Wren Question

[lperagallo]

Bell, Sara and Chris,

You may know the answer to this one. Have you ever experienced a loss (not great) of power as the temperature rises? Today on my fifth flight of the day I noticed that the turbine seemed to be not putting out the same power as at the beginning of the day. First flights the temps were at 70-75, last flight at 90. I didn't think that temps would effect the turbine that much.

I only noticed it during loops. All other flying around seemed OK, maybe with the exception of a stall turn. Rolls were fine, most other things fine. I run the max RPM at 135,000. Last week Chris Bergen suggested 137,000 and I didn't change it because I was lazy. This engine has about 13 hours on it. It's also got the new gear box that rocks!

I am waiting for a data cable to come in and then I can download the fadec to see if I can spot something going on. That is if I can figure out how to get it to run. Haven't had much luck getting it to run without the data cable. I thought it should at least do something other then giving me an error message in a language I can't understand.

Lou

[Sara]

Yes, power does decrease with an increase in ambient temperature. Our advertised engine thrust figures are taken at a "standard day" temperature of 15 degrees Celsius (60 degrees F)..
I asked Mike Murphy for a simple explanation as to why this happens. He says it's to do with air density and that higher temperatures make the air "softer" so the engine doesn't push against it as much.
We see these variations with UK weather - if it's a hot sunny day all the engines tested have thrust figures lower than usual, then we have a cold, miserable day and they are up again.

Sara Parish
Wren Turbines

[Bell Bloke]

Never noticed even a slight variation of power in different conditions..........hmm.
I notice it with my car and it is logial that air density is the key, could be more to do with how the main rotor blades work in thin hotter air than the effect it has on the motor. The only way to settle that one is dyno the engine on a hot day and then on a cold day.

[Sara]

Never noticed even a slight variation of power in different conditions..........

Lou experienced a 20-degree change during one morning - we don't get that sort of thing over here in the UK - so he could compare one flight directly with another. Maybe you wouldn't notice it if the flights were on different days.

The air density also affects power at altitude. Our customers in Denver, Colorado, have to use SuperSports (18 pounds thrust) to get the same flying abilities as others have with standard 54s (14 pounds thrust).

Sara Parish
Wren Turbines

[Danyboy]

Hello Sara, hello Lou 'n' Bloke... ;)

Actually, the whole thing would be about the density-altitude.

Density altitude is you current geographic given altitude in mbar, which is then corrected against local airpressure and temperature (15°C).
Whereas the temperature has way more influence than airpressure.

I've attached a density-altitude-chart from wikipedia below.
Note that the correction for airpressure is not included in the chart.

Let's take my flyingfield as an example. It is located at 1500ft AMSL. Today the local pressure is at 1013mbar and temperature will be 31°C in the afternoon. Since current airpressure is already at standard, there is no correction for pressure-altitude. If you now enter the chart at 1500ft on the horizontal lines and 30°C on the verticals, you can follow the diagonal lines toward the dotted line which represents "standard atmosphere". By doing so, you will see that though you're sitting at 1500ft AMSL, due to the temperature, air density is the same as it would be at around 4500ft on a standard day.
And that is quite some increase in altitude, as in 8000ft, there's only half the pressure you'd have at sea level on a standard-day, in 16'000ft there's only a quarter left...

http://upload.wikimedia.org/wikipedia/en/6/6f/Density_Altitude.png

And as we're not only relying on the engine for lift, but also for some blades, we'll have the decreased amount of power from the turbine, combined with the decreased lift created by the rotor...

Cheers,

Daniel

[Coolice]

Hey All,

I have noticed power differences depending on how warm or cold the day is I am flying on.
As with cars, cooler air is denser and hence contains more oxygen which in turn makes a flame burn brighter. This is one way turbo'd cars make more power with intercoolers, I would imagine the same can be said for our models with cooler air going into the combustion chamber.
.

[lperagallo]

Thanks to all those who replied. While I am a private pilot and do know the drill on density altitude, I didn't think it would play that much of a role when flying at such low altitudes. I also thought that turbines eliminated much of the issue with density altitude and thinner air as the turbine acts like a turbo charger. On a full size turbine I'm not aware of an option to lean out the engine, but I am not type certified to fly turbines.

Maybe I'll try to set the maximum power at 137,000 and see if that helps. I might also set up a flight mode that would let it go o 137,000 with the other modes set at 135,000 through the throttle curves.

I will also try to download the engine data when my data cable comes to see if maybe I have a fuel pump going bad or something like that. I hear you can tell a lot about the engine by watching trends.

Lou

[cbergen]

Lou,

While I have my Power set at 137,000, it never ACTUALLY gets that high...

The highest I ever saw in a download was 132,000, in a flat out go for broke high power, high speed pass, that allowed for a continuous full power stick setting. IOW I had the throttle stick bent forward!!

And it is not possible to change the power setting from the radio, in flight modes, only with the HDT on the ground.

[Bell Bloke]

Iperagallo, I'm going to have a go at water injection.............that should fix it.............will let you know what happens.........:YeaBaby:

[Coolice]

Iperagallo, I'm going to have a go at water injection.............that should fix it.............will let you know what happens.........:YeaBaby:

I got an idea what might happen..........

EJECT! EJECT! EJECT! ;-)
.

[lperagallo]

Chris,
And it is not possible to change the power setting from the radio, in flight modes, only with the HDT on the ground You are correct there, however, I believe you can do it through a throttle curve.


This is how I thought I would control the turbine speed through a flight mode switch.

1. Set the max rpm to 137,000.
2. Set radio to 0-50-100 in curves.
3. Reset the radio settings in Fadec.

Now according to Gaspar, the Fadec does the calculations as such.

Take the difference between idle setting and max rpm 137,000 - 42,000 = 95,000. That's the amount of RPM that will be spread across the entire throttle curve.

In idle 2 I would then start with this:
At low stick you get an idle of 42,000
At mid stick (50%) you get 42,000+47,500 (1/2 of 95,000) = 89,500
At top stick (100%) you get 137,000

Fix everything below 100% to get the proper headspeed

In Normal and Idle 1 I'd set it up this way:

To limit the max rpm to 132,000 the calculation is 132,000-42000 = 90,000. To get 90,000 from the throttle you need to give only 94.5% at top stick.
Of course load will determine actual rpm, but in theory you will never exceed 132,000 at 94.5% of throttle. To keep everything proportional in normal and idle1 I'd have to run the midpoint number and let the radio keep the response linear.

Comments please.

BTW, Robert ordered us some usb cables to connect the Fadec to the software to monitor the engine. That will help me see what is really going on with all the setting and run parameters.
Lou

[Bell Bloke]

That looks a bit complicated, I think what you are saying is this? This is exactly what I do:

1. Just set the turbine to 137,000

2. Have one thottle curve for cold days that goes upto say 90%

3. Have another throttle curve for hot days and high altitude that goes to 100%

I know this works because this is what I do when changing from small blades (less power)
to large blades (more power) I just flick a switch.
Note that your pitch curves don't change, obviously. :-)

[rkeith2]

I have my curves at 100 each end and then boost the mid range on hot days.

[Pinecone]

Water injection works on turboJET/FAN engines to increase mass flow and therefore thrust. Doesn't do as much for turboshaft engines.

And jet engines suffer for less thrust at high altitudes, just like piston engines. IN full scale, most turbo shaft engines are derated at sealevel due to the maximum torque the gearbox can handle. This allows them to continute to produce sea level power up to some altitude, as you are allowing the engine to run closer to it real full power. for example, if you take a 750 shaft HP engine and limit it to a maximum of 400 shaft HP (typical for a PT-6) you can continue to make 400 HP up to the altitude where the engine can only produce 400 HP.

With jet or fan engines, the thrust goes down with altitude, but the efficiency goes up. So they burna lot less fuel at altitude. I remember in the T-38 watching the fuel flow gauges swing drastically even doing a loop, there was a significant fuel flow difference (at the same throttle setting) from the bottom to the top of a loop. Another factoid, I understand the 777 engines burn more fuel at idle at sea level than at cruise at 35,000 feet.