# Why were the specifications of the Pipistrel Panthera downgraded?

I'm curious if this was a failure of the simulations or a result of the engine swap they did at the last moment. Originally, they said they could cruise at 200kts @ 10g/h fully loaded. Now its down to 155kts.

http://www.panthera-aircraft.com/technical-data

EDIT: Just wanted to add, for reference, a page which gave their original estimates. http://www.pipistrel-usa.com/models/panthera.html

You will also notice at the top of the page for the first link where they reference the 200kts with 4 people for 1000nm.

• it's not unusual for initial estimated specs to be off when it's actually tested (and the reason why tests are needed) – ratchet freak Aug 31 '14 at 13:54
• Yes, I just thought 47 knots was quite a mistake for a company with expertise in this area which is why I asked the question. – David Sep 1 '14 at 0:08

This sounds like an overly optimistic marketing department which wanted to make the aircraft look better. Engineering would allow to predict top speed within a few percent, so I would blame neither the simulations nor the engine swap.

The web page for the Panthera lists 155 kts only for the maximum range condition, which makes sense. Propeller aircraft need to fly rather slowly to maximize range. The cruise speed with maximum power is still close to 200 kts.

Please consider that the required engine power of propeller aircraft grows with the third power of flight speed. To give you the 200 kts top speed, the plane would need to run the engine with 215% of the power setting (and fuel flow) needed for 155 kts.

Once you know the installed power, you get reasonably good results just with the zero-lift drag and the propeller efficiency when you use this formula: $$v = \left(\frac{2\cdot P\cdot \eta_{Prop}}{\rho\cdot F_{ref}\cdot c_{D0}}\right)^{1/3}$$ Nomenclature:
$v\:\:\:\:\:\:\:\:\:\:$ speed in [m/s]
$P\:\:\:\:\:\:\:\:\:\:$installed Power [W]
$\eta_{Prop}\:\:$ Propeller efficiency; use 0.85 if not known
$\rho\:\:\:\:\:\:\:\:\:$ density of air [kg/m³]
$F_{ref}\:\:\:\:$ Reference area for coefficients, usually wing area [m²]
$c_{D0}\:\:\:\:\:$ zero-lift drag coefficient

The zero-lift drag coefficient for small propeller airplanes is around 0.02 to 0.025 if they have a retractable gear. A very well designed airplane might come down to 0.016, but that is already pushing it. To fly fast, a high wing loading is essential, and then the influence of all non-lifting components on the total drag is rather high. A well-designed airfoil might achieve a $c_{D0}$ of 0.0035, but the whole airplane will be closer to 0.02.

Things get even worse when you aim for higher flight Mach numbers. The Dornier 328 turboprop was initially calculated for 320 kts cruise speed, but Marketing felt the need to claim 340 kts. The difference does not sound substantial, but all engineers thought they must be crazy over in Marketing. In the end, the Do-328 needed 30% more installed power to fulfill the promises made by the (rather irresponsible) Marketing department. The increased engine power indirectly caused an accident in flight test when a propeller (with an unchanged, but now too weak propeller mount) broke off in sideslip and cut through 1/3 of the fuselage. Luckily, the aircraft landed without further incident after that ...

(Disclaimer: This is from the back of my head, and I think I remember the facts and figures correctly. If any of you know more, please edit the last paragraph!)

• Thanks for the detailed answer! I actually just started a Masters in Aerospace Engineering so it was particular interesting. I would agree with the marketing angle completely, but Pipistrel does have an amazing history when it comes to outstanding aerodynamics, and have multiple NASA awards to show for it. The Virus SW now cruses at almost 150knots @ 75% on a 100HP engine. That being said, I may leave this question open a bit longer just to see if anyone has actually talked to the company about this issue. Thanks again! – David Aug 31 '14 at 22:05
• @David: The specs look realistic, and I did not notice the Panthera before, so I do not know what their initial claims were. I guess they were intentionally vague to let anyone confuse maximum cruise speed and optimum cruise speed. – Peter Kämpf Aug 31 '14 at 22:09
• I just added a page with the original speed estimates up top. Here it is again for reference: pipistrel-usa.com/models/panthera.html . As you can see, their range was changed to 1000nm @ 200kts to 1000nm @155kts, which is quite a dramatic shift. – David Sep 1 '14 at 0:10
• @David: Thanks for the link! Yes, this page makes some incredible claims, and it suggests that >1000 NM range is achievable with 202 kts cruise speed. The 202 kts will need more engine power than 210 hp, however, and then those 10 gal/h are not right anymore. BTW, I find the claims for the fully electric plane even more incredible. – Peter Kämpf Sep 1 '14 at 21:40
• Are you talking about the electric Panthera, or this: pipistrel.si/news/… I'm very excited to see these planes starting to hit the market, but its going to be another 5 years or so until they have the range to make sense. This one looks like a battery version of the Virus SW. – David Sep 2 '14 at 1:13

They could get to their target if they pressurised the cabin, chose a turbo charged engine, and flew at FL25. Current max cruise speed is achieved at FL12.

In fact, if they went with the IO540 with 350hp they could have the record for fastest piston single, I estimate 262KTAS at FL25. Probably worried about heat dissipation. Also, it isn't what they want to do.

The SMA 400hp diesel would be interesting in this plane, despite weighing 600lbs.

• welcome to aviation.se. I'm sorry, but while interesting, I do not see how your post is answering the question. could you rephrase it? – Federico May 12 '16 at 5:09