Minimum acceptable self-launch climb rate

On 4/26/20 11:38 AM, Kenz Dale wrote:


The GP 15 Jeta has a ratio of 1:6.3, which is far lower than the rule of thumb for powered flight. Yet, I don't hear anyone complaining.


Given that there are approximately 0 Jeta's flying in the hands of real
customers, the lack of complaints may not be surprising.

You should always be cautious comparing specs of vapourware gliders to
those of gliders in the field, since in the first case the glossy
brochures are sometimes not encumbered by nasty things called facts.

-Dave

Even if CS22 only requires approximately 1.5 m/s (~300 ft/mn) at sea level ISA, this is really not comfortable.
I get bearly 250 ft/mn when taking off from high altitude airport (5700ft) with the Ventus 2 CM and it is OK but not a pleasant experience! Especially in mountainous environment.

Anything below 400 ft/mn at sea level will result in uncomfortable performance during hot days and/or high altitude.

Concerning electric performance, the motor itself does not lapses with altitude. Therefore, at given rpm, the thrust loss is pretty much proportional to the effect of air density variation on the prop.

The rule of thumb you propose is very rough and would only approximate sea level static.

For climb performance you really need to model the prop and get an estimate of thrust at typical climb speed.
Because FES and GP have relatively small diameter props, they are highly loaded and the GP has an efficiency advantage with the 3 blades.
If you run the analysis, you find the GP15 should have a sea level ISA climb rate of ~500 ft/mn at 470 kg, which is pretty good since you are taking off at 60 kg/m2!

kinsell wrote on 4/26/2020 11:37 PM:
On 4/26/20 11:38 AM, Kenz Dale wrote:


The GP 15 Jeta has a ratio of 1:6.3, which is far lower than the rule of thumb
for powered flight. Yet, I don't hear anyone complaining.


Given that there are approximately 0 Jeta's flying in the hands of real customers,
the lack of complaints may not be surprising.

You should always be cautious comparing specs of vapourware gliders to those of
gliders in the field, since in the first case the glossy brochures are sometimes
not encumbered by nasty things called facts.

The first one was at the SSA convention, then taken to it's new home in Oregon.
The virus situation has complicated and delayed the licensing, but the owner
expects to fly "soon". The factory (and many other businesses) in Poland has been
locked down (and still is) for over a month, which has delayed the April delivery
of the next two units to the USA, and that delay will likely ripple through the
other orders for many months. The prototype has flown for at least two years.

Bringing a new glider to market on time is difficult, even for established
manufacturers. 25 years ago, Schleicher surprised both myself and the US dealer
when they added a year to the delivery date for my ASH26E.

--
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

On Sunday, April 26, 2020 at 11:59:57 PM UTC-4, Eric Greenwell wrote:
Kenz Dale wrote on 4/26/2020 10:38 AM:
What's the minimum realistic climb rate that should be acceptable for self-launch? I've heard things like 1:4 ratio of thrust to MTOW (where thrust is approximated as 3x motor/engine power), but planes like the Silent 2 Electro and miniLAK FES seem to exceed this somewhat, with the miniLAK FES having a ratio approaching 1:5.5. The S2E's flight manual publish a climb rate somewhere around 400fpm, and just based on the numbers I would expect the miniLAK FES to be around 300fpm.

The GP 15 Jeta has a ratio of 1:6.3, which is far lower than the rule of thumb for powered flight. Yet, I don't hear anyone complaining.

And gas-powered craft might have 500fpm at sea-level, but they certainly can't attain anywhere near that at higher (density) altitudes.

So while all this sounds slow, when I compare to stall speeds I find that self-launch gliders' overall performance is generally 2-3x better than the FAA's minimum requirement of 200'/1nm for instrument flight (that's the closest I could find to a hard and fast rule for climb rates). So it would suggest that quick climb rates aren't necessary if the forward flight speed is very low.

So what's the right real-world response? Is there a certain absolute minimum for safe glider flight, or is it better to have a great climb angle rather than a great climb rate?

I don't think you should pay any attention to estimated thrust. What you want to
know is the climb rate, and that is always given by the brochure or handbook, and
most owners know about what their plane really does.

Based on 25 years of flying my ASH26E, I want at least 500 fpm at the weight I
would normally expect to fly. Don't use MTOW, as that gives you a very distorted
picture if you compare a Silent 2 Electro with a GP15, which can fly at a far
higher wing loading than the Electro.

The GP15 is rated at 865 fpm (large battery, 705 lbs takeoff weight, 8.4 lb/ft2).
That's a lot better than an Electro or miniLak, two of the gliders I considered
before deciding to buy the GP15. Compare it to my ASH26E, which is around 600 fpm
at the same wing loading, and drops off at higher density altitudes. So, the GP15
should be significantly better at places like Minden, Ely, and Parowan, as it's
electric propulsion does not lose nearly as much power at those places.

The maximum weight I expect to fly at (1010 lbs) gives a 12 lb/ft2 wing loading,
and I estimate the climb rate would be about 600 fpm.

--
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

At 8.5 lb wing loading an ASW-24E, on engine power of 17KW, gets 300 ft/ minute.
25KW should get it to about 450. These are the numbers I expect with my electric conversion. Color me skeptical that the GP15 is going to get 600. I don't doubt it will be very good, but think projections are optomistic.
FWIW
UH

On Monday, April 27, 2020 at 10:57:39 AM UTC+2, jld wrote:
Even if CS22 only requires approximately 1.5 m/s (~300 ft/mn) at sea level ISA, this is really not comfortable.
I get bearly 250 ft/mn when taking off from high altitude airport (5700ft) with the Ventus 2 CM and it is OK but not a pleasant experience! Especially in mountainous environment.

Anything below 400 ft/mn at sea level will result in uncomfortable performance during hot days and/or high altitude.

Concerning electric performance, the motor itself does not lapses with altitude. Therefore, at given rpm, the thrust loss is pretty much proportional to the effect of air density variation on the prop.

The rule of thumb you propose is very rough and would only approximate sea level static.

For climb performance you really need to model the prop and get an estimate of thrust at typical climb speed.
Because FES and GP have relatively small diameter props, they are highly loaded and the GP has an efficiency advantage with the 3 blades.
If you run the analysis, you find the GP15 should have a sea level ISA climb rate of ~500 ft/mn at 470 kg, which is pretty good since you are taking off at 60 kg/m2!

I can see how it's easy to do an analysis of climb performance relative to weight, but did you / do you need to account for impact of higher wing loading (increased stall speed / sink rate)?
I'm also curious whether there's a similar effect for higher altitude takeoffs via stall/sink speeds, other than air density on the propeller.

You will probably get more than the ASW24E.
The very high drag from the pylon with engine, like the ASW24E, reduces the glider L/D below 20, which start to have a significant effect on climb performance. This is probably a 100-150 ft/min difference.
It is more difficult to get the published power rating from 2 strokes carburated engines than it is from electrical motors.

Concerning GP15, its pylon is very low drag and most important, it is very light.
For climb performance it is not the wing loading but the weight which drives climb performance. To takeoff at 8.5 lb/ft2 with the ASW24E you need to be at 425 kg. With the GP15, you only need to 330 kg for the same wing loading.
Because of the available shaft power, drag and weight differences, I have no doubt the GP will have at least a 30% climb advantage with similar takeoff conditions.

wrote on 4/27/2020 6:08 AM:
On Sunday, April 26, 2020 at 11:59:57 PM UTC-4, Eric Greenwell wrote:
....

The GP15 is rated at 865 fpm (large battery, 705 lbs takeoff weight, 8.4 lb/ft2).
That's a lot better than an Electro or miniLak, two of the gliders I considered
before deciding to buy the GP15. Compare it to my ASH26E, which is around 600 fpm
at the same wing loading, and drops off at higher density altitudes. So, the GP15
should be significantly better at places like Minden, Ely, and Parowan, as it's
electric propulsion does not lose nearly as much power at those places.

The maximum weight I expect to fly at (1010 lbs) gives a 12 lb/ft2 wing loading,
and I estimate the climb rate would be about 600 fpm.

--
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

At 8.5 lb wing loading an ASW-24E, on engine power of 17KW, gets 300 ft/ minute.
25KW should get it to about 450. These are the numbers I expect with my electric conversion. Color me skeptical that the GP15 is going to get 600. I don't doubt it will be very good, but think projections are optomistic.
FWIW
The wing area of the ASW 24E is 110 ft2, for a weight of 935lbs (8.5 lb wing
loading); the wing area for the GP15 is 84 ft2 , for a weight of 714 lbs (also at
8.5 lb wing loading). The GP15 motor power is 30kW during the launch (the 25 kw is
maximum continuous operation). With 24% less weight and 76% more power, it should
easily beat 600 fpm.

I have compared the brochure values for the AS 34 and Gp15 as a "reality check",
and determined the GP15 values gave climb rates similar to the AS 34 when adjusted
for wing loading and weight. That's not the same as a flight test, but at least
the engineers at the two companies are using the same handbook :^)

--
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

"With 24% less weight and 76% more power, it should easily beat 600 fpm"
Don't forget the lower drag due to streamlined pylon compared to traditional installations like ASW24E.

The equation to determine climb rate is relatively simple, the main unknown is the thrust delivered by the prop at ~25-30 m/s climb IAS.

For the GP15, modeling a decent 1 meter 3 blades prop gives 550 N thrust at 30 m/s climb IAS with 25 kw on the shaft, sea level ISA.
AT 325 kg (722 lbs) TOM, L/D of 40 due to pylon, the climb rate should be ~4.3 m/s (~850 ft/mn).

Modeling the ASW24E with the same equation and following assumptions:
Thrust 450 N at 26 m/s IAS with 17 kw on the shaft (just a guess, using 20% higher prop efficiency than the GP due to larger prop diameter)
Weight 425 kg and L/D with pylon out 25
the climb rate is estimated at ~1.7 m/s (~340 ft/mn)

Even if these estimations are for sea level ISA, GP15 with such a high climb rate should perform very well even at high altitude in summer.
With the large battery packs and such a high climb rate, the GP15 should deliver an excellent climb altitude of ~4500 m with above assumptions and a 10% energy reserve. This is a very good differentiator!

On Monday, April 27, 2020 at 1:57:39 AM UTC-7, jld wrote:
Even if CS22 only requires approximately 1.5 m/s (~300 ft/mn) at sea level ISA, this is really not comfortable.
I get bearly 250 ft/mn when taking off from high altitude airport (5700ft) with the Ventus 2 CM and it is OK but not a pleasant experience! Especially in mountainous environment.

Anything below 400 ft/mn at sea level will result in uncomfortable performance during hot days and/or high altitude.

Concerning electric performance, the motor itself does not lapses with altitude. Therefore, at given rpm, the thrust loss is pretty much proportional to the effect of air density variation on the prop.

The rule of thumb you propose is very rough and would only approximate sea level static.

For climb performance you really need to model the prop and get an estimate of thrust at typical climb speed.
Because FES and GP have relatively small diameter props, they are highly loaded and the GP has an efficiency advantage with the 3 blades.
If you run the analysis, you find the GP15 should have a sea level ISA climb rate of ~500 ft/mn at 470 kg, which is pretty good since you are taking off at 60 kg/m2!

Another way to look at it is, how does it climb relative to a towplane? They are engine powered and suffer from altitude sickness as well. With a low drag electric, if you can out climb the towplane at high density altitude they you are relatively better off, regardless of the absolute numbers.

Last year I took two tows behind a Pawnee, climb rate to 1000 ft was 353 ft/m on one and 625 on the other, average 451. On the next 6 days I did self launches, averaging 573, the lowest was 439 and highest 796. This is all at Truckee, density altitude usually between 8800 and 9400 ft. A strong self launcher will beat the tow plane.

On Monday, April 27, 2020 at 2:22:15 PM UTC-4, jfitch wrote:
Last year I took two tows behind a Pawnee...

Reminds me of a western tow a couple years ago...

The task will open in 15 minutes.
CD, can you hold that, YO is still at 1600 feet on tow.
.... and 5 minutes later ...
The task will open in 15 minutes.
CD, can you hold that, YO is still on tow, 1500 feet and descending.
IIRC, I gave up at 1300 feet, pulled the release, and climbed out...

Some of you were in the air waiting...