvering speed) the loss of the horizontal stabilizer is frequently followed by in-flight breakup.
I spend a lot of time in AH looking at the sky floating down backwards. At that point I am usually thinking "I hope the b****rd flys down to look so I can nail him." (never happend yet, sigh). I never spend any time thinking "Ya know if this had been a real airplane it would'nt be exactly like this."
I think the flight modeling in this game is just incredible and merely see it as one of the little quirks of the game.
CFII, CFIG, AGI, A&P, IA
AS Aviation Maintenance
BS Aviation Technology
MS Aviation Safety
Adjunct Professor, Aviation (used to be Assistant Prof.) Central Missouri State University, Warrensburg MO
7000 hrs. Mostly in crop dusters.
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We all know that the horizontal stab creates lift downward to balance the plane in flight
This statement is incorect straiga. If you do a search it has been discused before. Planes can be perfectly stable with the horizontal stab producing up or down force.
Also your assumption that flaps always produce a nose down pitching moment is also incorect. They can produce a nose up or nose down pitching moment.
And finaly pilots are not the best people to ask this type of question .Most pilots only have a very fundemental understanding of stablity and control. And asumptions that they were taught for a basic understanding apply only in the "general case" but not always to specific cases.
HiTech
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From HiTech:
"Planes can be perfectly stable with the horizontal stab producing up or down force. "
So, its the Center of lift that decides which force you would need the stabilizer to do right?
Anyway, say your tail gets shot off in a 45 degrees upwards turn,- if the prop keeps running, what force is there to pitch you down?
However, losing the verical stab should lead you to rotate, and end up somewhere??
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Try this test of the nose pitch up. Buy a balsa wood glider. Assemble and fly note results. Then remove tail and fly again note results. Post back with findings.
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Anyway, say your tail gets shot off in a 45 degrees upwards turn,- if the prop keeps running, what force is there to pitch you down?
It all depends on the Wing CL combined with the tail CL, If after removing the lift from the tail, The CL of the remaining componets is ahead of the CG the plane will pich up. If it is Behind the CG it will pitch down. The force in either case is the lift generated by the wing.
HiTech
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I dont have measurable time in anything yet. Except ultralights. And while we may not have all the fun features and fancy guages of a full sized airplane, the fundamental aspects of flight are the same.
I can tell you as a witness exactly what happens when the tail comes off of an ultralight while it's flying. The lift produced by the wings sends the nose up, the rearend crashes into the ground, and the plane crumples and tumbles. Of course, this assumes forward motion to provide lift from the wings, I have no idea what would have happened had he been stalled when the accident happened.
An acquaintance of mine was bringing his bird in for a landing, and he didnt scope out things properly. Downdraft caught him, and as he tried to correct his tail smacked into a high tension powerline. It ripped off his stabilizers and most of his rudder.
Now I admit, being pushed around by that downdraft, he was probably hauling back like mad on the stick when the accident happened, that could have affected the outcome. Obviously, as I mentioned earlier, stall speed could have brought on a different reaction as well. I'm not a student of aeronautics. Matter of fact, I'm not a professional anything when it comes to flying. Its a hobby, and one I love. I'm not posting here to argue with any of your theories or ideas. Just providing some info from an accident I witnessed that fits the original question.
Thanks.
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Originally posted by icemaw
Try this test of the nose pitch up. Buy a balsa wood glider. Assemble and fly note results. Then remove tail and fly again note results. Post back with findings.
I was thinking something along those lines. See if I could find a cheap RC plane or someone with one. Rig a big firecracker to the empannage and see what happens.
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The ultra light situation is a special case. If we look at a representation of pressure distributions on a generic asymetrical airfoil under various conditions it can possibly illuminate the discussion.
The first picture illustrates pressures on various parts of the foil and is helpful in visualizing whence the negative pitch moment arises:
(http://pcspray.com/pics/pressdist.jpg)
As a matter of convenience we can resolve the forces acting on the wing as a single lift vector and negative moment around the aerodynamic center Moment is just another word for torque. A negative moment is counter clockwise and assumes that we are looking at the left side of the foil. This equates to pitch down:
(http://pcspray.com/pics/resultant.jpg)
In the following sequence we can watch the change in pressure distribution from zero lift (-3 degrees AoA for this foil)
(http://pcspray.com/pics/zerolift.jpg)
Through a typical cruise AoA:
(http://pcspray.com/pics/cruiselift.jpg)
To somewhere in the vicinity of CL Max:
(http://pcspray.com/pics/clmax.jpg)
Correction: The aerodynamic center is the theoretical point at which the wing responds to the moments acting upon it. It does not move with changes in AoA. The Center of Pressure is what is moving.
Although the Cp is not depicted you can see some things happening as we approach CL Max. One thing is that the Cp is moving forward. Under certain conditions it might be possible for the Cp to actually overtake the CG. Although we are looking at the wing in isolation this would be the Cp of the system.
It is therefore possible to lose the tail and have the aircraft pitch up. As HiTech said (and I agree) the CG merely has to be behind the Cp of the system.
If you do the balsa glider experiment it will pitch down if you throw it normally however if you keep launching it at progressivly higher angles of attack you can eventually get it to pitch up. Keep in mind that it would be hard to describe the airfoil on a balsa glider as asymetrical and you are entering the land of flat plate theory where I don't tread.
However, my earlier statement: "When the horizontal stabilizer departs a normally loaded conventional aircraft in the normal flight envelope it pitches down violently etc." is valid unless you consider angles of attack approaching Cl max as part of the normal flight envelope. You can't get the aircraft to anywhere near Cl max at speeds appreciably in excess of Va without waving goodbye to an important part of the aircraft structure. If you get the thing slow and greasy and yank back on the stick just before smacking a wire all bets are off.
I guess I flinch when HiTech says "Planes can be perfectly stable with the horizontal stab producing up or down force." While technically correct I can't think of an instance where there is not neutral to downforce at the tail in unaccelerated cruise flight. Flying an airplane around with the tail holding you up is possible but not a good idea.
Several years ago we were giving glider rides to Boy Scouts. I had several chunky scouts and the last scout of the day was a little tiny scout. I was too lazy to walk across to get the 25 pound ballast bar that was supposed to be installed for small people. The tail of the glider (SGS-233) would not come up during the takeoff roll and about the time I was reaching for the release to abort we became airborne. Once we had enough airspeed things were peachy it was in fact "perfectly stable". The landing was interesting in that as we bled speed in the flare I had to apply progressive forward stick and had it maxed out to the stop at touchdown. Needless to say that flying any slower would have resulted in an unrecoverable pitch up followed by a probably unrecoverable stall followed by waking up in a tube of light with the tiny scout and Elvis.
Apologies for the length of this post. I'm going to find something constructive to do with my time. Honest.
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Originally posted by Crumpp
Did any other planes in WWII have a trimmable stabilizer?
AFAIK the Fw190 and the Bf109... and maybe some other german plane.
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Originally posted by Casca
Under certain conditions it might be possible for the aerodynamic center to actually overtake the CG.
Yep, particularly if, due to a bug, the position of the CG doesn't move forward when the tail comes off, that might explain it.
Badboy
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Thanks for all the info guys.
But I think were missing the major point here.
In order to have a CG (Center of Gravity)we need have a complete airplane there. Now when the tail gets shot of you will no longer have a CG. But say we have a aft loaded CG and then we lose the enpennage, I cant amagine the CG being heavier than the tail section weight missing.
If the CG is aft on the CL the tail plane will not need to produce as much lift downward. But as airspeed changes so does the amount of lift the horizontal stabilizer produces. In either fact the tail is still providing balance. Remember the horizontal stabilizer is set on conventional airplanes to produce lift downward not upward.
Now remember that when the main wing is in a high angle of attack the tail plane is producing lift downward to get the main wing into a high angle of attack. So now shoot the tail off and watch the nose pitch down.
Airplanes like heaveys like have a little aft cg to keep the drag down from the tail plane. But the tail plane always produces lift downward. The amount of lift will very do to CG and airspeed,and angle of attack of the main wing.
The horzontal stab main function is to put the main wing into different angles of attack or balance is it not.
We all know that the center of pressure moves forward and aft along the main wing (for a given airspeed and angle of attack)so at any time CG can be forward or aft of the CP.
Most CG ranges are not all that big they are confind with in the fuselage.
We have two King Airs we fly the E90 pitches up its always aft CG loaded but the 200 it really pitchs down with a T Tail
The pitching up or down is dependent on airspeed, the amount of prop wash across the tail plane, CG location, and type flaps used, and wing plane form. Also is the gear down or up.
But for a general discussion shall we say an average airplane will pitch down with flaps.
Im not just a pilot I also have an aeronautical engineering degree. Pilots do get a limited knowledge, but pilots have the flight experience that others dont have. The more time you spend flying in different airplanes you learn from training how that airplane will fly. I think if you did not have a degree in aeronautics, pilots would have a bigger advantage about how airplanes fly even more so with high time pilots. Engineers have no idea how there airplanes will react until the test pilot tells them.
The horzontal stab main function is to put the main wing into different angles of attack or balance is it not.
Straiga
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AFAIK the Fw190 and the Bf109... and maybe some other german plane.
I know the FW190 does. Got the minutes of a meeting between Kurt Tanks design team and several of the JG pilots. In it they discuss the trim motor gearing. Thanks for the help. I was wondering if any other WWII fighters used it. Seems a good way to keep your elevator effectiveness.
Crumpp
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Originally posted by Straiga
In order to have a CG (Center of Gravity)we need have a complete airplane there. Now when the tail gets shot of you will no longer have a CG.
Just to clarify that point, the centre of gravity is a physical property of any body, it doesn’t matter how much of the aircraft is left behind it will always have a centre of gravity somewhere. The centre of gravity should move forward the instant the tail is lost.
Casca has only reminded us how the aerodynamic centre moves forward and he suggests that it might be possible for the aerodynamic centre to move in front of the centre of gravity causing a nose up pitching moment. I agree, but think that is generally unlikely, but I know it can happen to the P-51 under certain conditions. Even so, when the tail comes off, the centre of gravity should move forward instantly, keeping it forward of the aerodynamic centre and maintaining a nose down pitching moment.
I was just pointing out that if there was a bug in the flight model that meant the centre of gravity remained where it was even with the tail gone, that would cause the aerodynamic centre to be in front of the centre of gravity resulting in the nose up pitching moment we see. So I think that is a plausible explanation.
Just to temper this with reason, once the tail comes off, the fights over, the pilot’s dead… should we really worry too much about the contortions of the wreckage :)
Badboy
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We've done this at least twice already.
http://hitechcreations.com/forums/showthread.php?s=&threadid=95026&perpage=40
http://www.hitechcreations.com/forums/showthread.php?s=&postid=956394#post956394
http://hitechcreations.com/forums/showthread.php?s=&threadid=78917
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Originally posted by Straiga
Im not just a pilot I also have an aeronautical engineering degree.
Hey Straiga, wanna be my tutor when I get around to my advanced aerodynamics class?
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Well we put it to the a test.
Im here a flight safety international going through recurrent training. These are FAA approaved simulators here, some sims are boxes that sit on the ground and give you visuals and some are full motion sims.
We tested 3 sims one was a Gulfstream V a twin executive jet. One was a King Air 350 a twin engine low wing turbo prop with T tail. The other was a Pilatus PC-12 single engine turbo prop a low wing with T tail. Now what we did is that we told the computer to disregard the horzontal stabilator and elevator on these planes, like they were not even present. We set the planes in a cruise configuration and during the flight we moved the cg forward and backwards and then took the HS away (not the hole tail section) from the airplane to see what would happen.
The G-V, cruise Mach .89 at at 46,000 ft aft Cg limits. It pitched nose over to 55 degrees lost over 5000 ft in seconds.
G-V, mach .89 at 46,000 forward most CG, it pitched down to 120 degrees and broke up.
G-V short final 500 ft agl 140 kts in the landing configuration, (no slats, Full flaps) pitched over crashed in seconds.
B-350 cruise 20,000 ft. aft CG location 320 indicated nosed over after a 1/2 sec to 40 degrees then pitched straight down in a matter of seconds to mother earth.
Short final 500 agl 120 indicated landing configured, it pitched over nose down and crashed.
Pilatus PC-12 same thing.
3 different airplanes. Conclusion what we noticed was with an aft CG location it did not violently nose over but it still did.
It was explain to me by the sim engineer that when GC is aft it just reduces the amount of lift the HS (also less Drag) has to produce. But the HS still is produces lift downward, the HS would have a negative angle of attack or reduced lift due to the aft CG. The HS is nothing more than wing turn upside down. So trimming the elevator down would need to be used in this instance with an aft CG. Trimming the elevator down or up is going to increase or decrease lift on the HS.
So I asked, what if we took of the hole enpennage away what would happen. Everthing would happen even faster with out the mass of the tail section attached.
So this peak the interest of the sim engineer and he said he would check out AH and see what he thought of the aerodynamic modelling, who better than a sim man himself.
Jigsaw, do you really want me to tutor you I havent open a book in over 27 years.
Straiga
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Jigsaw: With Straiga as your tutor you would definatly fail.
HiTech
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Interesting post Straiga. The only results that really surprise me is finding out that the G-V is faster than the SR-71.
Just kidding. I know what you meant.
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Whoops it should be .89 mach sorry. I will change it.
Thanks Casca, some would not have figured is out anyway. LOL
I forgot more than some will ever know. 12 th in my class.
Look at it this way lets take a full loaded Me-109G and balance it on the head of a pin so its level this is the center of gravity. This could even be the aft CG limit. Then put the landing gear down and the CG moves forward and the nose points down. Now cut the enpennage off about about 4 ft. ahead of the vertical stabilizer now what happens the plane noses way over and way out of balance. Now how does this happen?
Straiga
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Flight simulators will never be perfect representations of how an airframe will react to any given situation. Perfect example is the F-15I that lost a whole right wing due to a mid air collision.
The designers of the airframe had tested that scenario out over and over in their simulator and said it was not possible at all. In ever test they tried on their simulator they could not replicate what the F-15I had done and swore it could never be done. That was until the Israelis produced the photos of the F-15I missing it's whole right wing. Just because it is tested on a "simulator" does not mean it is a 100% guarantee that it is correct.
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Engineers have no idea how there airplanes will react until the test pilot tells them.
I know what you mean. Just let the pilot prove what it can and can not do.
Straiga
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Another F15 (I think it was a Japanese pilot training with the USAF) flew into a line and cut a wing off. He landed it!!!!
Must have been an old Niki driver :D
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I've got a number of R/C airplanes laying around that are being neglected. I can see about running tests with them to test these options.
They're each JK Aerotech airplane. One 1/12 scale P-51 and a 1/12 scale Zero. I haven't finished the Zeke and the P-51 is only short radio gear which I can install with what I've got laying around. I can try to rig up a way to remove the horizontal stab (Currently they are "goop"ed on) but I'm not exactly sure how i'd do it. I can't figure out a way with a servo to remove a pin and i don't think a firecracker would be a good option. Any design ideas you can check the website at this location (http://www.jkaerotech.com) to see their design.
I'd take video of the flight tests as well and have plenty of wide open farmland out where I live.
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