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byeplane
1st Apr 2015, 21:51
The basics seem to be eluding me again.

I was looking at the Bae146 and wondering why its designers chose to use a high-wing turbofan configuration.

Were turbofans used because the 146 was intended to be capable of longer routes than its regional turboprop competitors (I noticed it has a 31000ft service ceiling compared to 25 and 24k respectively for the ATR and Dash 8)?

In relation to the high wing config I seem to remember that this gives better results on short field t/o's and steep approaches I.e. London City. However I cannot find much technical info to help explain this.

I do understand that the combo of high and swept wings make the ac extremely stabld about the longitudinal axis hence the need for wing anhedral.
And the incorporation of swept wings is beneficial in reducing drag resulting from relatively higher cruise speeds than a turbo prop.

Can someone help me fill in the blanks?

thanks!

Mechta
1st Apr 2015, 22:42
High wings allow a shorter, stronger, fuselage-mounted undercarriage. With the fuselage closer to the ground it easier for the aircraft to carry its own air stairs, thus speeding up turnarounds and allowing use of less well equipped destinations.

The choice of turbofans may have been influenced by their relative quietness compared to turboprops around at the time. I seem to remember that its low noise level was a factor in its selection by operators using Orange County Airport in California.

http://articles.latimes.com/1985-02-15/news/mn-3182_1_flight-path

Allan Lupton
1st Apr 2015, 23:05
I think we covered much of this before, however let me just say that in the 1970s (when we first came up with the 146) turbo-fan powered swept-wing aeroplanes were the norm.
We were designing a turboprop replacement aeroplane so of course it was bigger and faster and had longer range than (say) the 748 or F27.
It was intended to operate from the same short or hot or high airfields as the 748 - which it could and still does!
Mechta has explained most of the high wing logic and I'll just add the uninterrupted wing upper surface as another benefit.
At the time we were configuring the 146 there were no engines available that would have powered a new twin-engined aeroplane of that size so we went for (and took advantage of) four ALF502 power plants.
Hope this helps.

ETA the use of turbo-fan was beneficial to the noise footprint of course (and a lot of sales depended on it), but the initial reason was an Mmo of 0.73

Dash8driver1312
2nd Apr 2015, 03:55
Nothwithstanding that the ATR and DH8 didn't exist at the time.

Groundloop
2nd Apr 2015, 09:18
It was also designed to operate off gravel runways so the high mounted engines were clear of debris thrown up by the nosewheel.

byeplane
2nd Apr 2015, 19:38
thanks for the info. So just to be clear, the high wing config has negligible effect on t/o perf?

lakerman
4th Apr 2015, 11:59
For what its worth, have any of you ever seen a low/mid wing bird?

eckhard
4th Apr 2015, 15:08
As far as I understand ornithological physiology, the power to flap the wings comes from the breast muscles (pectorals) which are relatively large. These large muscles require a deep sternum (breast-bone) to provide sufficient anchorage and leverage. So I think the high-wing config of birds has more to do with muscle and bone layout than aerodynamics.

Apparently, if a man had the same power/weight ratio, he would need a sternum about six foot deep to anchor the huge pecs required!

Mechta
4th Apr 2015, 15:24
Mid-wing bird, (well, almost); the wandering albatross:

https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRepz9aefLy20aH1UhxVlIadQvrChSMhXvRHH4f_v0 PSRuv2O0-

With respect to wing position on take off performance; all other things being equal, the closer the wing is to the ground, the more ground effect there should be to enhance lift.

byeplane
4th Apr 2015, 18:36
Mechta,

that was also my understanding regarding low wings and ground effect. But the wiki page for the Bae146 suggests -

" The chosen configuration had a high wing and a T-tail to give good short-field performance, "

can anyone shed some light on this? I know that a t-tail config makes the stab and elevator more effective as it places it out the way of the downwash from the engines but I still fail to see how both benefit a short t/o

AerocatS2A
5th Apr 2015, 02:17
byeplane, the BAe literature says the high wing gives 4% more lift for the available wing area, the flaps fit snugly against the fuselage which makes them more efficient (not quantified), the tail plane is out of the wash which makes it more effective, it is also further aft being on top of the tail and acts as an end plate for the fin which means the fin is more effective and can be smaller hence less drag. All of these are small advantages that would add up to better performance in general.

skyhighfallguy
5th Apr 2015, 02:23
The 146 was supposed to be able to 3 engine ferry out of many small strips. I did a three engine ferry on this thing, amusing to say the least.

It seemed a robust airframe, was quite slow compared to many other jets, had NO leading edge devices and it only had one good feature in my mind. It was flying on the west coast of the USA! We took it in and out of 4000' runways with room to spare.

IT DID not have thrust reversers and I was glad we didn't fly it into icy runway airports.

Trying to think of something good about it. I'll get back to you if I find something

OH, it had a mechanical ice detector. Also amusing.

pattern_is_full
5th Apr 2015, 04:08
1. Ground effect occurs within one wingspan of the ground. So while the high wing of the Bae 146 (and C-17 and C5A/B) reduces GE a bit, there is still quite a bit of GE from an 86-foot wing 15 feet or so above the ground.

2. Ground effect is a two-edged sword. One can take off in ground effect without sufficient speed/lift to maintain flight outside ground effect. (cf. Ekranoplans, etc.) There have been a certain number of accidents tied to overloaded aircraft (not any particular type) that got off the runway in GE, but could not then climb high enough to clear obstructions.

I guess what I see in the 146 is that the reduction in ground effect due to a high wing, is countered by the increased wing and flap effective area due to the wing being less interrupted by a fat fuselage.

Wiki sometimes oversimplifies a bit, and I'd say the 146 design was intended to enhance good "unimproved airport" capability - no jetways, poor runway surfaces. And since such airports likely also have short runways - high overall TO performance (not necessarily through ground effect).

Whether it actually did so or not, I leave to others who've flown it.

skyhighfallguy
5th Apr 2015, 04:29
didn't the prince of wales crack a 146 up when he landed it somewhere (other than england)?

I don't remember the angle of sweepback, but it wasn't that swept, which would give better short field performance.

I'll say this, it didn't climb well above 10,000' and did not fly fast, I think we cruised it at .68

It did land smoothly though, the trailing link main gear made a greaser into the softest landing ever and a standard landing into a greaser.

AerocatS2A
5th Apr 2015, 09:06
15 of sweep and a Vmo of M0.72 (depending on the model). M0.68 for cruise is about right, maybe a little faster or slower depending on weight and what you're trying to achieve. TAS somewhere between 380 and 420 knots.

It does not have the performance of a traditional swept wing jet but it does have some redeeming features. The airbrake lets you fly it much like a turboprop. 250 knots to the localiser if your SOPs allow it and it'll slow down to 200 going down the slope with the airbrake out (not something I make a habit of, but I've done it and it worked.) Never had to do a three engine ferry though I get to practice it in the sim every 6 months. A non-event as far as performance goes.

The models with the 507 engines have better performance.

Sop_Monkey
5th Apr 2015, 09:29
Mechta

Good point with the Albatross. Nature would see to it the best "design" having the wing where it is. The top of the birds body would be blended in to provide the the extra lift.

As approximately 60% of lift is derived from the upper surface of a wing, it would make sense to me at least, a high wing design would give the best Lift all round. So best endurance and best t/o performance. IMHO.

Take these heavy lifters as an example. AN124, AN225, C5, C17, etc., etc.