FES climb rate question.

kinsell wrote on 10/12/2020 11:40 AM:
On 10/12/20 8:46 AM, kinsell wrote:
On 10/8/20 6:54 PM, 2G wrote:
On Thursday, October 8, 2020 at 1:14:30 PM UTC-7, Mana wrote:
You'll get at least 2m/s dry. But like PF says, better to take a tow to 400m and use the
FES straight and level until you find a thermal. It's much more efficient in level flight
than climbing.

On the Shark, based on a test flight I think that you will get less than 2m/s as it is a
heavier glider than Matthew's Diana 2 :-).

*From the FES manual:
5.3.4.1 Rate of climb
The* maximum rate* of climb* is* available* only* for* a* few minutes* with* fully* charged
battery packs. As battery voltage is reduced, the maximum achievable climb rate is lower.
The average rate of climb depends mostly on the type of sailplane and its take-off weight.
Maximum attainable altitude gain that in standard atmosphere conditions depends on the
type* of* sailplane,* its* weight* and aerodynamic* qualities.* To achieve the maximum
altitude* gain, use about* 15kW* of* power. Do* not* use* full* power* as* the efficiency
of* the system* is* lower. Usually, 80-85* km/h is* best for the* climb with positive flap
setting (the same setting as used while thermaling). Here are rough numbers:
•1600 m (5200 ft) for UL sailplanes at 300kg take-off weight, i.e. Silent 2 Electro
•1400* m (4500* ft)* for* the* 18m* class* sailplanes* at* 400kg take-off weight* (without
water ballast), i.e.LAK17A FES
•1200* m* (3900* ft) for* the* 18m* class* sailplanes* at* 450kg take-off weight* (without
water ballast); LAK17B FES, Ventus 2cxa FES, Discus 2c FES, HPH 304ES

Sounds like a very marginal self-launch system and is really just a decent self-retrieve
(turbo) setup.

Tom


These gliders are sustainers only.

Looks like the Silent 2 is rated for self-launch.

As is the miniLak FES, also a 13.5M glider. It works well for them because they are lighter
than the 18M gliders.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

As is the new LAK 17c FES https://www.youtube.com/watch?v=rxe6DIfPKb4

More prop clearance, 16s batteries and higher max RPM for 2.5 m/s climb rate. The angle of attack of the wing is smaller, back to what it was on the 17a (vs the 17b that was based on the fuselage of the LAK 19)

These gliders are sustainers only.

Looks like the Silent 2 is rated for self-launch.
As is the miniLak FES, also a 13.5M glider. It works well for them because they are lighter
than the 18M gliders.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)

On Monday, October 12, 2020 at 3:18:05 PM UTC-5, Mana wrote:
As is the new LAK 17c FES https://www.youtube.com/watch?v=rxe6DIfPKb4

More prop clearance,

Looks like keeping the tail down is interesting at 1:30 in the video.
What does the POH ask you to do with the stick?

If you use it to hold down the tail and hit a bump, then how high can the tail bounce before entering lawn mower mode?

Technically self launch, but with grass or tiger country, the tow line didn't look all that long and having the batteries full seems wise.

Take-off performance looks impressive. I was surprised by the amount of noise - sounded not unlike a petrol engined aircraft as it passed the camera, although presumably it is actually much less loud.

Any idea what the endurance is of that aircraft?

As the owner of a petrol self-launcher, I like to know that I have enough endurance for a launch to 2,100 feet (700 m), a relight if necessary, and a reasonable self-retrieve later if necessary. I will stick to petrol until batteries can cope with at least that.

On Friday, October 16, 2020 at 7:11:13 AM UTC-7, waremark wrote:
Take-off performance looks impressive. I was surprised by the amount of noise - sounded not unlike a petrol engined aircraft as it passed the camera, although presumably it is actually much less loud.

Any idea what the endurance is of that aircraft?

As the owner of a petrol self-launcher, I like to know that I have enough endurance for a launch to 2,100 feet (700 m), a relight if necessary, and a reasonable self-retrieve later if necessary. I will stick to petrol until batteries can cope with at least that.

I would like to know if it can even launch to 2,100 ft without the motor/controller overheating.

Tom

On Friday, October 16, 2020 at 8:11:13 AM UTC-6, waremark wrote:
Take-off performance looks impressive. I was surprised by the amount of noise - sounded not unlike a petrol engined aircraft as it passed the camera, although presumably it is actually much less loud.

Any idea what the endurance is of that aircraft?

As the owner of a petrol self-launcher, I like to know that I have enough endurance for a launch to 2,100 feet (700 m), a relight if necessary, and a reasonable self-retrieve later if necessary. I will stick to petrol until batteries can cope with at least that.

Much of the noise you hear from any propeller-driven aircraft comes from the tips of the propeller breaking the sound barrier. That won't change between an electric vs petrol engine. The longer the propeller, the faster the tips move.

On Friday, October 16, 2020 at 4:53:50 PM UTC-7, John Foster wrote:
On Friday, October 16, 2020 at 8:11:13 AM UTC-6, waremark wrote:
Take-off performance looks impressive. I was surprised by the amount of noise - sounded not unlike a petrol engined aircraft as it passed the camera, although presumably it is actually much less loud.

Any idea what the endurance is of that aircraft?

As the owner of a petrol self-launcher, I like to know that I have enough endurance for a launch to 2,100 feet (700 m), a relight if necessary, and a reasonable self-retrieve later if necessary. I will stick to petrol until batteries can cope with at least that.

Much of the noise you hear from any propeller-driven aircraft comes from the tips of the propeller breaking the sound barrier. That won't change between an electric vs petrol engine. The longer the propeller, the faster the tips move.

My ASH31Mi has a 1.55m diameter propeller that has a max rpm of 2500. If you do the math, this works out to 454 mph, well below the speed of sound. Few GA aircraft have props that go supersonic, and you can really tell those few that can.

Tom

The CalAir A-9b, for instance...

On 10/16/2020 8:50 PM, 2G wrote:
On Friday, October 16, 2020 at 4:53:50 PM UTC-7, John Foster wrote:
On Friday, October 16, 2020 at 8:11:13 AM UTC-6, waremark wrote:
Take-off performance looks impressive. I was surprised by the amount of noise - sounded not unlike a petrol engined aircraft as it passed the camera, although presumably it is actually much less loud.

Any idea what the endurance is of that aircraft?

As the owner of a petrol self-launcher, I like to know that I have enough endurance for a launch to 2,100 feet (700 m), a relight if necessary, and a reasonable self-retrieve later if necessary. I will stick to petrol until batteries can cope with at least that.
Much of the noise you hear from any propeller-driven aircraft comes from the tips of the propeller breaking the sound barrier. That won't change between an electric vs petrol engine. The longer the propeller, the faster the tips move.
My ASH31Mi has a 1.55m diameter propeller that has a max rpm of 2500. If you do the math, this works out to 454 mph, well below the speed of sound. Few GA aircraft have props that go supersonic, and you can really tell those few that can.

Tom

--
Dan, 5J

To follow up, if my simple arithmetic is correct, a 94 inch propeller
spun at 2,700 rpm achieves a tip speed of about 1,107 feet per second.Â*
According to NASA, 1,100 fps at sea level is the speed of sound.Â* At a
typical summer density altitude of, say, 8,800' MSL, the CalAir's prop
tips are supersonic.

But I may have screwed up the math...

On 10/17/2020 10:09 AM, Dan Marotta wrote:
The CalAir A-9b, for instance...

On 10/16/2020 8:50 PM, 2G wrote:
On Friday, October 16, 2020 at 4:53:50 PM UTC-7, John Foster wrote:
On Friday, October 16, 2020 at 8:11:13 AM UTC-6, waremark wrote:
Take-off performance looks impressive. I was surprised by the
amount of noise - sounded not unlike a petrol engined aircraft as
it passed the camera, although presumably it is actually much less
loud.

Any idea what the endurance is of that aircraft?

As the owner of a petrol self-launcher, I like to know that I have
enough endurance for a launch to 2,100 feet (700 m), a relight if
necessary, and a reasonable self-retrieve later if necessary. I
will stick to petrol until batteries can cope with at least that.
Much of the noise you hear from any propeller-driven aircraft comes
from the tips of the propeller breaking the sound barrier.Â*Â* That
won't change between an electric vs petrol engine.Â* The longer the
propeller, the faster the tips move.
My ASH31Mi has a 1.55m diameter propeller that has a max rpm of 2500.
If you do the math, this works out to 454 mph, well below the speed
of sound. Few GA aircraft have props that go supersonic, and you can
really tell those few that can.

Tom


--
Dan, 5J

Much of the noise you hear from any propeller-driven aircraft comes from the tips of the propeller breaking the sound barrier. That won't change between an electric vs petrol engine. The longer the propeller, the faster the tips move.

You're right that a fair amount of noise comes from the prop, and in particular that petrol vs electric won't change this.

However, I do not agree that it's because of breaking the sound barrier. Good prop design will keep the max tip speed below 0.6-0.7 Mach at max RPM and flight speed. It would be extremely hard to spin a well-specced prop fast enough that you'd see tips breaking the sound barrier. In an airplane which doesn't have a prop specifically designed to work transsonic (which AFAIK is pretty much any airplane outside of a handful of research ones), the prop loses so much efficiency as the tips approach Mach that you either actually slow down or dramatically increase the required engine power.