Should I expect Lower Airspeed at Lower Altitudes?

I have a 6A with an IO_320 and a Catto. Up at 8K, I get 190 MPH under 8 GPH. Due to weather, I have had to make several flights around 2500-3000 (DA around 4K or less). I routinely fly LOP, but have avoided doing so above 75%. I have done some research and now understand that I can go to 80%+ power if I can keep to 60* LOP.

At these lower altitudes, I can't seem to get much more than 2500 RPM, which gets me 175 MPH. If I go LOP (30-40* LOP) at 8 GPH, I should be netting 75% power and this gives me the 2500 / 175. I can't seem to find a way to get more RPMs without richening up. The only way that I can get higher RPMs / speed is to richen it up, which gives me more power, requiring that I go more LOP and can't seem to get there from here. I have tried adding more MAP via the throttle, but without more fuel, it doesn't net more RPM.

Is this just an aviation fact of life at lower altitudes? I could go 150-200 ROP, but don't want to throw that much gas at it for another 10-15 MPH.

I would appreciate some education in this area.

EDIT: I should add that my prop is just a bit under-pitched. I generally like this as it lets me turn more RPMs up high and give me a bit better climb. At 8K I can turn just a bit over 2800.

Yes it's a fact of life.
Specifically, if you pick an engine power and stick to it (let's say 75%), then as you go up air density decreases so drag decreases while power is fixed. So you'll go faster as you go up. Until you get high enough that full throttle no longer produces 75% power. Further climbing will usually result in a slow decrease in TAS. So if you fix the power:
Full throttle: max TAS at sea level (100% power)
75% power: max TAS around 8000' (density altitude)
50% power: best TAS around 18000' DA
These are the approximate altitudes where you use full throttle and still can get the desired power.

I think I'm understanding your question correctly.

The gist is that more dense air equals more air hitting your airframe equals more friction. As you climb you lessen that friction but your normally aspirated engine will eventually start to suffer. So if you plot aircraft efficiency through the air relative to density altitude and engine performance on the same graph there will be a single point where your aircraft is most efficient (this chart is actually in most certified POH's).

Jamie said:

I think I'm understanding your question correctly.

The gist is that more dense air equals more air hitting your airframe equals more friction. As you climb you lessen that friction but your normally aspirated engine will eventually start to suffer. So if you plot aircraft efficiency through the air relative to density altitude and engine performance on the same graph there will be a single point where your aircraft is most efficient (this chart is actually in most certified POH's).

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Yes. It's the density altitude where full throttle barely produces best glide speed (indicated airspeed). It's also maximum range, if you disregard the fuel burned in the climb. At these low speeds my simple model (above) is no longer true, because now induced drag cannot be ignored compared to parasitic drag.

But I thought he was just asking why 75% power is slower at 2000' vs 8000'. The simple explanation for that is more air resistance down low.

Are we to assume you are asking true airspeed?

Thanks for the feedback. A follow up question is how much trouble am I looking for if I push it to 80 - 85% at peak EGT or maybe 10-20* LOP? My CHT's have been low (under 330 or 340, depending upon OAT) at these speeds and I am guessing the risk above 75% is detonation, which I should be able to monitor for. With a FP, I am just not seeing a way to stay more LOP and get above 2500 at these altitudes. I am ok accepting my current speeds, but would like to know if I can do better.

Larry

lr172 said:

Thanks for the feedback. A follow up question is how much trouble am I looking for if I push it to 80 - 85% at peak EGT or maybe 10-20* LOP? My CHT's have been low (under 330 or 340, depending upon OAT) at these speeds and I am guessing the risk above 75% is detonation, which I should be able to monitor for. With a FP, I am just not seeing a way to stay more LOP and get above 2500 at these altitudes. I am ok accepting my current speeds, but would like to know if I can do better.

Larry

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I think the concern here is actually internal cylinder pressure (ICP). At 85% power you would need to be somewhere around 80* LOP to avoid the highest ICP's, and thus imposing excess stress on your engine. This is the "Red box" or "red fin" that Mike Busch talks about: https://www.jpinstruments.com/wp-content/uploads/2013/05/Mike-Bush-Red-BoxRed-Fin.pdf

Once you are around 60-65% power or lower, you really can't impose damaging ICP's at any lean setting, so it becomes less of an issue.

Chris

The red box presentations are OK conceptually but engines are too varied for the presentations to be useful in detail. What is the width of your red box? Article does not and cannot say because it is a generalization. For instance, as the chart Dan has posted many times shows, on an 8.7:1 IO-360 angle valve there are some power settings that invite mild detonation. I have never seen a similar chart for any 8.5:1 or lower compression parallel valve (but would like to). There have been comments on the lower compression engines that one can't make them detonate.

The red box article tells you to use 400F CHT as a guide to finding the width of the red box but then hedge it by saying if you have a good cooling system then 400F does not apply (needs to be a lower temperature). Frustrating. Is my cooling system good? Is yours?

As a result, I run it like I stole it. I'll let you know if I melt a piston.

rzbill said:

The red box presentations are OK conceptually but engines are too varied for the presentations to be useful in detail. What is the width of your red box? Article does not and cannot say because it is a generalization. For instance, as the chart Dan has posted many times shows, on an 8.7:1 IO-360 angle valve there are some power settings that invite mild detonation. I have never seen a similar chart for any 8.5:1 or lower compression parallel valve (but would like to). There have been comments on the lower compression engines that one can't make them detonate.

The red box article tells you to use 400F CHT as a guide to finding the width of the red box but then hedge it by saying if you have a good cooling system then 400F does not apply (needs to be a lower temperature). Frustrating. Is my cooling system good? Is yours?

As a result, I run it like I stole it. I'll let you know if I melt a piston.

Click to expand...

This is true but the OP mentioned running at peak EGT at 85% power. It would seem to me that if there WAS a red box for his engine, this would be in it. I also believe it is about more than detonation. ICP causes more strain on internal components and could contribute to earlier wear / failure. FWIW, Lycoming also recommends against leaning to peak at over 75% power.

Because of the fact that I don't know for sure, I operate mine fairly ROP at higher power settings. I'm not disagreeing with your practice, just erring on the side of caution due to my own ignorance. I have a low compression engine so it's probably not an issue anyways but I don't know. Below 65% I lean as much as I can.

Chris

neat question

This is a neat question, and just so happens that I got a neat story from Lee Logan over the weekend about a similar situation. I wont go through the details, but I'm sure it applies to recips as well as jets.

Man, its great to be able to fly with a mentor that has the wealth of knowledge, and the experience to make the learning situation more fun.
Tom

rzbill said:

The red box presentations are OK conceptually but engines are too varied for the presentations to be useful in detail. What is the width of your red box? Article does not and cannot say because it is a generalization. For instance, as the chart Dan has posted many times shows, on an 8.7:1 IO-360 angle valve there are some power settings that invite mild detonation. I have never seen a similar chart for any 8.5:1 or lower compression parallel valve (but would like to). There have been comments on the lower compression engines that one can't make them detonate.

The red box article tells you to use 400F CHT as a guide to finding the width of the red box but then hedge it by saying if you have a good cooling system then 400F does not apply (needs to be a lower temperature). Frustrating. Is my cooling system good? Is yours?

As a result, I run it like I stole it. I'll let you know if I melt a piston.

Click to expand...

Bill, I am probably a little closer to your style. I have concerns over detonation, but not as much about ICP, though that can be a pro-detonation factor. Would you run 80% at Peak EGT?

Larry

YellowJacket RV9 said:

This is true but the OP mentioned running at peak EGT at 85% power. It would seem to me that if there WAS a red box for his engine, this would be in it. I also believe it is about more than detonation. ICP causes more strain on internal components and could contribute to earlier wear / failure. FWIW, Lycoming also recommends against leaning to peak at over 75% power.

Because of the fact that I don't know for sure, I operate mine fairly ROP at higher power settings. I'm not disagreeing with your practice, just erring on the side of caution due to my own ignorance. I have a low compression engine so it's probably not an issue anyways but I don't know. Below 65% I lean as much as I can.

Chris

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I do agree that higher ICP's could possibly produce more wear, but I don't really believe that this is a factor for the Lycomings. If that type of wear were prevelant, you would be seeing worn bearings, broken ring lands, cracked rings, broken rods, cracked cyl heads, etc. Higher ICPs do contribute to detonation though, as do high MAPs. Detonation does concern me and I have learned that as MAP goes up, so does fuel flow to avoid detonation.

On a side note, my experience shows that "load" more than MAP alone invites detonation. The Lyc detonation charts show this as well (low RPM 29" = detonation, high RPM 29" = no detonation). If you drove some of the 70's vintage production cars, you saw this. Floor it without downshifting (i.e. "lugging it") and you'll hear the marbles rattling in the engine, but once the RPMs come up, the detonation stops.

85% power does seem a bit high to be runnning stoichiometric, but then so does 75% from my experience. I am guessing the peak torque range on these engines is much higher than the 2500 RPM we are talking about here and that is why they don't detonate, as it's 75% of "rated" power and not 75% of the engines potential power.

I was just looking to see if others are having any success with this type of configuration. I question how much margin there is around the 75% number and how much risk I would be taking to experiment with it.

Larry

YellowJacket RV9 said:

Because of the fact that I don't know for sure, I operate mine fairly ROP at higher power settings. I'm not disagreeing with your practice, just erring on the side of caution due to my own ignorance. I have a low compression engine so it's probably not an issue anyways but I don't know. Below 65% I lean as much as I can.

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This is just about exactly what I do in my -6A. I have the Skyview system and I almost always pull back to 65% power or below in cruise. Tha way as long as my engine runs smoothe I know I can't hurt it (as from another s is another reason to have MAP indication on a fixed pitch plane) with the mixture knob. A couple of times I have tried to convince myself to go higher power to just get there, but the fuel flow goes up so much (partially because I don't like to run LOP over 65%) and mpg goes down so much that I end up pulling it back again.

I flew 5.1hrs on the Hobbs today in 3 legs, one alone with full tanks at 8,000', one with a not-very-tiny passenger and 50+lbs of bags at 2/3rds fuel at 9,000' and the 3rd shorter flight alone with full tanks at 7,000'. I burned a total of 36.7 gallons and cruised the whole time at 140-148ktas. I set to 65% power and then set the mixture knob for speed, more fuel and TAS into a headwind and less with a tailwind.

lr172 said:

Bill, I am probably a little closer to your style. I have concerns over detonation, but not as much about ICP, though that can be a pro-detonation factor. Would you run 80% at Peak EGT?

Larry

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Hmmm. First a disclaimer. My comments are what I do, not a recommendation. Use this information at your own risk.

So... While I have done this on a couple of occations it is rare. Mostly because of opportunity rather that desire. I run 75% oversquare peak any time can get enough MP at altitude. Otherwise I get what I get. I run low RPM (even oversquare) to lower friction HP losses and slow the piston speed as much as I can to compensate for slower burn time of lean mixture. This is as opposed to buying an EI. My CHTs run from 300 to 350 depending on cruise power levels and OAT so I guess my cooling system is good per the red box article measure. I will go over 75% on descent but it is going leaner at the same time so the power readout is not really valid.

I did play with "80% peak EGT" trying to see if it actually perforrmed (airspeedwise) at 77% ROP power values per the Lycoming "HP from fuel flow" document. I can't report any results because my data gathering was unplanned and horrible. I rarely fly low enough to do this testing because of the rocks.

If you guys don't mind a follow up question as it seems doing this at Peak is not common. If I choose to run 80-85% down low ROP, how rich do I need to be. I know the red box guys say 250 and that seems like a lot. When running at these percentages, what is a recommended ROP setting?

Larry

I run LOP about 80% of my flying time, but when I do run ROP - say, in the 80% range - I run 100 degrees ROP as a minimum. I have seen that recommendation in numerous places talking about safe operation as relates to mixture. I'm fully comfortable at any ROP power setting at 125 degrees rich.

Because my IO-320 tends to run very cool, I watch CHTs mostly, whether LOP or ROP. If their inordinately high (say 20 degrees above usual) I richen a bit until they come down. On the lean side, they're always around 25 to 30 degrees cooler than ROP. "Running lean is cool."