Author Topic: For Real Pilots  (Read 1828 times)

Offline Straiga

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For Real Pilots
« on: January 01, 2005, 07:34:47 PM »
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.

Offline spitfiremkv

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« Reply #1 on: January 01, 2005, 09:25:47 PM »
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.

Offline Straiga

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« Reply #2 on: January 01, 2005, 11:57:23 PM »
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
« Last Edit: January 02, 2005, 03:19:41 AM by Straiga »

Offline Straiga

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« Reply #3 on: January 02, 2005, 02:18:02 AM »
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

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
« Last Edit: January 02, 2005, 02:46:08 AM by Straiga »

Offline Cobra412

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« Reply #4 on: January 02, 2005, 02:29:47 PM »
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.

Quote
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.

Offline spitfiremkv

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« Reply #5 on: January 02, 2005, 02:32:49 PM »
Quote
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.

Offline hawker238

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« Reply #6 on: January 02, 2005, 02:37:36 PM »
Quote
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?

Offline Cobra412

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« Reply #7 on: January 02, 2005, 03:29:08 PM »
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.

Offline Casca

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« Reply #8 on: January 02, 2005, 04:13:42 PM »
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.
I'm Casca and I approved this message.

Offline Angus

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« Reply #9 on: January 02, 2005, 05:32:09 PM »
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?
It was very interesting to carry out the flight trials at Rechlin with the Spitfire and the Hurricane. Both types are very simple to fly compared to our aircraft, and childishly easy to take-off and land. (Werner Mölders)

Offline 33Vortex

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Re: For Real Pilots
« Reply #10 on: January 02, 2005, 08:27:23 PM »
Quote
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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Offline Badboy

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Re: For Real Pilots
« Reply #11 on: January 02, 2005, 11:08:57 PM »
Quote
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.

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Offline Straiga

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« Reply #12 on: January 03, 2005, 01:22:58 AM »
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.

Quote
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
« Last Edit: January 03, 2005, 05:08:40 AM by Straiga »

Offline Cobra412

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« Reply #13 on: January 03, 2005, 03:25:57 AM »
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.

Offline Straiga

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« Reply #14 on: January 03, 2005, 03:32:48 AM »
Cobra 412,
 BINGO you got! to counter Torque, roll, pitch, and yaw. Built into the airframe.