Expert: Mark Janus - 1/26/2007

Question
In the chinook helicopter, there is power transfer unit that supplies hydraulic pressure of 3000psi at 3.5gpm  for the flight controls check on ground when the rotors are not turning. When the rotors are turning, the flight hyd pump which is driven by the transmission supplies 3000psi at zero flow and 2900psi at 16gpm to move the flight controls.

Question 1
Why is there a drop of 100 psi when the flt hyd pumps are supplying 16gpm?

Question2
The -10 says that whenever flight controls check is performed with the power transfer units on ground(without rotors turning) mixing of controls should be avoided. Each control has to be individually moved at a slow rate. When not adhered to, the operator experiences Hydraulic lock.Why is this so?  

Answer
Hi Clement

I'm not really qualified to answer your question (about the specifics of the Chinook helicopter) but I will give you a partial answer to your question....  a pump can provide a certain amount of pressure (head) and/or a certain flow rate...  there is a trade off  between these two...  basically  with no flow you are getting the maximum pressure the pump can provide (many times it (head) is quoted as a distance, i.e. the height of a column of fluid the pump could raise up)....  from Bernoullis equation we have a relationship between pressure and velocity (and elevation) so as the velocity goes up (i.e. 0 flow to 16 gpm) the pressure goes down...   There are also flow losses that cause a down in pressure (that you don't have when there is no flow).  For that pump there would also be a flow rate associated with 0 psi ....   an analogy with city water service is in order here....  if you measured the pressure at your home faucet it maybe around 60psi...  if you turn on all of your faucets in the house but one and measure the pressure again (on that one) you will find the pressure is much,  much less.... some of that fluid pressure is now part of what is called dynamic pressure (i.e. pressure associated with the fluids motion)....  the pressure we measure is static pressure (also there are losses in the pipes which account for some of the pressure drop)

Thanx,
mj