Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: TheCrazyOrange on July 02, 2014, 12:46:35 AM
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So while playing today, there were a few guys on vox complaining about how the spitfires aileron's would reverse at high speeds in real life, and that they don't do so in AH (they said "like 450mph+).
Is there any truth to this; I've never heard a thing about it. If so, what is the physical mechanism by which this oddity of aerodynamics occurs? Is it universal to all ailerons (and I suppose all control surfaces, given that they're all just altering the flow over the airfoils), or is it a quirk particular to the Spitfires? Or does the spitfire's design just happen to have a particularly low speed at which this occurs, similar to the 109's compression? Is it modeled in AH? I've never had this happen to me.
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So while playing today, there were a few guys on vox complaining about how the spitfires aileron's would reverse at high speeds in real life, and that they don't do so in AH (they said "like 450mph+).
Is there any truth to this; I've never heard a thing about it. If so, what is the physical mechanism by which this oddity of aerodynamics occurs? Is it universal to all ailerons (and I suppose all control surfaces, given that they're all just altering the flow over the airfoils), or is it a quirk particular to the Spitfires? Or does the spitfire's design just happen to have a particularly low speed at which this occurs, similar to the 109's compression? Is it modeled in AH? I've never had this happen to me.
Wing-twist due to large amounts of lift at high speed is usually the culprit. And no, AH's flight model doesn't actually calculate physics at the level needed for it to occur naturally ingame.
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Just so I'm clear;
The forces generated by the control surface are such that the wing would physically deform so as to keep the aileron at "neutral" relative to the flight vector? Wouldn't forces acting on the entire wing be greater than those acting on just the ailerons, resulting in "heavy controls", thereby preventing this from happening?
It just seems odd to me that even though the forces are sufficient to significantly deform the shape of the wing, the ailerons are apparently unaffected by the forces.
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It is caused by a twisting of the wing itself making the rolling motion mushy at best and particularly noticeable at higher speeds, even in the game. Its most evident in the Spit I, VIII and XIV. I'm not sure why the full span V and IX don't suffer from this as well. I suspect it's mostly the I with the VIII and XIV being more affected by higher output engine torque effects.
Regardless, the twisting of the wings in the I was a well documented occurrence.
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Just so I'm clear;
The forces generated by the control surface are such that the wing would physically deform so as to keep the aileron at "neutral" relative to the flight vector? Wouldn't forces acting on the entire wing be greater than those acting on just the ailerons, resulting in "heavy controls", thereby preventing this from happening?
It just seems odd to me that even though the forces are sufficient to significantly deform the shape of the wing, the ailerons are apparently unaffected by the forces.
An aileron rolls the aircraft by effectively changing the camber of the wing. This, in turn, creates more lift on the wing where the aileron is deflected downward and less lift on the wing where the aileron is deflected upward.
At very, very high speed on aircraft that have low wing loading (i.e. they have wings that produce lots of lift) and low torsional wing stiffness, the aerodynamic forces can cause the wing to bend/warp. This warping causes the wing, overall, to deflect the relative wind opposite the direction it normally would, causing a roll in the oppositely-expected direction. For example, an aircraft usually rolls towards the wing with the upward-deflected aileron, as that wing is producing less lift. However, at high enough speeds, that upward-deflected aileron can generate enough drag that it "twists" the wing down, therefore causing the wing to deflect the relative wind downward. This causes the aircraft to roll away from that wing, instead of towards it.
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There is also a fair amount of evidence that the Jug suffered a reversal of elevator control in very high speed turns which also appears not to be mapped in game. (Smell of Kerosene) :old:
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We don't have aileron reversal because our wings don't flex under load.
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Was this permanently damaging to the aircraft? Like if a new pilot panicked and continued to try to roll, instead of cutting throttle, and trying to pull out of the dive, would the spitfire need to be overhauled when it got on the ground?
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Was this permanently damaging to the aircraft? Like if a new pilot panicked and continued to try to roll, instead of cutting throttle, and trying to pull out of the dive, would the spitfire need to be overhauled when it got on the ground?
You have Vno and Vne speeds in aircraft. I'm not sure what they were in Spitfires, but as long as Vne airspeed wasn't exceeded, no overhaul or inspection should have been necessary. That said, I would suspect that aileron reversal happened close to or past Vne.
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"Was this permanently damaging to the aircraft?"
AFAIK there was never permanent damage to Spittys caused by aileron reversal. Usually the damage to wings (in any plane) was done in high speed pull-ups where the upper skinning of the wing would wrinkle and the wing would more or less permanently deform.
-C+
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At high speed the force of the airflow hitting the aileron twists the whole wing creating a net lift opposite of what was commanded. Aileron reversal can also happen near the stall if the downward deflecting aileron increases the effective angle of attack beyond stall. The outer wing stalls and the aircraft rolls in the opposite direction of what was commanded. I have not experienced any type of control reversal in AH.
http://en.wikipedia.org/wiki/Control_reversal
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There is also a fair amount of evidence that the Jug suffered a reversal of elevator control in very high speed turns which also appears not to be mapped in game. (Smell of Kerosene) :old:
Yes, aileron reversal was definitely an issue with the Jug. Like you guys have been saying, the local pitching moment created by the deflected aileron would be severe enough at high speed to produce enough wing twist to change the effective angle of attack at the wingtips. This twist could be severe enough to negate the ailerons and in some cases even roll the airplane in the opposite direction. Thin wings, because of their low torsional stiffness, can have a real issue with this.
The coupling of aerodynamic forces and structural stiffness can also produce destructive aeroelastic behavior like flutter, which can rip a plane to pieces in short order.
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The shorter span ailerons on the Mk VIII and Mk XIV were intended to address, or reduce, aileron reversal. If you look at those two Spitfires in AH you can see that their ailerons don't extend as far out towards the wing tip as the ailerons on the Mk I, V, IX, XVI and Seafire Mk II.
In AH this seems to not be modeled and the shorter span ailerons simply produce a poorer roll rate.
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The shorter span ailerons on the Mk VIII and Mk XIV were intended to address, or reduce, aileron reversal. If you look at those two Spitfires in AH you can see that their ailerons don't extend as far out towards the wing tip as the ailerons on the Mk I, V, IX, XVI and Seafire Mk II.
In AH this seems to not be modeled and the shorter span ailerons simply produce a poorer roll rate.
Which is weird because the lower rotation inertia and stiffer wing structure of low aspect ratio wings usually give them pretty nasty roll rates. Consider the roll rates of low AR planes like the F-104 or T-38. Those things will crack your head on the canopy!
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Yes, aileron reversal was definitely an issue with the Jug. Like you guys have been saying, the local pitching moment created by the deflected aileron would be severe enough at high speed to produce enough wing twist to change the effective angle of attack at the wingtips. This twist could be severe enough to negate the ailerons and in some cases even roll the airplane in the opposite direction. Thin wings, because of their low torsional stiffness, can have a real issue with this.
The coupling of aerodynamic forces and structural stiffness can also produce destructive aeroelastic behavior like flutter, which can rip a plane to pieces in short order.
This was referenced specifically as elevator reversal, assuming that similar forces were at work here. The test pilot describes a scenario whereby a Jug pilot was in a Lufberry with an Fw190 at very high speed and was forced to reverse his control input to stay in the turn
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Consider the roll rates of low AR planes like the F-104 or T-38. Those things will crack your head on the canopy!
:) There's a lot of bent T38's out there. But the problems usually manifest themselves as 'cranky trim'. I suppose the best documented aileron reversal in the modern era is the F86, but that was traced to an improperly installed actuator.
Isn't the roll rate on a T38 something like 720 degrees per second?
:salute
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Isn't the roll rate on a T38 something like 720 degrees per second?
:salute
Yeah, it's pretty ridiculous. :uhoh
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This was referenced specifically as elevator reversal, assuming that similar forces were at work here. The test pilot describes a scenario whereby a Jug pilot was in a Lufberry with an Fw190 at very high speed and was forced to reverse his control input to stay in the turn
I found some flight test data for the P-47D. It says there were no control reversal issues during the pullout from a dive. There might be some other data that says otherwise though.
Flight Test Engineering Branch
Memo Report No. Eng-47-1774-A
15 July 1944
FLIGHT TESTS ON THE REPUBLIC
P-47D AIRPLANE, AAF NO. 42-26167
USING 44-1 FUEL
I Introduction
Flight tests have been conducted at Wright Field on the P-47D airplane, AAF NO. 42-26167, at the request of the Power Plant Laboratory, Engineering Division. These tests were made to determine the increased performance of the airplane using the higher powers allowable by use 44-1 fuel as compared with powers allowable with the standard fuel, grade 100/130, Spec. No. AN-F-28. From 15 April to 30 June 1944 approximately 30 hours were flown by Captain R. B. Johnston.
The P-47D is a single engine, high altitude figher. It is equipped with a Pratt & Whitney R-2800-63 engine furnished with a water injection system and a four-bladed Curtiss Electric controllable propeller, blade design No. 836-2C2-18.
II Summary
Preliminary tests were run to clear the airplane for performance with higher powers with and without water injection. Detonation equipment was installed to determine if any flight condition became marginal as to detonation, cooling or improper operation of auxiliary parts. No detonation was observed in level flight up to 65.0" Hg. without water and 70.0" with water. No detonation was observed in climb up to 65" Hg. without water. Detonation occurred at 65.0" with water in climb but was remedied by using a No. 18 water jet. Cylinder head and carburetor air temperatures remained below the limits in level flight. Excessive cylinder head and carburetor air temperatures were encountered in climbs, limiting the duration of any climb to a point where limits are reached.
The airplane and engine handled well at all altitudes at the higher powers. At 70.0" Hg., water injection, a maximum speed of 444 MPH was obtained at 23,200 feet. At 65.0" Hg., with water a high speed of 439 MPH at 25,200 feet and a maximum rate of climb of 3260 ft/min. at 10,000 feet were obtained. At 65.0" Hg., without water a high speed of 430 MPH at 25,400 feet and a maximum rate of climb of 2850 ft/min. at 12,000 feet were obtained. At 56.0" Hg. without water a high speed of 418 MPH at 29,600 feet and a maximum rate of climb of 2330 ft/min. at 12,000 feet were obtained. At 52.0" Hg. without water a high speed of 412 MPH at 31,400 feet and a maximum rate of climb of 2030 ft/min. at 12,000 feet were obtained.
III Condition of Aircraft Relative to Tests
A. Flight tests were conducted at a take-off gross weight of 13,230 lbs. with the c.g. at 29.9 MAC, wheels up. This weight corresponds to the full combat weight of the airplane and includes full internal fuel, 15 gallons of water and ballast for 300 rounds of ammunition per gun.
B. All tests were conducted with landing gear retracted and wing flaps neutral. In level flight the cowl flaps were closed and the oil and intercooler flaps neutral; in climb cowl flaps, oil intercooler flaps were wide open. Gun blast tubes and wing racks were installed and all antennae were in place.
C. The airplane was finished with standard, service camouflage finish.
IV Flight Characteristics
A. Taxiing and Ground Handling
This airplane is easy to taxi and handles well on the ground as compared to other fighter planes with conventional landing gear. The brakes are touchy for the first one or two times used but after this they are smooth and respond well without excessive pressure. The tail wheel is full swivel when unlocked and responds very easily. There is a handle on the right side of the cockpit to lock and unlock the tail wheel., When in the "lock" position the wheel locks when returned to the center for taxiing straight and for take-offs and landings. At times it is hard to unlock the tail wheel after landing but it unlocks if the cable leading back from the handle is "flipped".
Crosswind has very little effect on taxiing and ground handling. The tail wheel reacts well and the airplane handles well in rough ground landings.
B. Take-off
Handling for take-off is good. With tail wheel in "lock" position, the airplane rolls straight and torque is very easily corrected for by using rudder and a few degrees right rudder trim. Aileron and elevator trim tabs are set in the neutral psoition for take-off. At lower power settings the take-off roll is very long but is greatly decreased by high power. Take-offs were not measured but they seem very short with War Emergency power and improve the pilot's confidence in the airplane. After the airplane is airborne, only a small change in rudder and elevator trim tabs is required to maintain climb.
All take-offs were made without flaps.
C. Stability
When trimmed in level flight for rated or normal power, the airplane is statically and dynamically stable directionally and longitudinally. Laterally, it has neutral stability. Also, when trimmed for 180 MPH glide, the stability is the same as with power.
D. Trim and Balance
Trim is easily maintained by using the aileron, elevator and rudder trim tab controls which are conveniently located on the left side of the cockpit. The trim tab controls work easily and are very sensitive. It takes considerable practice to trim the airplane and keep it trimmed because as on trim control is changed it requires changing others. At all altitudes and speeds the airplane can be trimmed to fly hands off. The auxiliary fuselage tank causes little extra trim when the gasoline from it is used first. If it is not used first, the airplane becomes slightly tail heavy. When opening shutters or putting down wheels and flaps, a medium amount of trim is needed.
E. Controllability
Control can be maintained in all attitudes and speeds up to the diving limits. Changes in speed require change in trim and if trim is not made or controls held, the airplane yaws violently. Thus, it requires quite a bit of practice for the pilot to maintain perfect control. For this reason, this airplane is not good on instruments. When the airplane gets out of trim it is very hard to retrim it on instruments.
F. Maneuvrability
No acrobatics were tried but rolling into turns and changing directions is very easily accomplished because of very light aileron forces. Rudder and elevator have somewhat heavier forces but not objectionably so. The radius of turn is large and the stick forces become very heavy in a turn requiring both hands on the stick.
The water control switch is objectionable because it must be held "on" by the pilot. This occupies the pilot's left hand and he cannot trim the plane or use both hands on the stick which is necessary to make a tight turn.
G. Stalling Characteristics
There is sufficient warning to the pilot of a stall. Slight buffeting of elevators can be felt in the stick, especially with cowl flaps open. Landing is not recommended with cowl flaps open because of false warning of stall and buffeting of elevator. Another warning of a stall is a jerking of the stick to the left. It will snatch the stick from the pilot's hand if he is holding it losely. The stall is normal and the nose falls straight forward and normal recovery is easy.
H. Spinning Characteristics
No spins were attempted.
I. Diving Characteristics
Acceleration in a dive is fast with control forces building up and becoming high above 350 MPH indicated. There is little vibration except as the speed approaches compressibility. A normal amount of trim is required but the forces on stick and rudder could be held by the pilot. However, the trim tab controls are easily accessible and the airplane can be trimmed easily for a dive.
In a pull out from a trimmed dive of approximately 450 MPH indicated the stick forces increase with the number of "g's" applied during pull out. At approximately 5 "g's" the forces are very heavy. There is no tendency toward stick reversals.
WWII Aircraft Performance P-47 Performance Trials
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Here's more than you ever wanted to know about the problem. (See page 131)
http://naca.central.cranfield.ac.uk/reports/1947/naca-report-868.pdf (http://naca.central.cranfield.ac.uk/reports/1947/naca-report-868.pdf)