[Tony Partain]

I have a question for all of the engineers and performance buffs. Say your plane is a 180 hp engine with a CS prop. It cruises at 200 mph at 7500' on a standard day. With this standard day you are showing 21.5" of mp. How much speed would you gain by increasing your manifold pressure by .5"? This would be done without increasing drag on the airframe.

 

[mgomez]

I'd expect about 1.5 mph more.

I did it as follows: I took the ratio of 22/21.5. That's roughly how much the power will increase by (about 2%). Then I took the cube root of that, which gives about 0.7%. That's the speed increase.

 

[Tony Partain]

A .5" increase in manifold pressure would be a considerable amount of performance. I did a test today and found that my intake is adding .3" in manifold pressure at 200 mph. (the airport manager wasn't very happy with my 200 mph fly by 20 feet over the runway but he'll get over it) I am going to toy around with a new system to try to get a larger number. Time to start working. 1.5 mph would be worth the time and effort.

 

[gmcjetpilot]

Cool how did you do your MAP rise check

A .5" increase in manifold pressure would be a considerable amount of performance. I did a test today and found that my intake is adding .3" in manifold pressure at 200 mph. (the airport manager wasn't very happy with my 200 mph fly by 20 feet over the runway but he'll get over it) I am going to toy around with a new system to try to get a larger number. Time to start working. 1.5 mph would be worth the time and effort.

How did you measure your RAM rise?

What air box , engine and induction do you have? Thanks

It sounds like you where using the difference in the ambient pressure on the ramp to that when you did your fly by, which is cool. That is correct.

It is a pet peeve when I hear folks claiming (and honestly believing) they are getting 1" or more rise. 0.30 at 200 mph is excellent and just wounder what your set up is.

TO answer the question the calculation the other Gent made sound about right. George

 

[Tony Partain]

I measured the ram rise by doing a fly by and recording the mp at 20 feet over the runway doing 200 mph. Then after taxing back to the hanger which is in the high end of the field shut the engine down and recorded the difference.

My air box is a modifyed version of Vans fiberglass unit. It is a carbon fiber unit with an oversize skeet tube running to the intake. Basically its a rectangular funnel to the sceet tube. I can post a picture on Monday after I get into the office.

After thinking about the difference in cylinder temperatures, I see that many planes have a air dam on the front cylinder to make it run in sync with the other temperatures. My though is to make a scoop that will block the air from this cylinder to bring it into line with the other temps plus possibly add to the mp rise.

Now with this said there will be a couple of concerns. First the opening of the scoop can be no smaller that the area of the sceet tube. Easy calculation. But it can't rob so much air that it effects the cooling on this bank of cylinders. Sounds like several prototypes to construct.

 

[mgomez]

I measured the ram rise by doing a fly by and recording the mp at 20 feet over the runway doing 200 mph. Then after taxing back to the hanger which is in the high end of the field shut the

Tony,

I'm not sure I understand. You're making one measurement with the engine running, and the other with the engine shut off?

I thought the way to do this is to make a measurement at speed, and another (with the same throttle setting and RPM) while stationary.

What am I missing?

Cheers,
Martin

 

[gmcjetpilot]

Thanks and yes

I measured the ram rise by doing a fly by................. taxing back to the hanger..............recorded the difference.

My air box is a modifyed version of Vans fiberglass unit...........I can post a picture on Monday after I get into the office.

After thinking about the difference in cylinder temperatures,...........air dam on the front cylinder............sync with the other temperatures.

Now with this said there will be a couple of concerns. First the opening of the scoop can be no smaller that the area of the sceet tube. Easy calculation. But it can't rob so much air that it effects the cooling on this bank of cylinders. Sounds like several prototypes to construct.

LOVE to see the pictures.

Yes your method is perfect and the only error is small MP gauge error.

Anything you can do to balance temps is good. I can help there, but not sure what you mean about the scoop.

This shows folks that the Van airbox installed properly can produce RAM and you don't need a forward facing induction to make it go.

Tony,
I'm not sure I understand. You're making one measurement with the engine running, and the other with the engine shut off?

I thought the way to do this is to make a measurement at speed, and another (with the same throttle setting and RPM) while stationary.

What am I missing?

Cheers,Martin

Right Martin think about this. What is the ambient pressure sitting on the RAMP? Lets say it is a standard day 29.92 inches of mercury (in-Hg) ( assume sea level, 59F) . OK, you read that on the MP gauge in the cockpit before start, right. Now start you engine and MP will be way less than 29.92 in-Hg. With the throttle at idle the MAP will be down in the low teens. On take off open the throttle wide open and the MP rises because there is less restriction. You are not moving fast so there is no "RAM" so you see the MAP gauge reads about 1 in-Hg less than ambient, say 28.8 in-Hg, at full throttle. Now do as Tony did, come back around and fly down the runway at 20-50 feet, 200 MPH, read say 30.20 in-Hg on the MP gauge. Since the ambient is 29.90 approx, the difference is 0.30, so you are getting a TRUE ram rise (ram air-pressure) above ambient.


Stock Cessna or Piper run a minus 1" all the time and a good system, like Tony has is about 1/4" plus is all you can expect over ambient, but Tony is doing a little better.

To get any ram is a real good trick. The traditional way to get POSITVE pressure in the induction is a turbo or super charger. The total RAM of the air craft going 200 MPH at sea level is 1.42 inches of Hg, so why not try to an get some of that dynamic pressure in the induction. Most scoops are hard pressed to have the ability to convert the velocity efficiently into static pressure, so you are going to loose 1/2 to 2/3-rds right there. Also you have interference of airflow from the prop affecting the way the air enters the scoop. If there is any non-smooth or abrupt transitions in the scoop and down stream "diffuser" you get internal turbulence which causes more losses. The filter creates about .25 in-hg loss, but for a Carb it helps straighten the airflow out going into the Carb's venturi throat. That is why I am surprised Tony is getting 0.30 in-Hg rise, I assume with a filter. Most filter "RAM" air systems run about NET. BTW net is GODD. You are overcoming the plus (1)in-Hg vacuum the engine is making. A 0.30in-hg gain is real good with an air filter. Lesson is keep the filter clean.

Now a piston engine is not like a jet engine. The intake flow starts, stops and pulses with each cylinder, especially in a 4-banger. So the trick is to get the correct induction pressure at the time that Jug is "sucking". Some have tried to time the prop pulse by running the induction scoop a fraction of an inch near the prop. Note however the area near the spinner is a bad place to get air for several reasons.

Bottom line the method of measuring the difference in the manifold and ambient pressure is the RAM. You can estimate the ambient pressure at altitude by knowing the corrected pressure and the temp at your altitude, but the way Tony did it is about as pure as you can do it. Also at altitude there is less RAM to be had.

The mistake people make is they are cruising along and they pull the alternate are and see a 1" drop and go, cool I am getting a 1" RAM rise. Well you really are only measuring how inefficient your alternate air is, usually a flap that sucks air from inside the hot cowl. Makes sense? Also as you fly higher and your indicated airspeed goes down, guess what, your RAM goes down with the air density. Since RAM is what makes your pitot and airspeed indicator move you know what I mean. If you can get net or ambient MAP in cruise you are doing good. Like I said Cessna's and Pipers are at least minus 1" to 1.5" from ambient. George

 

[Tony Partain]

If you go to the RV of the week you will see a side shot of the intake system. The air filter is the basic setup from Vans.

Vans setup works great, their system make very good numbers without the drag of a snorkel on the outside of the cowl. I would love to see if there is any boost from the stock setup. It seem that it would.

As far as my rise in mp, a rise of .22 in the right place on the scale could possibly register .3 on the readout of the guage since its digital.

I hope to make a prototype this week and try it out this weekend.

 

[Tony Partain]

One more thing, My baffling is not flat from the cylinder to the cowl. It has a bend in it about 1" from the back of the filter. This bend angles down and flexes the filter down in the front and presents more frontal area to the air stream. Instead of the air flowing over it in parallel, its hitting the filter at more of an angle than normal. Hmmmm.

 

[gmcjetpilot]

Wow dude nice

One more thing, My baffling is not flat from the cylinder to the cowl. It has a bend in it about 1" from the back of the filter. This bend angles down and flexes the filter down in the front and presents more frontal area to the air stream. Instead of the air flowing over it in parallel, its hitting the filter at more of an angle than normal. Hmmmm.

I SEE YOUR PICS, SWEET DUDE

Yes , I would not screw with it, if you are getting the numbers you are getting you are doing real good. Like I said guys try to put a short little tube facing forward with no filter. Yes it works and yes no filter helps. But due to the lack of room the tubes are nothing more than a toilet paper roll tube looking affair. No volume and really create a lot of air spillage on the outside of the cowl, which creates aerodynamic drag, but I don't have to tell you that. Looks like you hit the design right on. The only thing I would change to get 0.001% better efficiency would be change the SCAT to smooth inner tube. It may help, maybe not? May be worth an experiment. The only thing to be careful of the flex tube must be of a kind that it can NEVER collapse. The thing is anything you do will be small and hard to measure, so go back to plane A and stop screwing with it, fly safe and have fun. Very NICE plane and Love every choice you made, just about perfect. The only negative is it is not in my hanger. G

 

[Andy Duff]

MP vs. Altimeter Setting

I think the altimeter setting relates to a sea level pressure. Therefore, if your actual altitude (field elevation) is greater than sea level, than your MP will read less than the altimeter setting.... right?

 

[Tony Partain]

The thing is anything you do will be small and hard to measure, so go back to plane A and stop screwing with it, fly safe and have fun. Very NICE plane and Love every choice you made, just about perfect. The only negative is it is not in my hanger. G

I have been flying quite a lot but experimenting... is a big part of the puzzle. Keeps the mind fresh!

 

[osxuser]

If you can, make a composite duct (with a smooth inside) instead of the scat, then just join it with some scat on one side.

I've been educated recently on the aerodynamic challeges of making a decent intake with pressure recovery. And I must say that is the best setup i've seen recently for simplicity (and it works ). The fact that you can still run a filter is impressive as well. It reminds me of the setup on a FG cardinal quite a bit.

The main problem people with Fwd facing run into is room to make a decent plentum to collect the static pressure. You basically use the high pressure side of the cowl to accomplish this.

 

[mgomez]

Right Martin think about this. What is the ambient pressure sitting on the RAMP? Lets say it is a standard day 29.92 inches of mercury (in-Hg). OK, you read that on the MP gauge in the cockpit before start, right. Now start you engine and MP will be way less than 29.92 in-Hg. With the throttle at idle the MAP will be down in the low teens. On take off open the throttle wide open and the MP rises because there is less restriction. You are not moving fast so there is no "RAM" so you see the MAP gauge reads about 1 in-Hg less than ambient, say 28.8 in-Hg, at full throttle. Now do as Tony did, come back around and fly down the runway at 20-50 feet, 200 MPH, read say 30.20 in-Hg on the MP gauge. Since the ambient is 29.90 approx, the difference is 0.30, so you are getting a TRUE ram rise (ram air-pressure) above ambient.

OK, I misunderstood the metric. You guys are quoting the delta above AMBIENT pressure, and I thought you were trying to measure how much of the stagnation pressure was actually being recovered. Your metric includes both losses in the inlet and ram recovery.

My mistake.

 

[Kevin Horton]

I think the altimeter setting relates to a sea level pressure. Therefore, if your actual altitude (field elevation) is greater than sea level, than your MP will read less than the altimeter setting.... right?

Correct. You can have an altimeter setting of 29.92 at Denver, if you happen to have a "standard" day. The actual ambient pressure will be a lot less than that.

 

[Tony Partain]

Update on test

The picture will explain why the test is postponed. I have plenty of time to work the plane due to the lack of good flying weather! We received a total of 18 inches of snow.

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