Couple of interview technical questions

[duckbelly]

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.:D :D

[gravitysucks]

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!

[AtoBsafely]

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!

[Bus429]

Are high-wing aircraft not (more) inherently stable because of pendulosity?

[Gargleblaster]

"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.

[XPMorten]

High wing gets less influenced by Ground Effect
on landing.
Less space in the cabin due to the wing spars...
Longer gear and possibly extra worries with one gear landings.

MM

[Old Smokey]

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

[montys ex teaboy]

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.

[barit1]

...2/3 of the lift from a wing is producted by the top surface
An old-time racer designer told me that he designed his high-winged aircraft with as clean an upper surface (i.e. wing blending into a flat fuselage roof) as possible, because the air velocity is highest there, thus the most potential for drag reduction. He must have done it right, because 70 years ago he won more money than anyone else.

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.

[duckbelly]

Thanks everyone for your replies and info. Much appreciated!!

Duckbelly

[Captain Stable]

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)? :8

[Mad (Flt) Scientist]

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.

[bluepilot]

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!!

[DVR6K]

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 (?)

[Sheep Guts]

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

[Mad (Flt) Scientist]

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 point being that if the wings were AT 6 foot AGL, you would NOT have room for a decent height cabin. Therefore You don't get 6ft AGL wings on high wing aircraft, except for midgets.

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....

[XPMorten]

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:ok:

Cheers,

M

[Ignition Override]

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??:confused: Why does any of this help you become a very good employee or pilot?

[ww1]

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.

[doubleu-anker]

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.

[Piltdown Man]

I think the reason the F27 Mk50 ended up with high wing was to stop the propellers banging on the ground when the aircraft was in it's Mk1 state (well, it was either that or a ten foot undercarriage and ladders to get in and out). When the F27 was designed (rumour has it for radial piston engines) the props where so large that the wing had to be on top of the fuselage. Later on, when Fokker "updated" the F27 (to the Mk50 - aka F50), the only person building engines of the right size was Pratt and Whitney with the PW127 and they came with in a package with six bladed props.

But the best reasons for high wings are for ground handling. No special equipment required (except for ground coolers on hot days). It also makes it more difficult for those on the ground to bump into things.

And six bladed props - they are pretty quiet and can still absorb the power put out by the engine. In CLB and CRZ I think they do 800 RPM.

But when you fly a F50, you can help but ask - where does all this power go? The ATR 72 has less, carries more, goes faster and burns less.

[Checkboard]

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

... so all low wing aircraft will soon be seen with bumps added to the top of the feuslage, above the wing, in order to take advantage of this effect then :hmm:


- Duckbelly! Since when was the 146 above 60t? And it wasn't that long ago! Besides, as the 146 had a high wing, you should be an expert on it :D

[duckbelly]

Hmmmm. When was the last time we talked?? Been on the 73 for a few years now. Does that qualify??!!:ok: :ok:

I auto dumped the 146 the day I started my 73 endorsement!!

Send me a pm with contact details. Seeya!

[rigpiggy]

WRT the F27 it was originally designed to replace the DC 3, the use of the high wing allowed a lower fuselage "lip height allowing rapid on/offload w/o extra ground equipment. if you look at the HS748 you can see that it would have been possible to go low wing, with the same engines. however forklifts are necessary to load/unload. one of the reasons for the ATR's higher speeds are a higher wing loading, and a smaller fuselage, and more streamlined front end. The F27 vs the HS748 F27 is 10-15 kts faster, but the 748 will go into a 3000' strip w/ a full load. having flown both of them I prefer the 748 for it's human factors/systems. though I appreciated the "drag brakes" on the Fairchild version. Too bad they hadn't collaborated as a true high wing Hawker would still be in production with the Pratts and the ATR wouldn't have seen the light of day. Also the Darts if I remember correctly had a 10-1 reduction ratio and would have a prop speed of 1400-1500 rpm