Hi Tom,
I can't tell you about the intake filters on the 46 in RVN, but I can tell you a little about the H-34. The 34 had a dust filter installed in the carburetor duct. The filters were removed in country because it gave just that little added manifold pressure. I can recall an operation that we had during the summer of 1965 that the H-34s were refueling on the beach. Well, at the end of the day I think we had 3 or 4 helicopters on the beach awaiting engine changes.
S/F
Ernie Needham

Intake Filters

Thanks Ernie,

I guess it's one of those "Catch 22" situations -

Protect the powerplant or have max power available when you need it most -

I imagine the radial engines could withstand a lot more FOD than the turbines.

Thanks for the reply!

S/F

Tom

Barrier Filters

Barrier Filters is what they were called. Didn't have them stateside in '67, nor did we need them. Surely did need them in RVN though. Since I went from "D" models to "A" models in RVN, there was such a dramatic loss in load capacity between the two models that I didn't have any yardstick to measure a difference. For those that may not know, the "A" model CH-46 had significantly less power than the "D" and subsequent models due to engine upgrades in the T-58-8 in the Alpha and the T-58-10 in the Delta. Upon returning to CONUS and new "F" models, they worked well with and without barrier filters. So what I am taking a long time to say is that, they are very efficient filters and cause negligible loss of power, in my opinion.
Semper Fi
Joe

CH-46 Engine Intake Filters

I remember HMM-364 used the filters on our all-new CH-46D's in early 1968. Joe is right, there was no significant loss of power under normal conditions. However, under dusty conditions, as the filter began to clog there was a loss of power. If the clogging became severe the loss of airflow caused compressor stall and engine failure. During the Khe Sanh siege aircraft frequently had to fly through the clouds to deliver supplies from Dong Ha to the Hills. That meant they arrived in the zones with wet foam filters. The clay and dust of the Hill zones turned to mud when it hit the filters and caused significant clogging. Fortunately, there was an emergency procedure available in the form of a flap on the filter that could be pulled by the crew chief using a string that was routed into the cabin. That, of course, allowed unfiltered sand and dust to enter the engine but we were usually on our way out of the LZ by then. In general the filters worked very well and probably saved a lot of engines. We quickly learned to work around the clogging problem.

😮 :p As I reca;; there were three basoc fi;ters/ways to keeps sand, dirt, etc out of eng's. Witch's tits, Barroer Filters and later EAPS )engine air particale seperator+. Do not know much about witch tits, but barrier filter worked fairly well despite sometimes getting xlogged. The EAPS seemed to work we;; a;so Do not remember any real power loss with eaps? But then I flew mostly in states with them. But in winter thry could not be flown with 4 degrees C and Moisture, ice formed on the screens and cut off all air flow.

Barrier Filters

The barrier filters did work well, but could get you into troubble in a sandy condition. I remember standing by to pull the velcro sealed flaps many a time. Not fun when you are standing on the troop seats over a body bag and the pilot makes a quick bank pulling out of an LZ. Not something you want to fall on top of!
S/F
Tim

I think the differential between barriers and no barriers was about 2-3% when they were "clean", clogged?Like Al said, if it was bad enough it was like running any angine without air...no combustion!
Q

Barrier Filters

Maybe John Dulligan can weigh in on this -

Without belaboring the topic, the design of the filters and the string actuated breather hatch almost seem like the invention of an enterprizing crew chief or pilot.

You would almost think that the force of the rotor wash would keep most airborn particulants away from the top of the fuselage.

I don't remember getting alot of dirt/dust inside the cabin through the open hatches.

S/F

Tom

Air Intake Filters

Attached is the entry in the Natops manual. The power loss is propotionate to the pressure loss at the small losses incurred when the filter is clean. The book says 1% which is undetectable. In dusty conditions, the filters loaded up pretty quickly cutting the power available drastically but better that than all that crud going through the engine. The book says that 8 pounds of dust can wreck an engine (I seem to remember rear bearing seal as the weak point which showed up as a trail of smoke in the engine exhaust). If needed there was a tear off strip that would restore full power. So at the risk of ruining an engine, full power could be regained. Trade offs again. I have a picture of the filter which I will post later.

I'm inclined to agree that the filter was a field fix. Much too simple for an engineering solution. EAPS is much more like it.

Unfortunately there is a lot of dust and a gas turbine engine draws a tremendous amount of air from the surrounding area. A piston engine and a turbine engine both are 'air' engines, adding fuel to the air so that the oxygen in the air is burned up. (It's a bit hard but reverse your thinking. Does the engine burn fuel to which we add air so that it burns or do we burn the air but add fuel to burn it. The latter is more true). A piston engine burns pretty much 15 pounds of air for each 1 pound of fuel, any more and the engine leans out. The turbine, by contrast, can run very lean and burn a lot more air. It also uses the air for cooling at least. So for the same fuel consumption/power output, a turbine sucks up much more air.

I never was able to find out the logic behind keeping 'D' models back in US and leaving 'A' models in Vietnam. Not only did the 'A' have less power, it had less available lift from the power available. Above 105F the lift falls off pretty badly: true in all helicopters. It didn't make sense. But then as an old Navy friend used to say "It's the Gov't, John, it doesn't have to make sense. (he retired as a 2 star so he knew whereof he spoke.)