Slightly tail heavy DC-8?

[anderscn]

Hello,

I just downloaded and installed this plane (DC-8-50). But when flying at the same constant level, it appearently comes out a bit tail heavy.
Is this normal?

Best Regards,
Anders

[jimhalinda]

I would say yes that is normal, as you are quite heavy with 93% fuel in the tanks.

As fuel burns off, if you maintain the same speed, the nose should come down a bit, but even then it will probably always have a 2 or 3 degree nose up attitude to maintain your altitude.

I'm not pilot though, I'm sure many others can better explain what you can expect and why.

Regards,

Jim

[aerofoto - HJG Admin]

Can't say I've ever experieneced/oberved this .... and I've test flown "THE LOT" !

A couple of degrees "nose up" is the normal cruise attitude for a DC8.

That nose will begin to come up even more though .... if you're not flying fast enough !

MACH 0.82 is a good "ball-park" speed to aim for at FL310 .... and the DC8 will go both higher and faster too.

Mark C
AKL/NZ

[skyking]

As others have pointed out, some nose-up attitude is normal. But...we need to know the altitude and speed/mach you were flying at when you took that screen shot.

[jimhalinda]

It's right there on the screenshot - FL310, 301 knots indicated.

[skyking]

Feb 21, 2010 3:43:04 GMT jimhalinda said:

It's right there on the screenshot - FL310, 301 knots indicated.

Ooops. I didn't look at it close enough.

Well, I took my DC-8-50 up to FL310 and set it up as near as I could to what was displayed and got the same "appearance" as the posted screen shot. However, the 'horizon' gauge only showed the 'normal' 2 degrees or so. So, don't know why the photo makes it look like more. If it was like that for real, the stews would have a hard time pushing the serving carts from back to front !!

[Herman]

Hello Anders;

As stated in the above replies, a 2 to 2.5 degree nose up attitude is quite normal at the aircrafts normal cruising speed.

As far as I know, most jetliners, including today's, display that tendency. From what I have read, this was intentional and is part of the aerodynamic design of the wing, in order to take advantage of the air cushening effect,( can't remember the proper technical term ) similar to when a flat stone is thrown across a water surface.

Herman

[aerofoto - HJG Admin]

If I remember correctly ....

DC8's were equipped with a device known as the PTC ("Pitch Trim Compensator" .... not represented in our simulation though) which, if I understand correctly, was intended to assist the aircrafts pitch/trim attitude during the high-altitude hi-speed cruise regime .... because of the DC8's natural flying characteristics.

I can't remember though if that device was intended to raise or lower the nose .... but .... "SKY KING" might like to comment accordingly.

Mark C
AKL/NZ

[skyking]

I'll "comment" on the PTC [Pitch Trim Compensator] by quoting from the Training and Reference Manual on the three-position switch that is located on the Forward Control Pedestal.

TEST - Sends high speed signal to pitch trim compensator computer to test operation of the computer and actuator. An electrically driven actuator applies force to the First Officer's control column moving it aft. Used only for maintenance test and emergency or irregular procedures.

NORMAL - Allows pitch trim compensator actuator to apply force to Fisrt Officer's control column. This provides nose up elevator to counteract nose down pitch caused by aft movement of the center of lift at high speeds. The actuator starts to apply force at M.70 and is at full extension at M.88.

OVERRIDE - Disconnects the computer and provides power directly to the retract side od the actuator.

[skyking]

re. the PTC Switch -

I should add that on the Before Start Checklist the PTC Switch is checked to be in the Retracted, Override position. By "retracted" they mean to check the "snake" on the F/O's column to be retracted in the tube. This "snake" extends as the Mach increases. The PTC Switch is placed in the Normal Position when the After Takeoff Check is done. Then on the Approach Descent Check, the PTC is again placed in the Retracted, Override position.

It's called a "snake" because of the yellow bands on the rod make it look like a snake. It's the black looking rod to the left and behind the F/O's column that can be seen in the attached photo. You can't see the yellow bands because the PTC is in the retracted state.



Hope all this helps some.

Enjoy your flight !!

[Herman]

Thank you Captain for the info and pictorial display dealing with the DC-8 PTC.

The DC-8 seems to be the only jetliner that makes specific reference to the PTC,as part of their check list, however, the others must also be using a similar device, because all jetliners as they approach their normal cruising speeds are affected by a condition that is known as "tuck under".

I suppose, that both PT compensation and the 2 to 3 degrees nose up attitude at normal cruising speeds are related, in order to provide for lower drag values and there by improving the overall performance and efficiency of the aircraft.

Herman

[skyking]

Well, I'm not sure how Boeing, Convair, and others handled the "Mach Tuck" problem unless it was strickly by auto-trim somehow. The only other jet I ever flew was the 737-200/300 and I don't recall ever hearing anything about it. Maybe they didn't have the same kind of 'problem'.....don't know, can't remember. [and if the truth be known.....don't care any more !!] ;D ;D

[anderscn]

Thanks,

a lot for the replies. I will see what happens to the pitch as fuel burns, and/or by adjusting the Mach and altitude up and down.

By the way, when flying on the passengers airliners today, I have always noticed that it feels like I have to walk up-hill when walking in the aisle from the back towards the front of the plane. So even today planes are flying with some nose-up pitch.

Tomorrow I am actually flying home on a trans atlantic flight (probably in a B747); I will then evaluate if I can sense any difference between the beginning and the end of the flight.

Best Regards,
Anders

[Herman]

Many years ago when it was still allowed for people with private pilots licences to visit "up front" I had the opportunity to see the flight deck of a number of different jetliners, ( 747, DC-10, A320, 737 etc.)

Without exception, they all cruised with a 2 to 2.5 degree nose up attitude. I recall asking the DC-10 captain about this, stating that I would have thought that this would cause an increase in drag.

His answer was, as I have mentined before, he gave me the "pebble skipping across water" analogy where he compared the thin upper air imparting an upward force on the bottom of the wing surface and somehow decreasing drag as opposed to increasing it.

At the time it sort of made a bit of sence to me. Surely there must be someone amongst all of the HJG members who has an aerodynamics background to come up with a more scientific explanation to this.

Herman

[skyking]

Surely there must be someone amongst all of the HJG members who has an aerodynamics background to come up with a more scientific explanation to this.

It's really quite simple, Herman. You pull back on the stick and the airplane goes UP. You pull further back on the stick and the airplane goes DOWN. ;D ;D ;D