To Plenum, Or Not........

[RV10inOz]

Brian, I am very impressed with your speed results, and if you have actually achieved far less cooling drag, and gained some speed, and you can verify that, all at the same time as having reasonable and manageable CHT's you have hit a sweet spot.

Despite what your A&P said about too much info, data confirms that CHT's should be kept under 380, and that over 400 you really want to avoid this as much as possible. Remember the limits in the manual are certification limits, not where you should be running to if you want a long healthy engine life.

For the average punter, and I would say you are not, but the average guy is going to struggle. What we see is higher CHT's (less margin on hot and or heavy weight days) and we see no speed gain. In fact it could be said speed loss.

Guys like Dan Horton and yourself and a small group have probably cracked the sweet spot, but the vast majority are going to struggle.

When you say "good but not as good as standard" I don't understand. What is the goal you are trying to achieve. As cool as possible? Fast? Even spread?

What I am saying is that assuming most folk will not get a speed gain, or not significant, they should be focussed on making the cooling system deliver lower temps, ROP running around 350-360 max in the cruise is a good goal. Remember in a climb the aim is to keep them under 380F. When running LOP depending on altitude etc you would expect something like 300-330dF.

The standard baffles done properly achieve this. Anything less effective is obviously sub optimal. Is it going to kill your engine? No, but over the life of it it may make a difference to repairs or total life.

So summing up, unless you make significant gains over the standard baffles you are going backwards. For most people that is what needs careful thought. And the reason I think you should aim for the kinds of cruise numbers I am talking about is margin above left over for hot climbs and MTOW and not going over 380.


Jamie, you are a stirrer boy! Wait until you drop in later this morning, I'll set Dudley onto you

Tony, Ignore what the difference in EGT is, that is not critical, probe placement can do that alone. Your CHT's being within 20dF of each other is pretty good and typical. 350dF in the cruise is good, but only if that is ROP, a 10-20dF LOP cruise should yield a bit less. To make a value judgement more info is required.

% POWER on your EMS
To my knowledge, so far the only EMS that are on the market in the TSO world is the Auracle. I know why too, a former VAF contributor spent a lot of time helping them get their overall EMS package correct. In the experimental world the Dynon does it right, for a 8.5CR engine. I am not sure about the others like AFS and GRT but I suspect they may have either ROP or LOP calcs correct but not both. I would like to know if anyone has proof of it.

In simple mental maths terms to do a % power calculation in flight take USGPH x 15 (14.9 for most engines CR) but 15 is near enough. If you are ROP, then Mass Air Flow is the governing factor so a simple (and its rough but close enough) %age of MP x %age of RPM will do it. For example 25"/29.9 is 83.6% and 2430RPM is 2430/2700 or 90%. Answer is roughly 75% power.

In flight you could say .85 x .9 and you get 76.5%.....so who cares. Near enough

Brian, for your Dynon % power, go to your manual and find this section

E
ngine Type Configuration
W
ithin the GLOBAL menu, press DOWN▼ until you have selected ENGINE TYPE. Press SEL► to toggle between LY/CON, ROTAX, and OTHER.
ENGINE TYPE: LY/CON
Setting ENGINE TYPE to LY/CON enables a percent power display for normally aspirated/non-turbocharged Lycoming or Continental engines. Press DOWN▼ to select HP RATING and use the INC+ and DEC- buttons to set the value to the actual horsepower rating for your engine. Given the horsepower rating, an OAT, fuel flow, and altitude (from a connected EFIS-based product or GPS), the EMS-D120 will calculate percent power and lean-of-peak/rich-of-peak status, displaying both next to the Manifold Pressure gauge.
For percent power status to be calculated as accurately as possible, TACHOMETER (RPM) HI RED/YEL should be set to the RPM value specified by the Engine Manufacturer. (This is commonly referred to as ?Redline?).
I
f you do not have a fuel flow sender, the EMS-D120 will still display a percent power reading based on rich-of-peak calculations only. During lean-of-peak operation, percent power is displayed but is incorrect. While you are in LEAN mode, percent power is not displayed.

I hope that is helpful

[hydroguy2]

Thanks David. Following the Sport Air Racing League and folks like Bob Axsom, Dan Horton, and a few others pushed me to where I am. I knew many planes were leaving something on the table. Great all around performance but little things make a difference. I think there is another 3-5kts waiting for me to unleash. So, I have a few more things I want to implement, but my aviation budget is depleted these days.

Concerning the Dynon settings...I've entered the data things you mention, but must have an error somewhere. My engine was originally an O-360 180hp, but we rebuilt it with 9:1 pistons, AFP F.I. and Pmags, so figure it is closer to 190hp. What should I use for HP rating? Also not sure where I set the Tach red/yellow...I run a WW200 prop which is limited to 2600 continuous so may have set some parameters wrong based on that. I'll look tomorrow if I go to the hangar.

[Captain Avgas]

Efficiencies aside, the problem with full plenums is that they do not allow for easy inspection of the top of the engine when the top cowling is removed. There's potentially a number of items on top of the engine that are best monitored regularly including the fuel flow divider and associated injection lines, ignition coils, and plug leads. My guess is that with most plenum systems the top of the engine is inspected only at annuals.

Ease of access for inspection of an aircraft is in itself a safety factor.

[RV10inOz]

The Dynon is going to be a little confused on the LOP side of the curve, as it uses the fuel flow x 14.9 to calculate the actual HP and then as a percentage of what you enter as the full rated HP.

You are getting 15.47HP per USG.

For the ROP side of the curve, if you set your redline (RED/YEL) at 2700 and just set your Yellow/Green at the 2600 limit that will fix up the % power. On the ROP side the HP number you enter in the setup menu makes not difference. It is basically MP and RPM that dictate the numbers.

Now to get the LOP side somewhere near correct, I guess by basic numbers you are probably making 186 HP, and rough enough is good enough here, but to compensate for the higher HP/USG due to the higher CR, you need to fudge the system so set in 180 HP and see how it goes.

Get in flight, set a genuine LOP setting, and see what % it says, compare that to the USG x 15.47 (actual HP) compared to our guessed 186HP.

May not be perfect, but it should be damned close.

[DanH]

A lot of this conversation is confusing inlet performance issues (the conversion of dynamic pressure to static pressure) with leakage issues (allowing some percentage of cooling mass to bypass the hot parts). The confusion is natural as the two are linked; the method of connecting and sealing a plenum lid to the cowl inlets tends to affect inlet performance via choices of shape and size.

To the original question...a really well crafted set of silicone flap seals can seal as well as a so-so plenum. It may have better inlet performance; measurement suggests the Vans inlet performance is very good.

A really well crafted plenum will seal better than the best flap seals. Given equal inlet performance the effect will be to allow lower mass flows (allow a reduction in exit area) with the same total heat transfer....the exit air will be hotter.

Side note....if your CHT values are very low, even on warm days at slow speeds, you are to be congratulated. However, you're going a lot slower than you could go. Overcooling is, literally, a drag.....

[RV8R999]

Originally Posted by DanH

Side note....if your CHT values are very low, even on warm days at slow speeds, you are to be congratulated. However, you're going a lot slower than you could go. Overcooling is, literally, a drag.....

Absolutely! The question then becomes what basis determines the margin of "overcooling" needed to provide an educated, purposefully engineered reduction in exit flow required to minimize cooling drag? The answer, in my view, is the design, certified engine operating limits. While some would have you believe these limits are not valid I invite them to review 14 CFR Part 33.49
http://www.flightsimaviation.com/dat...art_33-49.html

Which specifies reciprocating engine endurance testing requirements. The most significant of which requires the engine to be operated at max rated power for a minimum of 35 hours with 1 cyl no lower than its max operating limit with the others no lower than 50 degrees below the limit. That is 1 cyl at or above 500 deg and the others at or above 450 deg for 35 hours at max rated power. The total endurance test is 150 hour upon which a tear down inspection must occur. See 14 CFR 33.57

http://www.flightsimaviation.com/dat...art_33-55.html

Upon conclusion of the test all parts must meet the design type specification - Which in FAA parlance means every part must conform to the approved drawings. This is the test for wear and conformity. Each part is measured and compared to the spec with data being tracked to determine wear rates which goes into the formula for determining TBO. In reality the major engine manufacturers conduct far more than the minimum 150 hrs during their development and have far more data than needed to substantiate their limits. The 150 hr test is conducted on what the manufacturer submits as their final conforming engine design at which point the specs are frozen and the test conducted. They KNOW what their engines are capable of with thousands upon thousands of controlled and well documented test hours.

This oft repeated notion CHT MUST remain below 380 value in climb, and 360 value in cruise for your engine to remain healthy is based upon anecdotal information and a bunch of clever marketing. However, if it makes you feel good it can't hurt

However, no matter what your belief about CHT limits, physics doesn't lie and the cooler you can operate your engine the more margin you have (against whatever limits make you happy) to reduce the exit and recover some of the lost momentum thereby increasing your speed.

Dan and I have both measured our Upper Plenum pressures using standardized techniques and equipment. He has a covered plenum with round inlets and I the standard Van's seals and inlets. Our upper plenum pressures are virtually identical through our entire speed range. Our lower plenum pressures vary based upon our exit sizes (at the time his was fixed and mine variable). Based upon our data it is my view when done well both offer nearly identical pressure recovery. The covered plenum has some advantages: Should be easier to do well, relieves stress upon the upper cowl hinge line, and probably requires less care and maintenance over time.

We plan to publish our full results in the future, but probably have more testing to conduct. We were hoping to get a few more folks with stock installations to participate, specifically parallel valve 0-360 with stock inlets and baffles. Any takers? Between Dan and I we can supply you with probes, test equipment, our test methodology and data analysis - FREE

[Toobuilder]

Originally Posted by DanH

...Side note....if your CHT values are very low, even on warm days at slow speeds, you are to be congratulated. However, you're going a lot slower than you could go. Overcooling is, literally, a drag.....

This is the boat I'm now in. With 20 or so hours since my cooling mods, I'm @300 or less in cruise on all cylinders. Climb is still pretty warm, but as soon as I pushover and go LOP, the temps just plummet. Still have some summer flying to verify, but I think this is pretty compelling evidence that I need variable geometry on the outlet.

It's time to trade in some of that cooling margin for more speed!

[DanH]

Originally Posted by RV8R999

Dan and I have both measured our Upper Plenum pressures using standardized techniques and equipment. He has a covered plenum with round inlets and I the standard Van's seals and inlets. Our upper plenum pressures are virtually identical through our entire speed range.

Again, lets not confuse inlet performance with what kind of lid closes the upper deck plenum. Ken is telling you the stock Vans inlet does a good job of converting Q to static, at least as good as the low Vi/Vo round inlet on my cowl.

However, equal upper plenum pressures does not necessarily mean equal leakage rates. Despite equal pressures we're recording significant differences in cooling efficiency, a measure of how much heat is being transferred to the cooling mass on its pass through the cowl. At this time we don't know how much of that difference is due to leakage (bypassing the hot parts), how much is due to the difference in fin area between Ken's parallel valve motor and my angle valve (less area of hot parts), and how much is due to differences in baffling (how closely and how long the cooling mas is kept in contact with the hot parts).

A really good plenum lid affects only leakage. That said, I remain convinced that a good plenum lid seals better than the best flap seals....and way, way better than average flap seals. That much is found in NASA CR3405.

Always something more to learn.

[tcone1]

[quote=RV10inOz;738254]Why go to all that trouble and expense and have higher CHT's? Unless you gained 10+ knots in cooling drag reduction I would argue it was not worth it.[quote]

I claim no personal genius on my setup...I adapted Sam James stuff onto a stock cowl. Maybe I'm the luckiest guy in the world because it just worked right out of the gate. I didn't even bother sealing stuff too well until I decided to run at the Reno Air Races.

I don't know about 10 knots faster, but my poor little ole stock, carb'd 8.5:1 o-360 was able to pull my -8 around the pylons at Reno at 218+ mph on a 8000' DA day. Back in phase 1 flight testing, about 900 hours ago, she'd do 193 KNOTs at Sea level using the NTPS 3 way gps test method.

The plenum did add about a week of build time and does add about 10 minutes to the time required to remove the top plugs. Remove 14 screws and viola, it's off.

Zero cooling issues means...it's never gotten hot despite living in a very warm climate. I saw 410 degrees once while taxing out for the third flight of the afternoon on a 108 degree afternoon. By 1,000 feet agl in the climb, the cht was back down below 360.

My oil cooler is on the baffle behind #4 cylinder...oil temp has never been above 210 and must be blocked in the winter to get temps above 180.

My experience with this plenum has been a net positive. I enjoyed adding it to the airplane, and it has more than paid me back in terms of increased cruise speed and hassle free operations.

[RV8R999]

Originally Posted by DanH

That said, I remain convinced that a good plenum lid seals better than the best flap seals....and way, way better than average flap seals. That much is found in NASA CR3405.

Always something more to learn.

With regards to inlet performance...lets not forget the upper plenum is also part of the inlet system from a thermodynamics point of view. Factor affecting pressure recovery efficiency includes the shape, size, radius of the inlet opening as well as the transition to and upper plenum volumes.

Concur wholeheartedly... In the standard baffling the upper cowl is required to absorb the load developed in converting dynamic pressure to static. Our cowls are flexible and only supported around the perimeter thereby allowing deformation when pressurized. Evidence is found in scalloping along the upper cowl perimeter during high speed flight - Dan has accurately described this as akin to blowing up a balloon. At 165ktas we measured about 14.25 inH2O which is about .51 psi. The upper plenum area is about (very rough mental measurement here) 360 square inches. This results in a force of about 186lbs exerted upward on the upper cowling. As the cowl deforms the standard baffles will not be as effective. Additionally, by their nature they rely upon pressure for a good seal which really means during those high power relatively low speed climbs they are at their worst condition for a good seal. The enclosed upper plenum, when designed appropriately, carries all the load developed in the pressure recovery process leaving the cowl to carry loads created by the lower plenum pressure which are much reduced.

As Dan mentions, the difference in engine cyl fin area between the angled (his) and parallel (mine) makes data difficult to compare for overall efficiency from a plenum vs baffle perspective. The angle valve has a significantly larger total fin area than the parallel valve which given the exact same baffle or plenum setup should result in better cooling with potential for even better drag reduction.