Scoop vs snorkel

[DanH]

Continued...

The traditional quick check of your MP gauge is to set field elevation, note the value in the Kollsman window, and subtract 1" per 1000 feet above sea level. For example, at 08A I'd set 200 feet, then subtract 0.2 from the Kollsman. In theory, if the Kollsman said 29.92, the MP gauge should read 29.72.

Like any rule-of-thumb, it's not super accurate, just close. The answer will be as much as 0.1" Hg too high below 6000 feet, and could be worse if your altimeter is out of calibration.

I dug around a bit this afternoon. Here's a simple web calculator which will work on your phone, at the airport. Seems to be pretty accurate.

http://www.csgnetwork.com/stationpressurecalc.html

Input the altimeter setting from your best local source and the altitude where your airplane is parked. Tap calculate, and compare "station pressure" to your MP gauge, engine off. The calibration risk is the accuracy of your local FAA equipment.

[RV8JD]

Originally Posted by DanH

Continued...

The traditional quick check of your MP gauge is to set field elevation, note the value in the Kollsman window, and subtract 1" per 1000 feet above sea level. For example, at 08A I'd set 200 feet, then subtract 0.2 from the Kollsman. In theory, if the Kollsman said 29.92, the MP gauge should read 29.72.

Like any rule-of-thumb, it's not super accurate, just close. The answer will be as much as 0.1" Hg too high below 6000 feet, and could be worse if your altimeter is out of calibration.

I dug around a bit this afternoon. Here's a simple web calculator which will work on your phone, at the airport. Seems to be pretty accurate.

http://www.csgnetwork.com/stationpressurecalc.html

Input the altimeter setting from your best local source and the altitude where your airplane is parked. Tap calculate, and compare "station pressure" to your MP gauge, engine off. The calibration risk is the accuracy of your local FAA equipment.

Thanks Dan. I'll check mine tomorrow.

[RV8JD]

Originally Posted by RV8JD

Thanks Dan. I'll check mine tomorrow.

I checked my (Dynon) MAP sensor today. It reads high by 0.45" HG. That is bigger error than I would have expected.



The Dynon MAP sensor is a repurposed auto sensor, if I'm not mistaken:

https://www.steinair.com/product/sen...fold-pressure/

So I reduced the MAP values in Post #17 by the 0.45" HG delta, but the numbers still seem a bit high, based on your comments in Post #19.

Here are the revised data points. RV-8, YIO-360-M1B, 1 Mag & 1 EI, Snorkel, at WOT:

21.5" MP, 2450 RPM, 9500' MSL, 10771' DA, 208 MTAS/181 KTAS.

20.0" MP, 2440 RPM, 11500' MSL, 11985' DA, 205 MTAS/178 KTAS.

[DanH]

Originally Posted by RV8JD

I checked my (Dynon) MAP sensor today. It reads high by 0.45" HG. That is bigger error than I would have expected.

So I reduced the MAP values in Post #17 by the 0.45" HG delta, but the numbers still seem a bit high, based on your comments in Post #19.

Here are the revised data points. RV-8, YIO-360-M1B, 1 Mag & 1 EI, Snorkel, at WOT:

21.5" MP, 2450 RPM, 9500' MSL, 10771' DA, 208 MTAS/181 KTAS.

20.0" MP, 2440 RPM, 11500' MSL, 11985' DA, 205 MTAS/178 KTAS.

Yes. Still too high. Why? Hmm. Maybe I'm missing something here.

(Review intake tract data)

Duh...I was sloppy. Sorry.

The 1.06" Hg (14.4" H2O) tract loss quoted previously (deltaP, airbox prior to filter vs MP at #3 primer port) was a value for 2500, WOT, derived (as a cross check) from two other deltas. However, I also measured the airbox/MP deltaP directly, at three different altitudes....and tract loss decreases with altitude. That detail would close some of the gap.

Here's a link to the original spreadsheet record:
https://www.danhorton.net/VAF/Scoop%20vs%20Snorkel/

Note Line 31. It's what I grabbed in a hurry while slopping down some coffee before work.

Now look at Line 18, the measured tract pressure loss for three altitudes.

Extrapolate from the 6500 and 10500 loss, and tract loss is roughly 9.2" H2O at 9500 and 8.4" H2O at 11500. That's 0.68" Hg and 0.62" Hg.

9500 ft static is 20.98..call it 21.0. Factoring density, Q is 1.13. 80% of Q is 0.9. 21.0 plus 0.9 = 21.9. 21.9 less a tract loss (mine, not yours) of 0.68 is an estimated MP of 21.2. You're reporting 21.5. Difference is 0.3" Hg

11500 ft static is 19.41..call it 19.4. Factoring density, Q is 1.05. 80% of Q is 0.8. 19.4 plus 0.8 = 20.2. 20.2 less a tract loss of 0.62 is an estimated MP 19.58. You're reporting 20.0. Difference is 0.42".

Right now I don't have an explanation for why you're still higher than the estimates. There are quite a few variables. That said, your new corrected MP's are a lot closer to reality, as are Steve's.

Steve, had a chance to check calibration?

BTW, as a reality check on the estimates, here are panel shots from a trip to PA last week. Note these MP figures are for a horizontal ram airbox, not a snorkel.

9500, peak EGT, WOT. Static would be 21.0 Including adjustment for 23F above standard, recovered Q would be 1.174 x 75% = 0.88 (the 75% is computed from the spreadsheet). The total before the filter is thus 21.88". The tract loss was 0.68", so the estimated MP would be 21.2". I'm actually showing 21.3, pretty close.



11500, 60 LOP, WOT. I show 19.5" at 180 KTAS. Static would be 19.41. Including adjustment for 30F above standard, recovered Q would be 1.03 x 75% = 0.77". The total before the filter is thus 20.18". The tract loss was 0.62, so the estimated MP would be 20.18 total less 0.62" tract loss = 19.56" I'm showing 19.5"

[wnplt]

Dan...

I?m the same MP as you at 9500. A little higher at 11,500. Not sure why my KTAS is only 165 to your 185? What engine and prop do you have? I know the 8 is a little faster than a 7 but dang you are smoking...

[DanH]

Thought about it, poked around in some papers. Let's assume Carl and Steve are reporting very accurate manifold pressure. That being the case, why would MP be higher than the estimates I provided? The obvious answer is one of the inputs to my estimates is wrong. The most likely potential culprit is the discount I've applied to Q, dynamic pressure. For both estimates I factored Q by 0.8, because for a good cowl, it's a reasonable estimate of Cp, coefficient of pressure, as defined in CR3405:

Cp = (local pressure at measurement point less freestream pressure) / available dynamic pressure

For example, if average plenum pressure is measured at 21.8, freestream pressure is 21.0, and available Q is 1.0, we would have:

Cp = (21.8 - 21) / 1 = 0.8

A factor of 1.0 (full dynamic pressure available at the face of snorkel filter) would add about 0.2" Hg to the snorkel estimate, bringing it closer to what Carl and Steve report. Is it possible?

Answer is maybe. The 0.8 factor is based on average plenum pressure. However, in reality the upper cowl pressure varies a bit depending on exactly where it is measured.

This is an upper cowl pressure survey from CR3405. Cp is as noted above. Note that in the low AOA cruise condition, Cp approaches or even exceeds 1.0 within the intake passage, right where the snorkel filter is located.

(Side note...the very good Cp in the climb condition reflects how well a large outboard inlet can harvest the dynamic pressure in propeller outflow.)



Returning to our OP's question (which is better, snorkel or scoop?), what we have here is a suggestion that a snorkel can outperform a horizontal ram. It's only a suggestion, because it rides on data taken with a cooling inlet unlike the stock Vans cowl inlet. If someone would like to actually measure static pressure at the face of a snorkel filter, I would be happy to help. Obviously I find this stuff interesting

[DanH]

Originally Posted by wnplt

Dan...

I’m the same MP as you at 9500. A little higher at 11,500. Not sure why my KTAS is only 165 to your 185? What engine and prop do you have? I know the 8 is a little faster than a 7 but dang you are smoking...

The same MP at the same altitude (assuming the same OAT), again suggests your reported MP is either (1) too high for reality, or (2) the snorkel's inlet location in the left cooling air ramp has a higher Cp. (see previous post).

Why? Available dynamic pressure is a function of density and velocity squared. You simply don't have as much dynamic pressure available at 165 KTAS as I do at 185 KTAS. In order to show the same MP, your gauge would be wrong, or you're capturing a higher percentage of the available dynamic pressure.

To be realistic, there some other possibilities, but first we should confirm or deny the pressures available with the snorkel.

My RV-8 has a 390, a tip-over fastback bubble, and some cooling drag reductions. In recent years my interest has been less about top speed, and more about tweaking for cheap cruise without losing much TAS. In the context of this thread, readers should note that it has a horizontal ram intake; a 4" dia inlet ring leading into an airbox with a large area filter, then an FM200. The rest of the intake tract (I think) is nearly identical to the M1B parallel valve engines you and Carl are flying. We should confirm that detail, as it may have an effect on tract pressure loss between the throttle and the intake valve.

[RV8JD]

Dan,

Thanks for all the data and info. Still chewing on it. BTW, I have the AvStar FI, if that makes a difference.

I'm surprised the Dynon MAP sensor in my airplane is so far off.

[chaskuss]

Carl & Steve,
You might want to read the thread linked below to learn more about some of the mods Dan has made to his 8s FWF. See

http://www.vansairforce.com/communit...d.php?t=173899

Charlie

[BillL]

As a data point,

10038' AltB, 8.1C OAT (+23.4F) 159 KTAS (yes, it is slow)

Ambient from Chart = 20.55 in-Hg absolute
Dynamic pressure = .8642 in-Hg
MAP from G3x = 20.7 in-hg
Pressure drop = .71 in-Hg or 9.72 in-H2O

Note: MAP not calibrated with static ambient . . . . yet.