Questions on the Speed trim

[cmbaviator]

Hi guys,

 

I have some questions regarding the speed trim feature of the 737CL.

 

The speed trim feature is available at high weight and high trust with flaps not up configuration, so basically it should be available during initial climb and during the approach.

 

* Initial climb :

 

The aircraft will try to maintain V2+20kts, but lets after 1500 fts AGL you want to start accelerating to 250 kts, if you only pitch down without trimming nose down, as speed is increasing but the trimmed speed is V2+20 kts, after releasing the yoke, the aircraft will pitch up. I'm wondering whats the technique to trim it correctly:

 

1) When  accelerating and reaching the target speed ( 250 kts), does just trimming for 1 sec will let know the aircraft that 250 kts is the " new trimmed speed", then the aircraft will trim nose down to maintain that target speed. In this scenario, the aircraft will automatically trimmed the correct amount to of trim.

 

2) When accelerating, we have to keep trimming nose down until the target speed is reached (250kts). In this scenario, only small correction will maintain the target speed.

 

 

* Approach

 

Same as above ?

 

I don't know if its clear enough, but i hope you will understand

[Hotdawg]

Speed trim system is armed 10 sec after liftoff, 5 sec following release of the trim switches, N1 above 60% autopilot not engaged, TE flaps extended, and airspeed between 100-300 kts.

 

It is a speed stability system designed to improve flight characteristics during operations with a low gross weight, aft center of gravity, and high thrust when the autopilot is not engaged. 

The purpose is to return the aircraft to a trimmed speed by commanding the stabilizer in a direction opposite the speed change.

Speed trim operates the stabilizer most frequently during takeoff, climb, and go-around. (high power and low airspeed causes pitch-up)

Speed trim satisfies certification requirement of a 1lb increase in stick force for 6 kt of airspeed increase.

 

In real life flying the 737 it is something that happens in the background and is a protection feature. If you're flying and trimming correctly speed trim rarely engages.   

 

If your hand flying you trim to remove control forces at your desired airspeed if you don't speed trim might do it for you.

Hope this helps

[Litjan]

[cmbaviator]

 

Thanks, but i've already watched it several times

[Litjan]

Sorry, I didnt read the original post carefully enough.

 

You want to trim pretty much all the time to alleviate any forces on the yoke. It is possible to accelerate to a new speed without trimming, but the further you get away from the "in-trim" speed, the more force you have to exert on the yoke. This makes precise flying hard.

 

If you accelerate a long time without trimming, and then "reset" the speed trim to the new speed by a very brief application of trim, then of course the speed-trim system will have to run for a while to achieve this new in-trim speed.

 

Does this make sense?

 

Cheers, Jan

[cmbaviator]

Sorry, I didnt read the original post carefully enough.

You want to trim pretty much all the time to alleviate any forces on the yoke. It is possible to accelerate to a new speed without trimming, but the further you get away from the "in-trim" speed, the more force you have to exert on the yoke. This makes precise flying hard.

If you accelerate a long time without trimming, and then "reset" the speed trim to the new speed by a very brief application of trim, then of course the speed-trim system will have to run for a while to achieve this new in-trim speed.

Does this make sense?

Cheers, Jan

Thanks, i get it now.

Yes i think i get it now, so lets recap:

* When speed trim system is not active:

You gave to trim manually to the target speed.

* when speed trim system is active:

You can manually trim to the target speed, if you are a little bit off then the trim speed system will make small correction for you.

You can either just briefly tap the trim switch so that the trim speed system register the new trim speed and then trim automatically to the target speed.

Of course the second choice is not ideal in real life.bbut as many of us doesn't have a force feedback, trimming the correct amount will be hard. Hopefully that speed trim feature will be useful as for example for accelerating from V2+20 to 250 kts, I'll trim down for 1-2s every 5 second. When reaching 230kts, I'll stop trimming and when 250kts is reached I'll just tap the trim switch so that the STS will trim it correctly for me

EDIT:

Well I guess that above 220kts, flaps will be retracted so the STS won't be available... Will press then CMD A button , at 220 kts lol

CMB

Sent from Tapatalk

Edited November 29, 2015 by cmbaviator

[Litjan]

The whole speedtrim system isn´t very intuitive, unfortunately. It wasn´t designed to "help" the pilot, it is more of a safety system. Like Hotdawg said, it was added to "enhance" the speed stability of the aircraft. Especially in some situations can become hard to handle during large power change, and if the pilot isn´t very attentive, a dangerous situation (low speed, high pitch) can develop that - due to low elevator authority, it can be hard to recover from (full forward yoke is not enough).

 

In practice the speedtrim system is often found working "against the pilot". If you want to fly faster, the speed trim wants you to fly slower! The plane will not start trimming unless you deviate from the "target speed", so if you fly with a wrong trim setting, but are at the correct speed, you still have to "push" on the yoke. It´s not an autotrim - and this plane is not an Airbus...fortunately .

 

Jan

[Hotdawg]

Nice video demo! Thanks

[Nils]

Not sure if it's helpful but I like to think of it as a system aimed to reduce phugoid oscillations