Return-Path: Received: from [24.25.9.103] (HELO ms-smtp-04-eri0.southeast.rr.com) by logan.com (CommuniGate Pro SMTP 4.2) with ESMTP id 394920 for flyrotary@lancaironline.net; Mon, 06 Sep 2004 08:06:23 -0400 Received-SPF: none receiver=logan.com; client-ip=24.25.9.103; envelope-from=eanderson@carolina.rr.com Received: from EDWARD (cpe-069-132-183-211.carolina.rr.com [69.132.183.211]) by ms-smtp-04-eri0.southeast.rr.com (8.12.10/8.12.7) with SMTP id i86C5oSH023253 for ; Mon, 6 Sep 2004 08:05:51 -0400 (EDT) Message-ID: <001b01c49409$ddb96db0$2402a8c0@EDWARD> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Manifold Pressure Date: Mon, 6 Sep 2004 08:05:56 -0400 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0018_01C493E8.5670DF30" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2800.1409 X-MIMEOLE: Produced By Microsoft MimeOLE V6.00.2800.1409 X-Virus-Scanned: Symantec AntiVirus Scan Engine This is a multi-part message in MIME format. ------=_NextPart_000_0018_01C493E8.5670DF30 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Message Repeat after me- static rpm and MAP :-)=20 By the way, I don't recall hearing any comments from you about any = increase in acceleration on take-off. Was there any? I know you = indicated the P effect increase was quite noticeable - but how about = acceleration? Oh yeah, plenty of acceleration, but I don't think I ever tried full = throttle again after that first takeoff. When flying level at lower = power settings, the acceleration is quite noticeable compared to what it = used to be. I've been pretty severely limited by my restrictive intake, = so it will only get better. =20 BTW, I finished the installation of the new oil cooler, and TB inlets. = I only managed a brief run to make sure nothing was leaking, but it = seemed like the mixture was way lean. Surely it didn't make that much = difference. Tomorrow, I'll tie it down, and check the tune, and static = rpm. I'm really happy with the way the inlets came out, and I'll be = even happier if they actually work. =20 Rusty (thanks for letting me use your thread for my update)=20 Ok, Rusty, I'll record the static and map when I get to the full = throttle runup stage. =20 This got me to thinking about manifold pressure and realized that = while I associate high manifold pressure with high power that it was not = necessarily so. In fact, it can be misleading. Manifold pressure (in of itself) does not indicate power unless other = factors are considered Example: At 2000 MSL you are drawing 20 "Hg Manifold pressure at 5000 = rpm (throttle is partially closed, so intake is restricted). By using = your mixture control (or prop control if variable pitch) reduce your RPM = to 4500 rpm without touching the throttle. You will see a sharp rise in = Manifold pressure. Does this mean more power, naturally not, you have = simply reduced the pumping speed and therefore the flow (suction) by the = rotors which permits the intake manifold pressure to increase without = having changed the throttle plate.=20 Example2: At 2000 MSL again drawing 20" Hg Manifold pressure at 5000 = rpm. Using your mixture control increase the engine speed to 5500 rpm. = You will see a decrease in manifold pressure because the throttle = position has not change (therefore it still restricts the air flow), but = the pumping speed of the rotors have increased "sucking" more air out of = the intake manifold. So manifold pressure decreases, however, we are = definitely producing more power as attested to by the higher rpm and = fuel flow. In the first example, increased manifold pressure actually represented = a power decrease and in the second example a decrease in manifold = pressure actually represented a power increase. So Manifold pressure interpretation requires consideration of other = factors before it is very useful in telling us about our power = production. Now for those of us with Fixed pitch props manifold pressure AND rpm = are a fair indication of power being produced. I say "fair" because you = really need to factor in fuel flow as well to get the best indication of = power. Normally, in straight and level flight, if we advanced the throttle = (reducing the throttle plate restriction) our manifold pressure will = increase. Since our engine is a positive displacement pump - meaning it = displaces the same VOLUME of air each rotation (note. I said volume not = mass) this means an increase in manifold pressure will increase the MASS = of air pumped through the engine each revolution. An increase in air MASS permits more fuel to be burned producing more = power, this increase in power will in turn enable the engine to turn = more RPM against the propeller load. This increased rpm will produce = more "suction" of the intake manifold until an equilibrium is again = reached. Lets not forget that this all starts with the ambient air pressure = surrounding us whether at Sea Level or 10,000 MSL. So Manifold pressure is a direct function of=20 1.. Ambient Air pressure=20 2.. Throttle plate position and=20 3.. Engine RPM It may also be a good indirect indication of power when other factors = are properly considered. Of the three parameters, throttle plate position is probably the one = we specific with the least accuracy - because unless you have calibrated = your throttle level - you really don't know the throttle plate position = except perhaps for full open and fully closed. FWIW Ed Ed Anderson RV-6A N494BW Rotary Powered Matthews, NC ------=_NextPart_000_0018_01C493E8.5670DF30 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Message
 
 Repeat after me- static rpm and MAP=20 :-) 
 
By the way, I don't recall hearing = any comments=20 from you about any increase in acceleration on take-off.  Was = there=20 any?  I know you indicated the P effect increase was quite = noticeable -=20 but how about acceleration?
 
 Oh=20 yeah, plenty of acceleration, but I don't think I ever tried full = throttle again after that first takeoff.  When flying level at = lower=20 power settings, the acceleration is quite noticeable compared to what = it used=20 to be.  I've been pretty severely limited by my restrictive = intake,=20 so it will only get better.  
 
BTW, I=20 finished the installation of the new oil cooler, and TB inlets.  = I only=20 managed a brief run to make sure nothing was leaking, but it seemed = like the=20 mixture was way lean.  Surely it didn't make that much=20 difference.  Tomorrow, I'll tie it down, and check the tune, and = static=20 rpm.  I'm really happy with the way the inlets came out, and = I'll be=20 even happier if they actually work.  
 
Rusty=20 (thanks for letting me use your thread for my=20 update) 
 
Ok, Rusty, =  I'll=20 record the static and map when I get to the full throttle runup=20 stage.
  =
This got me to thinking = about=20 manifold pressure and realized that while I associate high manifold = pressure=20 with high power that it was not necessarily so. In fact, it can be=20 misleading.

Manifold pressure (in of itself) does not = indicate power=20 unless other factors are considered

Example: At 2000 MSL you are drawing 20 "Hg = Manifold=20 pressure at 5000 rpm (throttle is partially closed, so intake is = restricted).=20 By using your mixture control (or prop control if variable pitch) = reduce your=20 RPM to 4500 rpm without touching the throttle. You will see a sharp = rise in=20 Manifold pressure. Does this mean more power, naturally not, you have = simply=20 reduced the pumping speed and therefore the flow (suction) by the = rotors which=20 permits the intake manifold pressure to increase without having = changed the=20 throttle plate.

Example2: At 2000 MSL again drawing 20" Hg = Manifold=20 pressure at 5000 rpm. Using your mixture control increase the engine = speed to=20 5500 rpm. You will see a decrease in manifold pressure because the = throttle=20 position has not change (therefore it still restricts the air flow), = but the=20 pumping speed of the rotors have increased "sucking" more air out of = the=20 intake manifold. So manifold pressure decreases, however, we are = definitely=20 producing more power as attested to by the higher rpm and fuel=20 flow.

In the first example, increased manifold = pressure actually=20 represented a power decrease and in the second example a decrease in = manifold=20 pressure actually represented a power increase.

So Manifold pressure interpretation requires = consideration=20 of other factors before it is very useful in telling us about our = power=20 production.

Now for those of us with Fixed pitch props = manifold=20 pressure AND rpm are a fair indication of power being produced. I say = "fair"=20 because you really need to factor in fuel flow as well to get the best = indication of power.

Normally, in straight and level flight, if we = advanced the=20 throttle (reducing the throttle plate restriction) our manifold = pressure will=20 increase. Since our engine is a positive displacement pump - meaning = it=20 displaces the same VOLUME of air each rotation (note. I said volume = not mass)=20 this means an increase in manifold pressure will increase the MASS of = air=20 pumped through the engine each revolution.

An increase in air MASS permits more fuel to be = burned=20 producing more power, this increase in power will in turn enable the = engine to=20 turn more RPM against the propeller load. This increased rpm will = produce more=20 "suction" of the intake manifold until an equilibrium is again=20 reached.

Lets not forget that this all starts with the = ambient air=20 pressure surrounding us whether at Sea Level or 10,000 MSL.

So Manifold pressure is a direct function of =

  1. Ambient Air pressure
  2. Throttle plate position and
  3. Engine RPM

It may also be a good indirect = indication of=20 power when other factors are properly considered.

Of the three parameters, throttle plate position = is=20 probably the one we specific with the least accuracy =96 because = unless you have=20 calibrated your throttle level =96 you really don=92t know the = throttle plate=20 position except perhaps for full open and fully closed.

FWIW

Ed

Ed Anderson

RV-6A N494BW Rotary Powered

Matthews, NC

 
 
 
 
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