It won’t take much drag to drop 10 knots. Bottom line, you have pretty much ruled out any other possibility. Fixed pitch prop running x rpm should move you at y airspeed. If you are getting the same RPMs and moving slower (no issues with the prop or pitch) you have a drag issue. Can’t really be anything else, can it?
Maybe an exhaust leak would allow the engine to produce same RPM but less % HP? Or maybe it's something or some interaction we haven't thought about yet.
Nope...you are making rated power....proven by your RPM gauge.
Nope, with a fixed pitch prop if you make more HP the RPM will go up and if you make less HP it will go down. Its exactly like what happens as you climb. Your engine makes less power as you climb so your RPMs will drop if you don’t adjust the throttle.
That's true, but the clutch would get very hot! very fast, if it was slipping all the time.
That's why I just ordered a TruTach II to measure my prop RPM. Should arrive early next week. But no, that's why I think it's unlikely there's been a drop in engine performance because my climb performance seems to be the same. This is the next thing I'm going to check though either this evening or tomorrow. I'm going to do a climb to 10,500', note the time it takes, compare that to the numbers I recorded previously for the climb to 10,500' and then also run at max continuous to see if I see IAS/TAS numbers comparable to what I had before. Because if I do, that means the problem is with Eco Mode, since I always pull back the throttle to enter Eco Mode on the 912iS engine in cruise. It's unlikely there's a problem with that, but just one more thing to rule out.
Pitch unchanged, but it is a new prop, right? So could be some minor difference.
New prop earlier this year, but I had been flying with it for 3+ months with consistent 120+ KTAS and confirmed the prop pitch hasn't changed.
Original prop was awful for the airplane and should have never been installed on it from factory. It was only producing 4800rpm in the climb, when Rotax says the minimum should be 5100. Couldn't pitch it for higher RPM in the climb or it would overspeed for Eco Mode. So it had way too big a RPM range for the 912iS engine. As a result, climb performance was awful. New prop achieves 5200rpm in the climb and does not overspeed for Eco Mode. So climb performance improved 30-50% and cruise performance remained unchanged. Vibration was also reduced. Starting throttle % also decreased from 45% to 35% because the new prop is so much lighter. The new prop was a fantastic upgrade and should have been on the airplane from the day it was built. It is an E-Props three-blade if you're wondering. The old prop was a Sensenich.
Unless you’ve got wrench fairies that come into the hangar at night and tighten stuff, I’d look for damage if the nose wheel is askew.
Mains and nose, with and without engine cowling installed, have been checked thoroughly by me and my A&P in the last couple of weeks. No damage detected anywhere.
I wasn't kidding when I said I have a minor mystery of the universe on my hands.Ok....now this is getting entertaining.
You didn't put them back on cattywampus?
Not sure I understand your question. There's no RPM increase needed. In fact, I fly in cruise around 5,400-5,450 rpm and 5,500 rpm is max continuous. Any faster RPM than I currently fly and Eco Mode would not kick in and I would be full rich all the time and burning a lot more fuel.
Did he also run the triangle before the change in AS happened? Because the whole premise of this thread is that changes in AS indication represent an actual change in AS, as opposed to a change in indication accuracy.
I was there. All of your questions about potential damage are perfectly reasonable, but I supervise this airplane at all times and no one moves it but me.
I did not run the triangle before the change in AS. However, before the AS change my GPS GS checked and I had verified that against 3 independent GPS units while in flight. I was getting 120+ knots GS fairly often. Now I never see that.
The only problem with that theory is I have pictures of my panel from before the AS change and I have always been in a slip. I was in the exact same degree of slip when the plane was doing 120-125 KTAS and when the plane is now doing ~110 KTAS. To me, that says that by correcting that slip I should only gain airspeed above and beyond the 120-125 KTAS it was flying previously. Also, the manufacturer told me today when I asked about rigging: "It is very rare that the actual control cables would need to be adjusted unless someone at some point has been into the system doing maintenance." And there's no logbook entry that suggests anyone ever changed the rigging on this airplane.
What I mean is, you were previously doing 120 at 5400 rpm (or whatever the actual numbers are). Now you're doing 110 at 5400 rpm. What rpm do you need, today, to do 120?
I'm not sure it will even do 120 unless I surpass max continuous (5500) since my cruise is only 100 rpm below max continuous anyway. I could go all the way up to 5800rpm so long as it's for less than 5 minutes if this information will help you somehow and could enter a shallow descent at full power to see at what RPM it would take to get me going 120 KTAS. How helpful would this information be as to what RPM today it will take to get me going 120 KTAS?
So the nosewheel askew isn’t the problem. Look elsewhere.
It did, but not by much. The new prop is about 11 pounds lighter than the old prop. But we just added 5 pounds back to the engine bay with a larger battery. So I'm about 6 pounds lighter in front. But that didn't effect this airspeed problem because I was flying 120+ KTAS after the new prop and still flying at slower airspeed after the larger battery install. Maybe some things changed slightly in the baggage compartment, but not by more than 10 pounds or so.
I can officially confirm we can rule that out.
I sure am.
And temperature, and pressure.
Well, if you’re flying around in a slip to stay straight and level something is off. Even if this isn’t contributing to your speed loss it’s definitely something you should address. If your rudder trim is at max deflection that’s just another reason to get the rigging looked at.
Yes and yes.and flaps....
I agree with you and it's something I will address after this airspeed problem is resolved and I'm flying the 120+ KTAS I was flying before.
Being lighter in front can make you slower.
