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707 drag (Read 3562 times)
windplayer
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707 drag
Jul 7th, 2012 at 4:08pm
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Guys, especially real pilots, as i started to fly 707 i noteced that it takes MUCH more time to slow down in level flight then, say light 727.
727 slows much as it should by boeing docs. But i dont have numbers for 707. Plane have big wings, thats right, but how do you feel about that?

In passenger 707 in about 200 000 lbs landing weight it takes forever to slow it down from 250 to 200 KIAS without spdbrakes to start flaps extension. Heavy freighter in 240 000 lbs slows a bit faster, but still looks like plane want to fly.

Also there is no way to descend at 250 KIAS clean. You have to use full extension of Speedbrakes to get V\S more than 1000 fpm. And as you set flaps 14, you cant use spdbrakes, and it still slows too slow Shocked

Is that right for real plane? How do you feel about that?
Did you use speedbrakes with flaps?

And also what does GEAR DOOR UNSAFE light shows? Does it mean that you shouldnt set idle power inflight? (coz idle power setting light up that light)
  
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LOU
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Re: 707 drag
Reply #1 - Jul 8th, 2012 at 3:12am
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I agree the 707 is a bit slick compared to the 727. I think the 727 is a bit too draggy so somewhere in the middle would be more realistic. We resisted using speed brakes in the 707 because it shook the plane too much. Sometimes, if you did need to use the drag, you could turn of the inboard spoilers so it would not shake the tail, but in practice most pilots would avoid the speed brakes in the 707. The 727 with the "T" tail did not have the shaking problem. The 707 did take a bit longer to slow than the 727, but you are correct this model is a bit too slick. At light weight the 707 clean, would not descend as fast as the 727 which had higher wing loading. Using speed brakes with flaps was a no, no in both planes.

The gear red light was just a reminder that the gear was not extended, sort of like the gear horn in the 727.
  

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windplayer
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Re: 707 drag
Reply #2 - Jul 8th, 2012 at 9:20am
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Thanks Lou! So that shaking comes from turbulence hitting elevator?

Also i noticed in general, that extended speedbrakes makes plane much more sensitive in roll. Looks as it should be. is that right?

And is there any bank angle restriction when turning with speedbrakes extended? I noticed that if i try to descend on A\P, with spdbrakes.(especially in 727) In turns (using HDG HOLD mode) it dangerously drops the nose! It turns with high bank angle, and it looks like lack of lift and it drops the nose down to  -10, -20  Shocked So i quit using A\P and speedbrakes together.
Did you had some rules about that?
  
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LOU
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Re: 707 drag
Reply #3 - Jul 9th, 2012 at 6:40pm
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We were limited to 30 degrees bank for passenger comfort. The 727 had an interesting quirk, the selection of spoilers from the cockpit went through a device called the aileron mixer spoiler ratio changer . This was a complicated device that mixed the ailerons and spoilers to obtain a rapid roll rate during various phases of flight. With the spoiler handle full down or full up the roll of the 727 was fairly snappy but controllable. If you placed the spoiler handle in something less than full up or full down the roll rate was VERY fast and not considered controllable. The only time in any procedure that the spoiler handle was used in a mid position was during landing with one main not extended. The idea was that the enhanced roll rate would help to keep the wing without the gear off the runway a bit longer. Never had to try it in real life, only in the simulator. However, since the activation of the spoilers was manual, the speed of the movement of the handle was critical. If you moved the handle slowly from down to up during landing and any aileron input happened it was a pretty wild ride and could result in a scraping of the wing tip or flap. Later planes (747, 767, 757) did not have this problem.

As you bank the plane, lift generated by the wing is used to turn the plane.

Here, have a look...



With the plane in level flight the lift is perpendicular to the wing and all is balanced.



As you bank the wing, the lift generated by the wing remains perpendicular to the wing and is used to move the wing sideways. The lift loss needs to be made up or the plane will loose altitude. This is why you need to increase the angle of attack as the bank is increased to make up for the loss of lift in the turn.

If you have the speed brakes/spoilers extended you are loading the wing and the stall speed will increase. If you are at high altitude and heavy weight and either increase the wing load by a steep bank (remember, 60 degree bank turn produces 2 G's) or make a quick level off which also increases the G load you could induce buffet and a stall. The autopilot authority could be exceeded with such a maneuver and may not be able to control the plane and could trip-off. Remember the operating space between stall buffet and Mach buffet at high altitude and heavy weight is very narrow. It is called "coffin's corner" where if you slow down just a bit you stall and if you speed up a bit you are into Mach buffet.

The result of either is not good!  Shocked  

Pilots were very careful making abrupt maneuvers or loading the wing at high altitude in the 727 since it had fairly high wing loading. The 707 had a bit more in the way of wing and was a little bit less critical. That is why it was iffy to try to climb over a CB (cumulus build-up) since the higher you go the narrower the operating range is, and a few knots either way results in buffet. In turbulence at high altitude it was recommended to turn OFF the altitude hold and accept a slight loss or gain of altitude since increasing the G load would result in buffet and stall.    

Lou

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windplayer
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Re: 707 drag
Reply #4 - Jul 9th, 2012 at 9:36pm
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Thanks Lou! I saw the term "wing loading" in articles about stalls etc. Now it's time to read about it! And general aircraft aerodynamics.

That "coffin's corner" reminds me of West Caribbean Airways Flight 708.  MD-80 stalled because they flew close to stall speed, with anti-ice on, and sudden updraft pitched nose up, so they exceeded max AoA.

I flew 707 at 36 000 ft max. at about 280 000 lbs at FL360 it looks like it pretty close to stall speed at cruise power setting.

BTW, what would you prefer in situation like this:

You climbing in 707, and you've choose maximum cruise altitude for your take-off weight. Say it 37000 ft. and while at 35-36 000 ft you flew into some turbulence. And since FL370 your max alt for weight, will you decide go lower because of turbulence at max alt can be dangerous because of narrow operational speeds interval?
That question arises in almost every flight i fly in 707.

P.S. I just landed after simulating failures in 707. Worst I had: 2 eng failure + airspeed indicator failure at the same time. Landed safely at flaps 14. The shot i failed today: pitot tube failure. I exceeded Vne and overstressed plane while making turn. Now i'll read, try one more day, and then im sure i'll have a ton of questions  Wink
  
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LOU
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Re: 707 drag
Reply #5 - Jul 10th, 2012 at 2:09pm
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Windy,

Each plane has a recommended speed for turbulence penetration.
It's been a while, but I seem to remember Mach .78 to .80 was a good number in the 707 and 727.
What you don't want is to be at the edge of the operating range of the aircraft for that weight and available power.

A good example is mountain wave. If you're at high altitude and run into a wave, the plane my not have enough thrust to maintain altitude in the down portion of the wave.




As the plane enters the wave it is pushed upward causing the autopilot to pitch down in order to hold altitude.
The auto throttles pull the thrust way back trying to maintain speed.
Then (red plane) the plane enters the part of the wave pushing the plane down.
The autopilot pitches up trying to maintain altitude.
The available thrust is not enough to maintain speed and the aircraft approaches a stall. Mountain wave can be turbulent or smooth.

Either way, a plane at the edge of it's operating range is looking for trouble.  Shocked

Lou




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Re: 707 drag
Reply #6 - Aug 8th, 2019 at 11:17pm
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Hi,

Is it possible to edit the flight model in order to adjust the drag to a more realistic value? Has anyone tried it?

Thanks!
  
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