Jump to content
sbox23

Ch47 transmission oil system

Recommended Posts

Hi, anyone know why only the aft vertical shaft and combining transmission auxillary low pressure lights up at 10 PSI whereas the rest of the transmission low pressure lights up at 20psi?

Share this post


Link to post
Share on other sites

I have asked this same question and never really gotten a for sure answer. So heres my best guess; the aft shaft is so much higher then the aft transmission that even though its a 20psi pump, by the time it gets all the way up there all you can expect is 10. As for the c-box maybe we can get someone with more 47 experience to weigh in.

  • Like 1

Share this post


Link to post
Share on other sites

Thanks for the reply @AS350 pilot. was really thinking whether both system got to do with the low amount of oil flowing through that why it only require 10 psi.Still hoping that someone has the answer.

  • Like 1

Share this post


Link to post
Share on other sites
On 7/18/2019 at 4:18 AM, sbox23 said:

why only the aft vertical shaft and combining transmission auxillary low pressure lights up at 10 PSI whereas the rest of the transmission low pressure lights up at 20psi?

 

On 7/23/2019 at 6:15 AM, sbox23 said:

was really thinking whether both system got to do with the low amount of oil flowing through that why it only require 10 psi.Still hoping that someone has the answer.

 

We need to understand the system we’re dealing with. The pump doesn't pump pressure. The pump delivers a rate of flow and that rate of flow meets with and hopefully overcomes resistance in the system. What you’re reading on the gage is not the amount of pressure the pump is putting out. What you’re reading is the amount of resistance being overcome downstream of the gage. Contrary to your quote, 10 PSI represents low resistance and adequate flow rates. The difference between pressure and flow is often misunderstood.

 

The CH47 aft transmission lubrication system, see photo below, is a parallel-series system where the pump is servicing multiple branches. The branch could have multiple series loads or additional parallel flow paths. These parallel and series combinations behave differently and have branch pressure that differs from the overall system pressure.

 

The transmission pressure is taken downstream of the filter. The filter and transmission resistance to flow causes a pressure drop. The physics of this series branch states the sum of the pressure drop must equal the system pressure. Under normal operations, 6-10 PSI is needed to overcome filter resistance resulting in a 6-10 PSI drop across the filter. The remaining 14-10 PSI is dropped across the transmission.

 

Problems with the system similar to filter blockage or blockage downstream of the filter will increase the pressure drop across the filter, and reduce flow into the transmission. The physics are satisfied by an increased pressure drop across the filter and decreased pressure drop across the transmission. The total of all pressure drops in the branch remains equal to system pressure.

 

Without the transmission pressure information, we would never have known there was a problem, since the system pressure is still at 20 PSI, see photos below.

 

click photo to enlarge

spacer.png

 

391hGi6.jpg

 

 

[media]https://youtu.be/AyizWUpPt28[/media]

'>https://youtu.be/AyizWUpPt28

 

'>https://youtu.be/F4VM_Xlp-SU

Edited by iChris
  • Like 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...