C208 Vy

[Lumps]

The book figure for the C208 best rate of climb is 107 knots at sea level, scaling down to 91 knots at FL200.

The two 208 operators around this patch both use 95 knots as their company best rate of climb, and one is a skydive operator the other charter. What do they know that Cessna doesn't?

95 knots is mentioned occasionally in emergency procedures as glide speed, but otherwise it seems it has no use until you get to FL150 or so...

[wateroff]

Most speeds in AFM/POH's are predicated on certain criteria. Light aircraft will usually have a published speed based on Max weight, 12 knots seems a big stretch though....... Aircraft modification?

If in doubt, fly the published speed.

[Jabawocky]

Well if they actually did their homework :hmm: they would not be using Vy or Vx……instead Vz.

If I was an operator Vz would actually be the answer.

I wonder why very few folk know what that is and why it is not in a POH.

You guys need to stop thinking the POH is the bible. It is an example of certain things only and ONLY section 2 is the certified part despite what anyone who thinks otherwise says.

So if y'all did some homework y'all would not use "the published speed UNLESS that speed was the result you wanted to achieve such as the best rate or angle. :ugh::ugh:

[UnderneathTheRadar]

Hey Jabba

Hopefully the skydivers wouldn't be using Vz - unless their drop zone is a long way away.....

UTR

[Lumps]

yeah I thought Vy would be THE speed for skydive ops.

Although I think I've found the answer:

Basically if the POH says Vy is 107KIAS at maximum continuous power (say 675hp) then any setting less than this will decrease Vy. Suspect these operators are running their -114's at a reduced power setting.

thanks to:

http://www.pprune.org/tech-log/343259-vx-vy.html

What is Vz? Are you referring to Carson Speed?

I don't see any reason not to believe a POH that states Vy = xxxKIAS. Why would they make that up? Surely it was determined with data.

[Jack Ranga]

Y'all?? WTF? Too much time in Alabama boy :=

[Radix]

............

[Wally Mk2]

Too many variables when it comes to perf figures in a light machine. diff story for heavy weights though.
Airframe anomalies, prop degradation, pilot technique, temp on the day, engine condition all add up to no one figure fits all.
The idea in a jump plane (silly buggers!) is to get to TOC ASAP due cost so am sure these operators have been trying many dif numbers to achieve just that.

Wmk2

[the_rookie]

Wouldn't Vx be THE speed for skydive ops?

[5-in-50]

flying Vx would take more time to reach the drop flight level than Vy, so no.

[BlatantLiar]

SOPs that are approved by the regulator. I'm sure they have a good reason.

Seeing as skydiving is a private op the SOPs would not have been approved by CAsA.

I'm perplexed the answer is not obvious enough for someone to have answered it already.

[Creampuff]

'Skydiving' is 'a private op'?

Regulatory reference please.

[BlatantLiar]

'Skydiving' is 'a private op'?

Regulatory reference please.

https://www.youtube.com/watch?feature=player_detailpage&v=cOeKidp-iWo#t=67

Hopefully things never change.

[haughtney1]

The operator I flew Vans on meat bombing ops used 105 KIAS for the climb to jump alt, 2 knots more yielded VERY little in the way of time saving.

[Lumps]

The operator I flew Vans on meat bombing ops used 105 KIAS for the climb to jump alt

Must've been using 675hp on the way up from what I have gathered. Or they missed something other skydive 208 operators didn't.

Start with the question 'why does Vy decrease with altitude' and the answer lies within. (and yes, caveats for turbines with low flat ratings/high thermodynamic rating or turbocharged pistons)

[evilducky]

Another consideration is that the published speeds are for a new, completely clean aircraft. Any exterior modifications that increase parasitic drag (such as a skydiving door, step and rail) result in the parasitic drag curve becoming steeper, resulting in an intersection with the induced drag curve at a lower airspeed. Best lift to drag ratio occurs at the TAS corresponding to this intersection, hence a lower actual Vy on an aerodynamically dirtier aircraft.

[Jabawocky]

Sorry y'all…..I was referring to charter ops and not grass darts :ouch:

[UnderneathTheRadar]

Jabba

Did you see Norman Howell's presentation at Oshkosh? Vz for a 601P proven at 156kts.....

UTR

[Lumps]

evil ducky, interesting, makes sense. I wonder how much difference this makes or if it is even measurable. The 12 knot difference cannot be attributable to the drag increase but I don't think you were implying that.

Is it the GA8 that when tested with a cargo pod stuck under it made not one knot of difference?

[Jabawocky]

UTR

No I did not, however I suspect it was from the work of Prof David Rogers, who used an Aerostar for doing proving flights.

I do have a copy of his report….long heavy read, but right up your alley :ok:

[kingRB]

Most van's have some form of the AeroAcoustics APE system fitted as well. Cannot remember the supplement well enough if there was any variation in there for Vx or Vy, but could be a reason.

I've flown the van for an operator as well which had company Vy far lower than whats published in the POH.

[Lumps]

ED hope that 206 had super good aftermarket baffling or a suitably inaccurate CHT gauge so you didn't know how hot that conti was getting!

If Vy decreases as excess thrust decreases, then why does the ROC decrease in turbocharged aircraft such as the GA8-TC going from 1000ft/min down to 600ft/min at 10k? Presumably the TIO-540 can maintain 38" MP at 10000ft...

[evilducky]

The prop blades are like wings - drop the density and you lose thrust. Yes the CSU can make up for this by increasing the pitch (effectively increasing the coefficient of lift for the blade) but this also leads to an increased amount of induced drag on the blade.

[Lumps]

makes sense. Big effect, but then I guess the thrust margin of 300hp hauling 1900kg is quite small so wouldn't take much of a drop in excess thrust to make big difference in RoC