BigGiraffe
Topic Author
Posts: 256
Joined: Sat Jan 29, 2000 7:58 am

707 Secondary Air Inlet Doors

Wed Mar 01, 2000 11:11 am

The engines on the 707-300C's had secondary air inlet doors around the circumference just behind the engine inlet. Some airlines, such as TWA, chose openings which were long in the direction of the circumference and short from fwd to aft (so they looked tall and skinny from the side). Other airlines, such as QANTAS, used a design which was closer to being square. Why were there two different designs, and what were the advantages to one over the other?
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: 707 Secondary Air Inlet Doors

Wed Mar 01, 2000 11:48 am

I don't know the precise reasons's for the difference but my guess would be the more square doors were for engines with the higher mass airflow.

What I do know is the rectangular doors were associated with engines that had a long pointed inlet hub cover while the square doors were on engines that had the inlet hub cover, dome shaped (P & W JT3D-7)
 
User avatar
DL_Mech
Posts: 2074
Joined: Fri Feb 18, 2000 7:48 am

RE: 707 Secondary Air Inlet Doors

Wed Mar 01, 2000 6:21 pm

Hey BigG- I keep hoping you get an answer on this. My father was a mechanic for TWA and I can tell you that all their 707s had the small doors. This was probably so that nose cowls could be interchanged between -131Bs and -331B/Cs. I know that Pan Am and QF had both types. Later- DL_mech.
This plane is built to withstand anything... except a bad pilot.
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: 707 Secondary Air Inlet Doors

Thu Mar 02, 2000 4:35 am

Having done a bit more digging the large doors were on JT3D-7's that delivered 19,000 Lbs thrust while the small rectangular doors were on JT3D-3's that delivered 17,000 Lbs thrust.

All the 707-320's I worked on had -7 engines with the large doors.
 
BigGiraffe
Topic Author
Posts: 256
Joined: Sat Jan 29, 2000 7:58 am

RE: 707 Secondary Air Inlet Doors

Thu Mar 02, 2000 6:47 am

Thanks, VC-10. Good answer! The increased thrust would require more airflow, wouldn't it?

But I still wonder why the old nacelle designs (707, early 737/747) needed these doors and the new ones don't...?

BigGiraffe
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: 707 Secondary Air Inlet Doors

Thu Mar 02, 2000 7:29 am

I have found in the B747 F/E's Tech Manual that the Inlet By-pass Doors were removed from the 747 to comply with noise requirements.
 
BigGiraffe
Topic Author
Posts: 256
Joined: Sat Jan 29, 2000 7:58 am

RE: 707 Secondary Air Inlet Doors

Thu Mar 02, 2000 7:46 am

That's interesting... I wonder if they took a performance cut to reduce noise, or whether they came up with a design change which 1) allowed more air while the aircraft was slow with high thrust, and 2) still maintained optimum airflow in cruise? (I'm assuming they came up with that neat design change, but am still curious about what it was).
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: 707 Secondary Air Inlet Doors

Thu Mar 02, 2000 8:12 am

Looking in the SRM the fixed geometry inlet on the 747 is 10 inches longer than the variable geometry inlet and I would venture a larger diameter, as the SB to replace the VG inlet say's a new pylon fairing above the inlet is required.
 
User avatar
DL_Mech
Posts: 2074
Joined: Fri Feb 18, 2000 7:48 am

RE: 707 Secondary Air Inlet Doors

Thu Mar 02, 2000 8:18 am

Wouldn't the longer inlet also reduce the noise?
This plane is built to withstand anything... except a bad pilot.
 
BigGiraffe
Topic Author
Posts: 256
Joined: Sat Jan 29, 2000 7:58 am

RE: 707 Secondary Air Inlet Doors

Thu Mar 02, 2000 11:01 am

I sure appreciate you digging into this for us, VC-10! It never occured to me that those inlet passages would make a lot of noise although it makes sense now that you've said it, so getting rid of them would make the engine quieter. The increase in duct diameter would make up for the deletion of the secondary passages.

I'm still thinking about what effect the lengthened duct would have, and tend to agree with DL_mech that it would mask some noise from the front fan and therefore make things quieter.
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

BigGiraffe

Fri Mar 03, 2000 5:33 am

I don't understand the point you are making about the longer intake. I understand there will be some noise reduction from the increased length but I don't know where you are going from there
 
User avatar
DL_Mech
Posts: 2074
Joined: Fri Feb 18, 2000 7:48 am

RE: BigGiraffe

Fri Mar 03, 2000 5:50 am

Hey VC-10- I think I started this mess about the longer inlet. On the 737-200 Nordam hushkits that Delta has installed, there is a two inch extension between the inlet guide vanes and the nose cowl. I assumed that this was done for noise reduction. Didn't the 707 hushkits have a similar system? I have never seen a hushed 707 up close. DL_mech
This plane is built to withstand anything... except a bad pilot.
 
BigGiraffe
Topic Author
Posts: 256
Joined: Sat Jan 29, 2000 7:58 am

RE: 707 Secondary Air Inlet Doors

Fri Mar 03, 2000 6:27 am

I guess we were thinking out loud that the increase in length had something to do with reducing noise as did the deletion of the secondary passages. They might also have been lengthened to help channel air without the passages. It is interesting that the 737 ducts also lengthened a little. I don't know anything about the 707 hushkit except it also causes a reduction in thrust.
 
VC-10
Posts: 3546
Joined: Tue Oct 26, 1999 11:34 am

RE: 707 Secondary Air Inlet Doors

Fri Mar 03, 2000 8:23 am

I never worked on hushkitted 707's so I can't answer that.

Thinking further about the 747 inlet, I only ever saw one variable geometry inlet & I don't think it had any acoustic panels, it was just a scaled up JT3D-7 inlet. There I think is the answer, the fixed geometry inlet duct is all acoustic panels less the inlet lip.