Couple of interview technical questions
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Couple of interview technical questions
Long time since I've flown anything less than 60t, and am going for a position with a F50 carrier. I can't find the answers to a couple of possible questions. Anyone help out?? They're probably somewhere in my mound of books somewhere, but I don't have the time to trawl through them:
1. What are the advantages of high wing aeroplanes? And disadvantages to, I suppose.
2. Advantages of 6-bladed props vs 4 blades? Got a couple for this one: Less noise, lower prop speeds, more thrust. Any more?
Ta.
1. What are the advantages of high wing aeroplanes? And disadvantages to, I suppose.
2. Advantages of 6-bladed props vs 4 blades? Got a couple for this one: Less noise, lower prop speeds, more thrust. Any more?
Ta.
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Duckbelly, high wing usually means that props and engine intakes are up out of the sh@* reducing problems with ingestion and providing greater ground clearance.
For non-swept wing machines there would be stability implications in flight but without delving into my notes I'll not elaborate on that one.
Hope this helps, or at least prompts a correction from someone more knowledgable!
For non-swept wing machines there would be stability implications in flight but without delving into my notes I'll not elaborate on that one.
Hope this helps, or at least prompts a correction from someone more knowledgable!
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I've been flying dash 8's for too many years...
The main advantage of the high wing is the ground clearance, but also it allows the cabin to be closer to the ground (less stairs for commuter aircraft). Other than the max bank angle, I'm not aware of any advantage/disadvantage for crosswind but there is less ground effect in the flare.
Increasing the number of prop blades allows the prop to absorb (convert to thrust) more power for a given diameter. Smaller diameter is better at high speed (due to compressibility at the blade tip), but more blades tend to create interference and lose efficiency.
Good luck, and maybe your company will buy dash 8's soon!
The main advantage of the high wing is the ground clearance, but also it allows the cabin to be closer to the ground (less stairs for commuter aircraft). Other than the max bank angle, I'm not aware of any advantage/disadvantage for crosswind but there is less ground effect in the flare.
Increasing the number of prop blades allows the prop to absorb (convert to thrust) more power for a given diameter. Smaller diameter is better at high speed (due to compressibility at the blade tip), but more blades tend to create interference and lose efficiency.
Good luck, and maybe your company will buy dash 8's soon!
"Disadvantage of high wing for cross-wind landings!"
Could I be educated on that ?
To my understnaind a high wing can bank more close to the ground due to a) higher wing and b) engines and props farther from the ground.
Could I be educated on that ?
To my understnaind a high wing can bank more close to the ground due to a) higher wing and b) engines and props farther from the ground.
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The advantages of 5/6 bladed propellers become evident at higher speeds. The disadvantages are evident at lower speeds.
The low speed disadvantage arises from the prop tip vortices, and their associated drag, which absorb some of the engine power. A 6 bladed prop will have twice as much loss from vortices as a 4 bladed one.
At higher speeds, where the prop tip speed is a trigonometrical function of TAS and propeller RPM, the prop tip reaches Mcrit, and propeller drag increases rapidly thereafter. From a performance aspect, this will limit the cruise speed available, and from a limitations aspect, will impose a lower Vno/Vne on the aircraft. If now, the same blade area is to be maintained (in order to absorb all of the power), increasing the number of blades will either allow for a lower propeller radius, a lower prop RPM, or both. Either will reduce the propeller tip speed, allowing higher cruise speeds and Vne/Vno before the prop tips become trans-sonic.
If the increase in number of propeller blades is also accompanied by an increase in blade area, the propeller will operate at a lower pitch/blade angle for the same power delivered. As Torque is proportional to the Sine of the blade angle, and Thrust is proportional to the Cosine of the blade angle, less available power is lost to Torque, and more is available for useful Thrust.
Advantages of a high wing? - Better view outside for the passengers, much increased lateral stability (perhaps too much), less damage to the airframe in the event of a wheels-up landing (particularly if the aircraft has a strong keel beam a la F27), offset by you being underwater in the event of a ditching.
Regards,
Old Smokey
The low speed disadvantage arises from the prop tip vortices, and their associated drag, which absorb some of the engine power. A 6 bladed prop will have twice as much loss from vortices as a 4 bladed one.
At higher speeds, where the prop tip speed is a trigonometrical function of TAS and propeller RPM, the prop tip reaches Mcrit, and propeller drag increases rapidly thereafter. From a performance aspect, this will limit the cruise speed available, and from a limitations aspect, will impose a lower Vno/Vne on the aircraft. If now, the same blade area is to be maintained (in order to absorb all of the power), increasing the number of blades will either allow for a lower propeller radius, a lower prop RPM, or both. Either will reduce the propeller tip speed, allowing higher cruise speeds and Vne/Vno before the prop tips become trans-sonic.
If the increase in number of propeller blades is also accompanied by an increase in blade area, the propeller will operate at a lower pitch/blade angle for the same power delivered. As Torque is proportional to the Sine of the blade angle, and Thrust is proportional to the Cosine of the blade angle, less available power is lost to Torque, and more is available for useful Thrust.
Advantages of a high wing? - Better view outside for the passengers, much increased lateral stability (perhaps too much), less damage to the airframe in the event of a wheels-up landing (particularly if the aircraft has a strong keel beam a la F27), offset by you being underwater in the event of a ditching.
Regards,
Old Smokey
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If memory serves me correctly, approx 2/3 of the lift from a wing is producted by the top surface. So for a given wing span the wing that has the most top surface would be more efficent. ie., the high wing.
Just my pennies worth and I stand to be corrected.
Just my pennies worth and I stand to be corrected.
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...2/3 of the lift from a wing is producted by the top surface
But more to the point: when the wing area is quoted in the spec sheet, do your own math and determine whether or not the center section area (i.e. root chord x fuselage width) is counted in their figures. My own sums are inconclusive; sometimes this center section area seems to be included, and sometimes not.
This is obviously not an answer, but an expanded question.
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Can anyone explain to me why a high wing would give less ground effect?
Given the same wing area, the same prop diameter and the same prop/ground clearance, you park a wing at, say, 6' AGL, does it matter if you hang the fuselage off the bottom or put it on top? Or am I just being stupid (again)?
Given the same wing area, the same prop diameter and the same prop/ground clearance, you park a wing at, say, 6' AGL, does it matter if you hang the fuselage off the bottom or put it on top? Or am I just being stupid (again)?
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If the wing were at 6' AGL and you hung a fuslage under it, either the pax would be crawling down the aisle (bad for meeting the evacuation time regs!) or you'd need a trench down the centreline of the runways and taxiways to avoid excessive friction drag.
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Hmmm, the F27 and the F50 are two aircraft i know that have the wings over 6' above the ground and the cabin hung beneath it!! cabin seems pleanty high enough to me!! and with the exception of the odd gear collapse have not heard of them making a trench down the runway!!
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Ground effect occurs at a height of approximately half the wingspan above the ground and becomes more of an issue the closer the wing is to the ground so high wing aircraft suffer less from the effect as the wings are always further from the ground at the same stage of flight as a lower winged aircraft. Also the F50s wing isn't very swept, thus decreasing the amount of drag created by wing tip vortices, and therefore decreasing ground effect.
With regards lift, high wing aircraft are able to produce some lift from the bump on the top of the fuselage at the wing root. If you look at the 146, the bulge on the fuselage produces enough lift to negate the use of leading edge flaps or slats at low speed, less weight from all the mechanisms etc.
Also, high wing aircraft are more laterally stable (due to wing angle of attack in a crosswind situation) than low wing aircraft hence why aircraft such as the 146 have anhedral to destablise them slightly and prevent dutch roll. The F50 doesn't seem to have any an/dihedral so it's high wing is "naturally" stablising but not sure about possible increased susceptibility to dutch roll (?)
With regards lift, high wing aircraft are able to produce some lift from the bump on the top of the fuselage at the wing root. If you look at the 146, the bulge on the fuselage produces enough lift to negate the use of leading edge flaps or slats at low speed, less weight from all the mechanisms etc.
Also, high wing aircraft are more laterally stable (due to wing angle of attack in a crosswind situation) than low wing aircraft hence why aircraft such as the 146 have anhedral to destablise them slightly and prevent dutch roll. The F50 doesn't seem to have any an/dihedral so it's high wing is "naturally" stablising but not sure about possible increased susceptibility to dutch roll (?)
Props are for boats!
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Have to agree with DR6K on the reason ground effect is smaller on High Wing Aircraft.
Would disgaree that HighWing has less head room as most Modern High wing Turbo props have the fuselage tube attched to the wing on top with a large bump ie as in C130 or Dash8 and 7 also all the Russian Marks AN24,AN26,AN8 and AN12.
Talking about anhedral all of the above ex Soviets have anhedral tips which must be for extra stability or instability what ever the case.You notice this stuff in the hot sun when you are very tired.
Regarding Props. The more the better for high speed is an advantage, but this same advantage transmits to a considerable yaw change compared to less blades during 1INOP Flight. Hence most of these models have Rudder Boost or Auto Feather No-go options in their POH s and SOPS etc.
I know in a King Air 200 that going from 3 to 4 Blades on some of the newer models means no auto feather no go.
Sheep
Would disgaree that HighWing has less head room as most Modern High wing Turbo props have the fuselage tube attched to the wing on top with a large bump ie as in C130 or Dash8 and 7 also all the Russian Marks AN24,AN26,AN8 and AN12.
Talking about anhedral all of the above ex Soviets have anhedral tips which must be for extra stability or instability what ever the case.You notice this stuff in the hot sun when you are very tired.
Regarding Props. The more the better for high speed is an advantage, but this same advantage transmits to a considerable yaw change compared to less blades during 1INOP Flight. Hence most of these models have Rudder Boost or Auto Feather No-go options in their POH s and SOPS etc.
I know in a King Air 200 that going from 3 to 4 Blades on some of the newer models means no auto feather no go.
Sheep
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Hmmm, the F27 and the F50 are two aircraft i know that have the wings over 6' above the ground and the cabin hung beneath it!! cabin seems pleanty high enough to me!! and with the exception of the odd gear collapse have not heard of them making a trench down the runway!!
The original question put the wings AT 6ft AGL for high and low wing and it's not practical for a high wing aircraft to have a wing that low. You can comfortably walk under a dash8 wing, for example....
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Some clarifications on ground effect (GE).
there are TWO types of ground effect;
1. Span Dominated GE
This occurs at about 1,0+ span altitude.
Due to less vortex buildup, we get DRAG REDUCTION.
The reduction is logarithmic and is very significant close to ground. A high wing aircraft will not get
as much affected as a low wing.
2. Chord Dominated GE
This occurs at about 1,0 chord altitude.
Due to an increase in RAM pressure below the wing,
we get an INCREASE in LIFT Cl. The peak is around 0,05-0,2 chord - depending on airfoil etc.
The F50 WILL NOT even enter this ground effect,
because of the high wing, short chord and long gear. So, e.g. on landing one has to do a much more pronounced flare.
The result of these two is an increase in L/D ratio
and thereby efficiency of the wing
Cheers,
M
there are TWO types of ground effect;
1. Span Dominated GE
This occurs at about 1,0+ span altitude.
Due to less vortex buildup, we get DRAG REDUCTION.
The reduction is logarithmic and is very significant close to ground. A high wing aircraft will not get
as much affected as a low wing.
2. Chord Dominated GE
This occurs at about 1,0 chord altitude.
Due to an increase in RAM pressure below the wing,
we get an INCREASE in LIFT Cl. The peak is around 0,05-0,2 chord - depending on airfoil etc.
The F50 WILL NOT even enter this ground effect,
because of the high wing, short chord and long gear. So, e.g. on landing one has to do a much more pronounced flare.
The result of these two is an increase in L/D ratio
and thereby efficiency of the wing
Cheers,
M
As long as you make every effort to avoid ditching with a high wing plane, does it matter to the interviewers? C-130 Initial Training never discussed the advantages or dis-. of a high wing plane.
Do they want you to second-guess engineering design or be a pilot?
If you fly it at the correct speeds (aren't these developed by test pilots?) and follow THEIR procedures, then why does the decision of the original Fokker engineers, in the 50s or 60s, to use a high wing make a difference??
Why does any of this help you become a very good employee or pilot?
Do they want you to second-guess engineering design or be a pilot?
If you fly it at the correct speeds (aren't these developed by test pilots?) and follow THEIR procedures, then why does the decision of the original Fokker engineers, in the 50s or 60s, to use a high wing make a difference??
Why does any of this help you become a very good employee or pilot?
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Much easier on a high-winger to land in a stiff crosswind - aileron into the wind, rudder the nose onto the runway canterline. Upwind main gear touches first, then downwind, then nose. Not really feasible with say, a 737, which has just 18 inches or so of clearance between the engine cowl and the runway.
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Ignition Override
Good point you raised. Unfortunately there is a disturbing trend in aviation now. They tend to be looking for "rocket scientists" to man cockpits. I was informed by a follow captain recently that "stick and rudder skills" were well down the list of priorities that were needed for the job.
I would have thought that some of this basic theory belongs with the initial principals of flight examinations.
Good point you raised. Unfortunately there is a disturbing trend in aviation now. They tend to be looking for "rocket scientists" to man cockpits. I was informed by a follow captain recently that "stick and rudder skills" were well down the list of priorities that were needed for the job.
I would have thought that some of this basic theory belongs with the initial principals of flight examinations.