Minimum acceptable self-launch climb rate

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

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

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!