Archive for the ‘ cl ’ Category

NLF215F considerations, Cl for different conditions

My earlier post about the NLF215F simulations with XFLR5, the related parameters for aircraft would be in the use case (one iteration of thinking):

– low altitude cruise:
* altitude = 12000 ft
* W/S = 22 lbs/sqft
* Clcruise = 0.41
* NLF215F flap in the -10 degrees position, gap seals closed

– high altitude cruise:
* altitude = 36000 ft
* W/S = 22 lbs/sqft
* Clcruise = 0.96
* NLF215F flap in the 0 degree position, gap seals closed

– extreme high altitude cruise
* some fuel burned already -> W/S reduced to 21 lbs/sqft
* altitude = 46000 ft
* W/S = 21 lbs/sqft
* Clcruise = 1.48
* NLF215F flap in the 0 degrees position, gap seals closed

– approach
* 1 slot open
* W/S = 15 lbs/sqft
* altitude = 1000 ft
* Cl = 1.1, V = 75 kts (at gross weight, W/S 22 lbs/sqft)
* Cl = 1.1, V = 65 kts (when fuel tanks nearly empty, W/S 15 lbs/sqft)
* NLF215F flap in the +10 degrees position, 1 slot open

– landing
* 2 slots open

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>NLF215F considerations, Cl for different conditions

>My earlier post about the NLF215F simulations with XFLR5, the related parameters for aircraft would be in the use case (one iteration of thinking):

– low altitude cruise:
* altitude = 12000 ft
* W/S = 22 lbs/sqft
* Clcruise = 0.41
* NLF215F flap in the -10 degrees position, gap seals closed

– high altitude cruise:
* altitude = 36000 ft
* W/S = 22 lbs/sqft
* Clcruise = 0.96
* NLF215F flap in the 0 degree position, gap seals closed

– extreme high altitude cruise
* some fuel burned already -> W/S reduced to 21 lbs/sqft
* altitude = 46000 ft
* W/S = 21 lbs/sqft
* Clcruise = 1.48
* NLF215F flap in the 0 degrees position, gap seals closed

– approach
* 1 slot open
* W/S = 15 lbs/sqft
* altitude = 1000 ft
* Cl = 1.1, V = 75 kts (at gross weight, W/S 22 lbs/sqft)
* Cl = 1.1, V = 65 kts (when fuel tanks nearly empty, W/S 15 lbs/sqft)
* NLF215F flap in the +10 degrees position, 1 slot open

– landing
* 2 slots open

What is wrong with sailplane airfoils for powered planes

Everything might look very obvious at first, but after digging more and more, it becomes clearer and clearer what kind of compromises all aircraft are made of and why.

A known thing is that the more efficient the airfoil the higher L/D ratio it has and vice versa. So one could go and find that sailplane airfoils produce very high L/D ratios. There is a little but on that though: Sailplane airfoils commonly achieve the best L/D ratio at higher Cl than what is optimal for a powered aircraft with reasonable wing loading where the cruise Cl is between 0.15 and 0.20. E.g. NLF 215F seems to achieve its L/D max at around Cl 0.5 which is unusually low compared to some other airfoils that require Cl being close to 1.0. That is acceptable for a sailplane that is thermalling at close to the stall speed. However, that is not where one wants to cruise with a powered aircraft, there is usually a requirement to get somewhere in a reasonable time, thus speed has some importance.

I have previously mentioned that the wing loading and cruise Cl has direct relation. The higher the wing loading, the higher the cruise Cl vice versa. Then the speed where the best L/D ratio occurs has a relation to the previous and it also tends to have relation to the top speed.

Diamond DA40 uses Wortmann FX 63-137 airfoil. It has best L/D ratio higher than the optimal < 0.2 (for light wing loading). Therefore the best L/D speed is the same as the approach speed on the aircraft. Similarly on Diamond DA42 Twin Star the same airfoil was used but the wing loading is as high as it is on Cirrus SR20. The result is that the best L/D speed is higher than on DA40, the top speed is higher (it is not only because of the two engines, the two engines produce also more drag than one). Because of the substantially heavier wing loading, the DA42 cruises at higher Cl than the DA40 and it gets closer to the airfoil optimum resulting better aerodynamic efficiency.

Cirrus SR20 is very similar to the DA40 but it has a different airfoil and higher wing loading. That results best L/D ratio speed being 96 kts. SR22 has that value even higher, it is over 100 kts, but it can be misleading that the best glide speed mentioned in the operating handbook is lower than on SR20. That is the best glide speed, it is not the best L/D ratio speed of the airfoil, it is a compromise of the airfoil + fuselage + propeller and in the SR22 the propeller is braking a lot more than on SR20, which alone is enough to explain the lower best glide speed – because of the propeller braking, the SR22 sinks faster, but if there was no propeller, SR22 could have higher glide speed than the SR20. But what this has to do with the topic? The interesting thing is that the Cirrus has different airfoil and higher wing loading and the optimum glide speed is higher than on DA40 which results potential to faster cruise speed than DA40 (whereas it is not exactly the airfoil’s best L/D speed because of the mentioned reasons). Providing that there is enough power available, the Cirrus airframe is faster although the larger fuselage cross section and wetted area most likely pretty much diminishes the benefit from the wing, that is also partly a reason why the best cruise speed performance of DA40-180/XL and SR20 is not that much different, SR20 is just slightly faster – the Diamond has better fuselage shape and it simply is a lot smaller aircraft than the Cirrus and size does not tend to come without penalty when it comes to aerodynamic drag.

However, it would be beneficial for efficiency to have an airfoil which could achieve higher L/D ratio at the cruise Cl of the DA40 already. It does not come without penalties of course, the airfoils which have high L/D ratio at low Cl don’t necessarily always produce optimal Clmax (which then has also relation to the required wing area which gets back to the stall speed and wing loading).

And it is not all in that, Daniel Raymer notes in his book that usually only 90% of the theoretical Clmax of the airfoil gets realized in practice. Therefore it is a interesting compromise between the wing sizing, and the best L/D at cruise Cl. Daniel Raymer notes in high book that the Cl is one of the hardest things to estimate without experimental data from test flights, and often test flights result in the need of modifications (e.g. if the Clmax in practise is not as good as was predicted, a larger wing is required to meet the maximum stall speed criteria, which is for single engine aircraft 61 kts).

It would be really interesting if someone would have a batch processing functionality in a airfoil program that would ingest the UIUC airfoil database data and simulate through all airfoils and put them into a correct order for the given specification (cruise Cl below 0.2), as high L/D at cruise Cl for a low wing loading, and at the same time, as high Clmax as possible, and at the same time, gentle stall charasteristics at low Reynolds number. And of course, the pitching moment also has some importance, high pitching moment tends to cause more trim drag which reduces the achievable Clmax (of the total airframe) considerably – if the wing can achieve e.g. Clmax 2.2, the airframe may be left to below 1.5 in total because of the download in the tail that is negative lift.

>What is wrong with sailplane airfoils for powered planes

>Everything might look very obvious at first, but after digging more and more, it becomes clearer and clearer what kind of compromises all aircraft are made of and why.

A known thing is that the more efficient the airfoil the higher L/D ratio it has and vice versa. So one could go and find that sailplane airfoils produce very high L/D ratios. There is a little but on that though: Sailplane airfoils commonly achieve the best L/D ratio at higher Cl than what is optimal for a powered aircraft with reasonable wing loading where the cruise Cl is between 0.15 and 0.20. E.g. NLF 215F seems to achieve its L/D max at around Cl 0.5 which is unusually low compared to some other airfoils that require Cl being close to 1.0. That is acceptable for a sailplane that is thermalling at close to the stall speed. However, that is not where one wants to cruise with a powered aircraft, there is usually a requirement to get somewhere in a reasonable time, thus speed has some importance.

I have previously mentioned that the wing loading and cruise Cl has direct relation. The higher the wing loading, the higher the cruise Cl vice versa. Then the speed where the best L/D ratio occurs has a relation to the previous and it also tends to have relation to the top speed.

Diamond DA40 uses Wortmann FX 63-137 airfoil. It has best L/D ratio higher than the optimal < 0.2 (for light wing loading). Therefore the best L/D speed is the same as the approach speed on the aircraft. Similarly on Diamond DA42 Twin Star the same airfoil was used but the wing loading is as high as it is on Cirrus SR20. The result is that the best L/D speed is higher than on DA40, the top speed is higher (it is not only because of the two engines, the two engines produce also more drag than one). Because of the substantially heavier wing loading, the DA42 cruises at higher Cl than the DA40 and it gets closer to the airfoil optimum resulting better aerodynamic efficiency.

Cirrus SR20 is very similar to the DA40 but it has a different airfoil and higher wing loading. That results best L/D ratio speed being 96 kts. SR22 has that value even higher, it is over 100 kts, but it can be misleading that the best glide speed mentioned in the operating handbook is lower than on SR20. That is the best glide speed, it is not the best L/D ratio speed of the airfoil, it is a compromise of the airfoil + fuselage + propeller and in the SR22 the propeller is braking a lot more than on SR20, which alone is enough to explain the lower best glide speed – because of the propeller braking, the SR22 sinks faster, but if there was no propeller, SR22 could have higher glide speed than the SR20. But what this has to do with the topic? The interesting thing is that the Cirrus has different airfoil and higher wing loading and the optimum glide speed is higher than on DA40 which results potential to faster cruise speed than DA40 (whereas it is not exactly the airfoil’s best L/D speed because of the mentioned reasons). Providing that there is enough power available, the Cirrus airframe is faster although the larger fuselage cross section and wetted area most likely pretty much diminishes the benefit from the wing, that is also partly a reason why the best cruise speed performance of DA40-180/XL and SR20 is not that much different, SR20 is just slightly faster – the Diamond has better fuselage shape and it simply is a lot smaller aircraft than the Cirrus and size does not tend to come without penalty when it comes to aerodynamic drag.

However, it would be beneficial for efficiency to have an airfoil which could achieve higher L/D ratio at the cruise Cl of the DA40 already. It does not come without penalties of course, the airfoils which have high L/D ratio at low Cl don’t necessarily always produce optimal Clmax (which then has also relation to the required wing area which gets back to the stall speed and wing loading).

And it is not all in that, Daniel Raymer notes in his book that usually only 90% of the theoretical Clmax of the airfoil gets realized in practice. Therefore it is a interesting compromise between the wing sizing, and the best L/D at cruise Cl. Daniel Raymer notes in high book that the Cl is one of the hardest things to estimate without experimental data from test flights, and often test flights result in the need of modifications (e.g. if the Clmax in practise is not as good as was predicted, a larger wing is required to meet the maximum stall speed criteria, which is for single engine aircraft 61 kts).

It would be really interesting if someone would have a batch processing functionality in a airfoil program that would ingest the UIUC airfoil database data and simulate through all airfoils and put them into a correct order for the given specification (cruise Cl below 0.2), as high L/D at cruise Cl for a low wing loading, and at the same time, as high Clmax as possible, and at the same time, gentle stall charasteristics at low Reynolds number. And of course, the pitching moment also has some importance, high pitching moment tends to cause more trim drag which reduces the achievable Clmax (of the total airframe) considerably – if the wing can achieve e.g. Clmax 2.2, the airframe may be left to below 1.5 in total because of the download in the tail that is negative lift.

Relation of cruise Cl and wing loading

It is interesting to look the parameters of different airfoils. One notable thing is that the glide ratio of the given airfoil is in relation to the Cl at cruise condition, and the wing loading has a direct effect to the cruise Cl of the airfoil, the higher the wing loading (the smaller the wing area in relation to weight), the higher is the cruise Cl. With some aifoils, this relation is stronger than with others, since the low drag laminar bucket is at some specific Cl, and it is not always in a favorable position for use in a light aircraft, it may be quite often designed for airliners which have very high wing loadings (and very high stall speed as a result as well, unreasonably high for a personal aircraft, which makes surviving a crash unlikely (which would be unacceptable for a personal single engine aircraft)).

I calculated some rounds of weights, wing areas, wing loadings (I have been calculating with wing loadings between 18 lbs/sqft to 25 lbs/sqft (e.g. Lancair Legacy has 23 lbs/sqft)) and subsequently the Cl-cruise and the L/D at the given Cl. Some airfoils are particularly poor at low Cl whereas at least, if Javafoil is at all to be trusted (the methods it uses aren’t very accurate), the already mentioned NLF414F is a rare exception. It has excellent lift/drag relation exactly where it should be in a light aircraft with low wing loading. It would be easier to say for sure, if I could see some wind tunnel data for the NLF414F but so far I haven’t found enough information. Also it would be interesting to compare to the Wortmann FX63-137. According to Carmichael [1] it does have good L/D charasteristics, but would be great to be able to determine, how good exactly at Cl 0.1, 0.15 and 0.2 (as this is the usual range in light aircraft). The low drag potential is wasted if it can be only utilized at Cl higher than e.g. 0.4, which is not practical or even quite possible in a lightweight personal high performance aircraft and it is also interesting, that many aircraft that are using airfoils which have very low drag potential, may be operating the airfoil outside the best cruise Cl area, and the result is not that good, not that different, or in many cases worse, than if the airfoil was a low drag traditional one, like NACA 66-212. I haven’t found so far the wind tunnel data for the Wortmann either, seems like it is not at least available in the Internet, at least not for free.

>Relation of cruise Cl and wing loading

>It is interesting to look the parameters of different airfoils. One notable thing is that the glide ratio of the given airfoil is in relation to the Cl at cruise condition, and the wing loading has a direct effect to the cruise Cl of the airfoil, the higher the wing loading (the smaller the wing area in relation to weight), the higher is the cruise Cl. With some aifoils, this relation is stronger than with others, since the low drag laminar bucket is at some specific Cl, and it is not always in a favorable position for use in a light aircraft, it may be quite often designed for airliners which have very high wing loadings (and very high stall speed as a result as well, unreasonably high for a personal aircraft, which makes surviving a crash unlikely (which would be unacceptable for a personal single engine aircraft)).

I calculated some rounds of weights, wing areas, wing loadings (I have been calculating with wing loadings between 18 lbs/sqft to 25 lbs/sqft (e.g. Lancair Legacy has 23 lbs/sqft)) and subsequently the Cl-cruise and the L/D at the given Cl. Some airfoils are particularly poor at low Cl whereas at least, if Javafoil is at all to be trusted (the methods it uses aren’t very accurate), the already mentioned NLF414F is a rare exception. It has excellent lift/drag relation exactly where it should be in a light aircraft with low wing loading. It would be easier to say for sure, if I could see some wind tunnel data for the NLF414F but so far I haven’t found enough information. Also it would be interesting to compare to the Wortmann FX63-137. According to Carmichael [1] it does have good L/D charasteristics, but would be great to be able to determine, how good exactly at Cl 0.1, 0.15 and 0.2 (as this is the usual range in light aircraft). The low drag potential is wasted if it can be only utilized at Cl higher than e.g. 0.4, which is not practical or even quite possible in a lightweight personal high performance aircraft and it is also interesting, that many aircraft that are using airfoils which have very low drag potential, may be operating the airfoil outside the best cruise Cl area, and the result is not that good, not that different, or in many cases worse, than if the airfoil was a low drag traditional one, like NACA 66-212. I haven’t found so far the wind tunnel data for the Wortmann either, seems like it is not at least available in the Internet, at least not for free.