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How much of a performance loss would you notice / worry about?

There have been various threads mentioning this… a 10kt or 20kt speed loss.

Anything like that indicates a massive problem somewhere, and I would regard the plane unairworthy.

Normally one should be getting the same IAS, plus or minus 1kt, for a given fuel flow, say at 2000ft, and at a given MP/rpm.

Administrator
Shoreham EGKA, United Kingdom

I would be very alarmed to see a 10 knot change in performance on a GA airplane, which was not explained. Yes, that aircraft would be unairworthy.

However, there may be reasons for a performance loss, which is why the pilot should be comparing their observations to the same airplane, not necessarily another of the same model. Examples (all of which I have personally experienced) could be: Wheel fairings on or off, I have never seen flight manual performance charts showing both configurations; A different pitch fixed pitch prop, most fixed pitch prop GA airplanes allow different pitches within a range, and that will produce different performance; Mods, like STOL kits, bubble windows, speed fairings, changed exhausts,or different tires, few of which are often accompanied with changed performance data; Really poor paint condition, that’ll cost a few knots anyway, and, for strut braced Cessnas, the wing cam adjustment for angle of incidence, to differentially adjust roll, can have the unintended result of sweeping both wings forward or aft (or worse, one each way!), and this has a slight effect upon performance. This was explained to me by a Cessna Tech Rep, when I asked him why two identical C 150’s cruised at different speeds. I checked the wings, and he was right!

Get to know the plane you fly, and note differences in it – hoping the only differences are those you intend!

Home runway, in central Ontario, Canada, Canada

Had a burnt exhaust valve once and hence lost all compression on #3 cilinder. It shames me to say, but I didn’t notice the performance loss immediately, only after a couple of take-offs I noticed the take-off run was slightly longer than usual and I did lost a few knots in cruise. Lost the waste gate controller once, I did notice that immediately…

EHTE, Netherlands

Something in relative terms of VNE-VS0 or VNE-VA or VA-VS0 with floor being instrument precision would make more sense in general?
A 10% threshold on the above is roughly those 10-20kts you mentioned for a GA aircraft

+/-1 kts is very small to give anything meaningful, I had an interesting story on Archer/C172P, both POH claims 2-3 kts improvement with spats, I tried testing their speed performance with and without while accounting for the same density & % power, Cessna did +3kts while Piper lost -5kts go figure (did not fully check on fuel consumption numbers tough)

Paris/Essex, France/UK, United Kingdom

The greatest reason for unexplained performance loss (or gain) in my experience is mountain wave, which can exist hundreds of miles from the nearest mountains. I had it last week over Belgium, for example.

EGKB Biggin Hill

Peter wrote:

here have been various threads mentioning this… a 10kt or 20kt speed loss.

Anything like that indicates a massive problem somewhere, and I would regard the plane unairworthy.

Normally one should be getting the same IAS, plus or minus 1kt, for a given fuel flow, say at 2000ft, and at a given MP/rpm.

I wouldn’t worry about a few knots as differences in aircraft weight could well have that effect and you usually don’t weight that in you planning as it has a very minor effect on cruise speed of light aircraft. (The recent Cessna 172S POH actually does state that cruise speed will increase by 1 kt per 60 kg decrease in mass.)

But around 5 knots I would start wondering and 10 knots indeed indicates a major issue.

ESKC (Uppsala/Sundbro), Sweden

Timothy wrote:

mountain wave

How does mountain wave affect indicated airspeed ?
Since I am flying by the numbers, any loss in indicated airspeed of >5kts at a given setting would be realized. If the engine runs at a certain setting, at a certain altitude, at a certain temperature, with usually known weight, with certain RPM, with a certain fuel flow, and a certain EGT and CHT, the speed must be within a very well defined range. If it isn’t, I’d ground the plane and have it checked.

Last Edited by EuroFlyer at 19 Dec 16:28
Safe landings !
EDLN, Germany

EuroFlyer wrote:

How does mountain wave affect indicated airspeed ?

Probably, he meant,
- Wave lift downdrafts: you should add power (aka climb) to maintain level flight on same ASI, or keep power/height & lose ASI, or keep power/ASI and lose height? but the same apply when flying the wave lift updraft segments or in any up/down parcels of air :)
- Turbulence/rotors: unless you have a rocket, you should not fly there with wings full stop….

None of the numbers regrading the performance of “sinking/floating in up/down parcels of air” appear in POH for a GA aircraft (only “flying in still air”), but overall the updrafts strength in ft/min is highly correlated to the delta in speed under S&L flight (total energy = speed + height + updraft strength + archimedes force stays the same), I don’t think these matter a lot for aeroplanes: I never saw a publication of a GA aircraft sink/float rates at zero speed in still air but some aircraft (e.g. airships/balloons, gliders) do publish these numbers in their manuals

Last Edited by Ibra at 19 Dec 17:00
Paris/Essex, France/UK, United Kingdom

How does mountain wave affect indicated airspeed ?

Because of the vector – it’s up or down as well as lateral. Thus in order to maintain altitude there is a change in pitch angle (attitude) which results in a change in indicated airspeed. The pitch change may not be immediately noticeable, especially if on autopilot. We had this over Iceland a couple of years ago at around 7,000’ agl, with pitch angle increasing and IAS decreasing. Disconnected a/p, could not easily maintain altitude, requested block, then caught the updraft at the other side. Quite a ride.

Another reason for decreasing airspeed, with no apparent power reduction/loss, is airframe ice, but that might be more noticeable

NeilC
EGPT, LMML

But aren’t we talking about loss of engine performance ? Anything related to weather would be temporary and has an obvious reason. If I’m in a downdraft because of strong winds or turbulence, why would I think the engine has a problem ? Maybe I just don’t get it…. ;)

Last Edited by EuroFlyer at 19 Dec 18:47
Safe landings !
EDLN, Germany
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