AeroVee performance issues

[mike.smith]

You may have alluded to one of the problems. When I got rid of the Nikasil cylinders I think it took about 30 to 40 hours for the temps to get back to "low" They would still hit 400 to 410 on climb even solo on really hot days. When I was normally aspirated 390 was a good day climb temp at 80 OAT. Solo climbs were still around 600 fpm though with a Prince prop. Another thing I did was look at the bottom opening and realize the engine mount crosses the firewall right above the exit, I added about 1.25 inches to the opening to compensate for this and saw a slight reduction in temps. I think I suffered from too much anxiety over some things that breaking in the engine fully solved. Trying to cool a new(er) engine is just impossible until it is ready. As far as the climb performance I bit the bullet and went turbo. All My problems with cooling and climb went away and I rarely see climb temps above 380 now. It's a $4000 fix but to me was worth it, the Jab was a lot more than I was willing to go at the time.

I actually have about 35 hours on the new cylinders, not the 20 I stated. The turbo is certainly a possibility. Of course I'll need to find hangar space that costs less than $500/month, first! I know VW never published max CHTs for these engines, but I feel like 410 deg is not a good way to treat these aluminum heads. I know a VW is not a Lycoming or a Continental, but those are aluminum heads too, and to quote Mike Busch:

Both TCM and Lycoming specify CHT limits (460°F and 500°F, respectively) that should be considered emergency limits, not operational limits. Allowing your CHT to get anywhere close to those values for significant periods of time will most likely result in premature exhaust-valve problems and increased incidence of cylinder-head fatigue cracking. I do not like to see CHT above about 400°F, which is the temperature at which the aluminum alloy from which your cylinder head is made loses one-half its tensile strength. (The strength decreases rapidly as the temperature rises above 400°F.) For legacy aircraft, I recommend a maximum target CHT of about 380°F just to provide a little extra cushion, and consider any CHT above 400°F to be grounds for "doing something right now" to get it down.

and

... a common misconception that burned exhaust valves are caused by high EGTs. This is not correct. Burned exhaust valves are caused by valve-guide wear and valve-stem wear, and the best way to keep that from happening is (1) to keep CHTs down, and (2) to run a lean mixture to minimize build-up of combustion byproducts on the valve stem. The leaner you operate (while keeping CHTs at prudent levels), the happier your exhaust valves will be.

So I like to set 390 as my warning area, and 400 deg as my upper limit, while striving to keep the CHTs at 380 or less whenever possible. Call me a granny, but those are my feelings about it.

[gammaxy]

- Ever since then I usually average about 4.5 gph. So to me, 4.09 gph is too lean and may be part of my heat problems. It can't hurt to check so I'll richen the mixture a bit and do some tests.

The very least fuel burn I ever saw was over 4.75 gal/hr in cruise flight at altitude with the mixture leaned severely. Making power I always saw over 5 gal/hr.

Sonex says their factory planes average 4 to 4.5 gph, so my usual numbers seem to be in line with that.

An 8:1 compression ratio engine like ours gets about 14.3 hp per gph fuel flow lean of peak (which I believe would be best case for us since most of us are unable to operate LOP). 4.5 gph corresponds to about 65hp (14.3*4.5). I often fly WOT for the entire flight and burn over 5 gph--especially if I'm flying low. Probably not difficult to go as high as almost 5.5 if you are a little too rich or flying at low density altitudes.

The 4-4.5 gph number is a reasonable average for typical local flights where due to takeoff and landing you spend a significant portion of time operating lower than 80hp.

[radfordc]

I really wish I understood how Sonex gets 150mph at 8000 feet.

Check out Jeff Shultz's chart showing performance numbers for his plane/engine: http://www.sonex604.com/percent_power.html

He shows a TAS of 151 mph at 8000 ft.

Also note that at low altitude turning 3200 rpm requires 5 gal/hr. This is in line with what I used to see.

[gammaxy]

I really wish I understood how Sonex gets 150mph at 8000 feet.

Check out Jeff Shultz's chart showing performance numbers for his plane/engine: http://www.sonex604.com/percent_power.html

He shows a TAS of 151 mph at 8000 ft.

Also note that at low altitude turning 3200 rpm requires 5 gal/hr. This is in line with what I used to see.

That's a nice chart and the numbers seem correct near sea level (maybe even slightly slow), but I don't believe the values at higher altitudes are correct. He doesn't appear to make the claim that the higher altitudes were measured, so I think there is a math problem.

When you fly higher, the air is thinner, so there is less drag, but there is also less power available. These effects tend to cancel each other out so you end up close to the same speed, but then you have to fly at a higher angle of attack in the thin air to generate lift so drag is greater and the airplane flies slower.

A lot of airplanes are RPM limited and/or mixture limited at low altitudes, so there can be an increase of speed as you climb, but I've never experienced that in my Sonex since I happily fly at WOT at any altitude.

Here's some math:

The power required to overcome drag is: Power required = 1/2 * air density * velocity^3 * drag stuff
If I make the same assumption that drag area doesn't change when we climb to altitude, then 1/2 * (drag stuff) can be treated as a constant, C, when comparing performance at different altitudes.

Here, I solve for C:
76HP = C * (air density @ SL) * (130 TAS @ SL)^3
C = 76HP / ( (air density @ SL) * 130^3)

Now, I use C at 8000 feet to solve for the expected velocity due to reduced drag and engine power when the air is 0.77 times as dense as sea level air:

76HP * 77% = C * (air density @ 8000) * V^3
76HP * 77% = 76HP * (air density @ 8000) / (air density @ SL) * (TAS @ 8000)^3 / 130^3
(TAS @ 8000)^3 = 76HP * 0.77 * 130^3 / (76HP * 77%)
(TAS @ 8000)^3 = 130^3
TAS @ 8000 = 130

Notice how the decrease in engine power cancels with the decrease of drag to leave the expected TAS at 8000 feet the same as sea level. Trouble is, the lift generated will be 77% as much at that speed due to the thin air, so the angle of attack will need to be higher (more drag) and the airplane will fly even slower.

[radfordc]

You math wizards will have to have a discussion among yourselves.

I do know that I admire Jeff's work and I put a lot of trust in the Sonex guys, too.

There are plenty of references about TAS and they all seem to say that TAS is faster as you go higher.

http://stoenworks.com/Tutorials/Underst ... speed.html

https://www.vatsim.net/pilot-resource-c ... -airspeeds

http://www.pprune.org/archive/index.php/t-518735.html

[gammaxy]

You math wizards will have to have a discussion among yourselves.

I do know that I admire Jeff's work and I put a lot of trust in the Sonex guys, too.

There are plenty of references about TAS and they all seem to say that TAS is faster as you go higher.

http://stoenworks.com/Tutorials/Underst ... speed.html

https://www.vatsim.net/pilot-resource-c ... -airspeeds

http://www.pprune.org/archive/index.php/t-518735.html

The difference between TAS and IAS does increase as you go higher. Jet engine develop more power at faster speeds and really do fly significantly faster at higher altitudes with less drag. Normally-aspirated engines don't, and lose power at about the same rate as the decrease in drag. Mixture settings and propeller limits combine to make it possible to fly a little faster at moderate altitudes, but I don't think people fly the Aerovee anywhere close to RPM redline.

I somewhat regularly end up as high as 12,000 feet and have never noticed 20mph extra at 8000 feet. Has anyone else? I'd be happy to learn that someone does see this effect, because then I have room to figure out how to fly/improve my airplane.

I don't mean to hijack the thread unless this conversation is interesting to anyone. I'm pretty happy with the performance of my Aerovee and think Mike would be happier with his if we could figure out what the problem is.

[sonex1374]

Hi Chris,

Thanks for your interest in my power chart! Developing a useful reference is never easy, and there are a bunch of variables that come into play. For my chart I used a combination of theory, based on empirical data and generalized curves, and tweaked it using actual flight test data. Although I can't say I tested every value in the chart, I have found it to be quite close for those that I have tested. I have dozens of flight test logs that I've pored over and I've found that the old adage of TAS increasing 2% per thousand ft density altitude holds pretty close.

I generated a similar chart for my current Sonex with Jabiru 3300 and regularly refer back to it. It's dead-nuts on for TAS and fuel flow, and I've come to really enjoy having it available in the cockpit. My current chart can be found in my POH. Feel free to review it at your leisure.

http://www.sonex604.com/misc/N604CM_POH.doc

I encourage you to get out and run your own tests. There's no substitute for actual results, and first-hand knowledge beats theory nearly every time!

[samiam]

Chris, your post uses a lot of algebra, but basically comes to the conclusion:

x = x

Or, in this case:

IAS = TAS. But science tells us that this simply isn't true.

[gammaxy]

Chris, your post uses a lot of algebra, but basically comes to the conclusion:
x = x
Or, in this case:
IAS = TAS. But science tells us that this simply isn't true.

If I said anything about IAS, it is a typo. The point of the math is that drag decreases (if you make assumptions about angle of attack being the same at both altitudes) at the same rate that power available decreases as you fly higher. Best case is for your true airspeed at altitude to equal the true airspeed at sea level (not exceed it by 20 mph without some sort of explanation).

I think the reason why my argument seems so obviously wrong to people is that in a typical airplane like a Cessna 172, you can't do 100% power in cruise near sea level without exceeding the redline, so you artificially limit the power to ~75% until you can finally go full throttle at 8000 feet (which then is only ~75% power). Because of this, power stays constant until 8000 feet while drag decreases, so you definitely do get a speed gain.

My Aerovee Sonex does not exceed red-line near sea level, so I happily fly WOT at all altitudes and power decreases as I climb. This is very unusual compared to typical GA airplanes and means expectations carried from more typical aircraft don't apply.

Thanks for your interest in my power chart! ... Although I can't say I tested every value in the chart, I have found it to be quite close for those that I have tested.

Thanks for making your POH and all your other data so available. I've benefited from it enormously. The first thing that is surprising about your chart is that it shows 3300rpm at all altitudes. At WOT in cruise, I don't expect to be able to get the same RPM at all three altitudes (for the same reason that a Cessna 172 at WOT near sea level will exceed redline, but not at 8000 feet). I think what the chart is attempting to show is that IF you ended up at 8000 feet in cruise at 3300 rpm, you expect 150 mph, but it's not necessarily intended to indicate that you WILL get 3300 rpm. If that's the case, then this chart shouldn't be taken as evidence that 20mph+ TAS increase is possible between SL and 8000 feet.

Maybe your propeller allowed you to spin way faster than 3300 rpm at sea level and you throttled the engine accordingly. If this is true, then the chart makes more sense to me, but I think the percent power numbers should be adjusted to reflect throttling the engine down low and I'm going to buy a new propeller :-D.

[Bryan Cotton]

I saw the resurrected canopy seal thread. Is the canopy fit pics he posted typical on other Sonexes? Seems like that could be a boatload of drag. Hard to get a good feel from a picture though.