Ram Air boost numbers

hevansrv7a said:

While testing for other things today I made a point of observing my MAP at a given pressure altitude because it dawned on me that density altitude does not affect MAP even though it affects available oxygen.

At 7000 feet pressure altitude my MAP (which has been calibrated) was 24.3". The Navy says the standard atmosphere has 23.09" at that altitude. I figure that means I got 1.21" of boost net of any induction losses through my SJ airbox and K&N filter.

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Very interesting. I don't know how fast you were going at 7000 ft, but if you were doing 175 kt TAS, and the temperature was standard, the theoretical maximum amount of ram pressure rise would be 1.21" Hg. I wonder if you are getting any pressure increase from the prop. I recall that Roy Lopresti claimed that he could achieve some additional MAP by placing the induction inlet close to the prop, and clocking the prop to synchronize the pressure pulses from the prop with the engine's intake strokes.

It must be so, then.

Kevin Horton said:

Very interesting. I don't know how fast you were going at 7000 ft, but if you were doing 175 kt TAS, and the temperature was standard, the theoretical maximum amount of ram pressure rise would be 1.21" Hg. I wonder if you are getting any pressure increase from the prop. I recall that Roy Lopresti claimed that he could achieve some additional MAP by placing the induction inlet close to the prop, and clocking the prop to synchronize the pressure pulses from the prop with the engine's intake strokes.

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The prop is very close to the intake inlet on the SJ snout and I was going closer to 180 KTAS for that observation. Free boost! who'd have thunk it? I say you are right about the additional boost from the prop. because no induction system could be drag-free, even with the K&N filter that faces directly into the airstream in a cone shaped housing, large end aft. I wonder, then, if I'll lose that with the 3-blade prop? I'll look for it. Thanks, Kevin.

I'm not sure about the temp, but at 8000 DA my PA was 8550 and at 7800 PA the DA was 7500 so the variation from standard temp was not a lot.

I must say I am a bit skeptical of your numbers because you have a fixed pitch prop. Any time RPM changes MP will change inversely, ie, reduce RPM MP will go up, if the throttle is left alone as is the case with a CS prop.

Since you have a FP prop, you really can't get valid results from trying this test. I have a CS prop and have done this a few times: Start at an altitude, say 3000 ft. Note RPM and MP. Climb 2000 ft without touching prop or throttle, and stabilize to cruise speed. Then zoom back down to the initial altitude and level off at 3000 feet, at that point you may have and additional 50 kts. of IAS. You'll find that even with the additional speed there is no increase in MP from ram air effect. My MP is always the same as the starting condition.

In fact I only see 0.3" of MP drop when I pull the carb heat on.

The best measure of boost is to slow to near stall speed at altitude, take a reading, then accelerate and compare readings.

The boost a low speeds is very small so you rise calculation will be pretty accurate.

We see slightly more than one inch boost at 8000 feet in the Rockets, operating at 170 KIAS.

Some well set up engines benefit from "pressure time impulse" effect as the prop blade pressure wave passes the intake, hopefull with the valve open, but this takes carefull design to successfully execute.

Bob

At 200mph TAS, my three-blade prop imparts about a 5-7 ft/sec avg. velocity increase to the air in the rearward direction; that is nowhere near enough to give any appreciable pressure increase. At 8000' dalt and 200 mph TAS, stagnation pressure is 1.136". With an added 6 ft/sec, it would be 1.183".That is the increase that goes on all around the prop arc, not just during the small fraction of the arc when the prop is in front of an inlet, which is only a short part of the total inlet valve opening time. Plus, my blade design exerts much more velocity increase in the root area than does a typical design. In Jack Norris' book on prop design, he has a section in which he calculates the slow-down of air ahead of the cowling on an RV; he shows a very dramatic velocity decrease in the vicinity of the prop hub. I have always felt, but have never measured, that there is a definite free-stream velocity increase into inlets that are close to the base of the spinner since the spinner definitely displaces and speeds-up the air flowing over it on a continuous basis, not a pulse into the inlet as from the prop passing. Unfortunately, the spinner-cowl interaction is not covered in Jack's book. Whether the spinner would give an appreciable pressure increase is something that needs to be measured or calculated. You don't say whether you have a carb or FI; that's important since the carb gives much more pressure drop than does the FI throttle valve. The formula for ambient pressure vs pressure altitude is 29.92[(1-PA*6.88E-6)^5.256]. For your 7000' PA I get 23.083" which is about the same as your 23.09" Navy source. In all the tests I've run with my plane my MAP is always what I would get considering the stagnation pressure and the carb pressure drop, and I've got lots of flight data to back that up. What's it all mean, Alphie?

Data for Commenters and Doubters

Since my MAP measurements were at WOT for the most part, changing RPM will not change the reading except as it changes airspeed. Yes, it would for partial throttle.

My system is vertical induction FI - the basic one sold with Superior engines as noted on my website. Air comes in through the SJ airbox. There is some additional air possibly coming in through the flapper in the top of the airbox in front of the filter which is there for safety. If there is high pressure air inside the cowling, perhaps from the spinner gap, then that could be involved, too. That said, if the pressure in the cowling, outside the plenum were that high, then I'd expect cooling problems and just the reverse is true.

The snout of the intake is about midway along the length of the blade - see picture on website.

Since I was able to see some rise in MAP as airspeed increased (at WOT and level) I think it is reasonable to conclude that airspeed caused MAP to increase.

Just to repeat, I did everything reasonable to ensure the two instruments - altitude and MAP - were accurate. I don't have the knowledge nor the information to dispute those who say this cannot be true, but I don't think we've gotten to the bottom of it.

Bumble bees can't fly, either.

"If there is high pressure air inside the cowling, perhaps from the spinner gap..."
I'm not sure what you mean by that. One big source of drag is the high pressure air within the cowling going forward around the crankshaft into the area behind the spinner then exiting the spinner-cowling gap radially. This interferes-with and causes separation of the flow from the spinner onto the cowling increasing drag. One way of dealing with this without sealing around the crank, which many racers do, is to form a 1/4" to 3/8" radius on the cowling just outboard and behind the spinner to take the air emerging from the gap and cause it turn it to the rear.

More Data

Today's data:
Pressure Altitude 8480 and DA 8100. WOT. Speed around 150-160 kts, 22.5", speed at 175 kts, 22.85".

This is about 1.0" above ambient and less net boost than I got last time. This was with the 3-blade prop.

Maybe there was a boost from the 2-blade prop or maybe something else was different. Maybe I was not going as fast. I don't know.

BTW - A close friend who is an AI says that the C-150 prop can yield about 50 rpm more if mounted in a different position than the one called for by Cessna. It's certified, so they don't do it. If true, this would tend to support the idea of prop pulses helping the ram air, but we're talking about much lower speed numbers.

"BTW - A close friend who is an AI says that the C-150 prop can yield about 50 rpm more if mounted in a different position than the one called for by Cessna. It's certified, so they don't do it. If true, this would tend to support the idea of prop pulses helping the ram air, but we're talking about much lower speed numbers."

OK! We've got a 110HP C-150 at a true airspeed of 110 mph at 5000' dalt with a 68" diameter prop of 82% efficiency. Its altitude HP will be about 92, and the thrust HP will be 75.5. At its forward speed of 161.3 fps, its drag(thrust) will be 75.5X550/161.3=257.4lb. Each blade will intercept (68/12)^2Xpi/4X161.3=4069 ft^3 of air, or, at the 5000' dalt, that's 8.3 lb mass each for a total of 16.7 lb mass. Since thrust is mass times delta V, 257.4 lb/16.7 gives a delta V from the prop of 15.4 fps far to the rear of the vena cava flow behind the prop, or half that at the prop disc, 7.72 fps. Dynamic pressure at 161.3 fps is 26.65 psf or 0.377" hg. With the additional 7.72 fps, it becomes 29.27 psf, or 0.414" hg. 'Hardly seems like the additional 0.037" of MAP would be all that consequential in producing enough extra power to gain that 50 rpm. I hate to be a bubble-burster, but that incredible blast of air you get at full power on the ground just isn't there in cruise! I'm really surprised that this notion of these hi-velocity pulses hasn't been de-bunked long before this. Now, that being said, I wouldn't be surprised that there might not be some sort of interference in the flow from the prop into the inlet aperture, either from blockage or from the helical flow tripping on the edges of the aperture, which could lead to the engine producing less that its rated altitude power! My experience with my prop design is that it causes engines to run cooler since it promotes, rather than blocks flow in the root region. On Jim Smith's RV-6, it appears that he had more air entering his cooling inlets which got heated, flowed back out of the cowl and down the sides of the fuselage and entered his cabin air inlets where his OAT sensor was mounted, so that there was almost a constant OAT indicated regardless of altitude. This also took place on an RV-9 I tested! This didn't happen on either of these planes with their other props.

More data - boost still there

Ellipse, I was repeating something I had heard about the C-150 in the hope it would be helpful, but I can't defend it. Nice exposition, though. It's an interesting idea that rather than boost, the other position interferes less!
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Back to my RV-7A: Today I flew at pressure altitude = 9380, density alt = 8730, RPM = 2650, temp=-9 C, power = 75-76% and TAS = 175 kts per NTPS 3-way method.

MAP was 22.3. Once again, I watched it climb as the airplane gradually built up that last 30 kts or so, once level. My Navy chart shows 9,000 is 21.38 and 10,000 is 20.57. The differences between 8 and 9, and then 9 and 10 are .84 and .81 respectively. In other words, the difference per altitude declines with altitude. So for rough purposes, use .8" between 9 and 10, take .38 of that for a result of .3 and subtract that from the 21.38 at 9 to get 21.08 ambient for 9380. Not perfect, but probably pretty close.

Mine = 22.3, ambient = 21.08, difference = 1.22. This was with my new 3-blade Catto prop.

elippse said:

On Jim Smith's RV-6, it appears that he had more air entering his cooling inlets which got heated, flowed back out of the cowl and down the sides of the fuselage and entered his cabin air inlets where his OAT sensor was mounted, so that there was almost a constant OAT indicated regardless of altitude. This also took place on an RV-9 I tested! This didn't happen on either of these planes with their other props.

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I find that rather surprising...It should be rather common knowledge by now that mounting an OAT probe in the side NACA inlet (or any other location that exposes the interior side of the sensor to the cabin atmosphere) is probably the worst location possible.

Pretty much all OAT sensor bodies have most of there mass on the interior side of the surface it is mounted to. This puts a lot of thermal mass in the cockpit area. Other than flying at low altitudes in the hot summer, the cabin area is usually always warmer than the outside air temp. Sometimes it is much warmer. This will cause major errors in OAT readings if the sensor body is exposed to the cabin air.
A much better location is to at least mount it in the bottom skin extension that bridges to the bottom of the wing. An ideal location is to mount it in an outboard wing inspection cover or the bottom of the wing tip. This location puts it outside the cabin and it wont be effected by warm air leakage at the cowl / firewall intersection, etc.

My Mooney had a filter by-pass, which became just about a straight through ram-air system. I would see about 1" more MP when opened.
Personally, that's why I put a larger engine in my -9A (180hp), so I wouldn't have to worry about getting more power!!!
Regards
Jack
N99552
Ran engine today
C of A next week

Scott: Keep in mind that their OATs showed the normal temperature drop with altitude with their other props, but not with mine. If you don't want to get the stagnation rise which makes the OAT higher than true OAT, mount it out of the freestream. On mine, Dick Shapley's -9, and Smith's -6, the OAT sensor is in the tailcone and shows no rise with speed.

Evans: I calc that the dynamic pressure at your dalt and TAS is 1.13", so with no pressure drop in your FI induction system, what you got sounds pretty close!

elippse said:

On Jim Smith's RV-6, it appears that he had more air entering his cooling inlets which got heated, flowed back out of the cowl and down the sides of the fuselage and entered his cabin air inlets where his OAT sensor was mounted, so that there was almost a constant OAT indicated regardless of altitude. This also took place on an RV-9 I tested! This didn't happen on either of these planes with their other props.

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elippse said:

On mine, Dick Shapley's -9, and Smith's -6, the OAT sensor is in the tailcone and shows no rise with speed.

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Ok...but in your previous post you said "cabin air inlet where his OAT sensor is located"...Which was the reason for my comment.
It is true that high speed airflow over the sensor induces an error, but that error is minuscule compared to the inaccuracy caused when builders locate the sensor so that it is exposed to the cabin environment on the inboard side.