RV-4 Indicated Speed Question

[Xkuzme1]

Seems that recently I have noticed a decrease in speed from my -4. My RV-4 used to regularly indicate about 177 indicated while in cruise (3-4000 msl). Recently it seems that the speed from my -4 has dropped off, and I am backing it up with ground speed and it has dropped too. I am averaging about 160-165 mph.

Airplane is straight, clean, and well maintained. Prop is in good shape. Gear ferings and wheel pants are straight.

I recently did the conditional. The owner/builder hadthe timing at about 28-29 degrees. We adjusted it to 25 degree btdc. It doesn't start quite the same as it used too. It used to start in 1/2 a blade. Now it starts in 3/4 to 1 bade, but still runs like a sewing machine. That's the only thing I could possible think of. Doesn't seem like timing should be the culprit.

Any ideas?

X
O-320
Sensenich metal prop
Carburetor
BOW 1030

[lr172]

We all know that 25* is not optimal timing at cruise power settings and bumping it to 30 will improve performance (lack of variable timing however provides this same advance at full power where it introduces detonation risk). However, I can't see that type of increase. I am sure someone who has converted to EI will jump in with real data, but I don't see 15 MPH improvements being realistic for 3-4 degrees of advance.

Have you done a compression check. Coincidental timing gets all of us from time to time. I would keep looking for another cause of the speed loss.

Larry

[lr172]

[quote=Xkuzme1 It doesn't start quite the same as it used too. It used to start in 1/2 a blade. Now it starts in 3/4 to 1 bade, but still runs like a sewing machine. That's the only thing I could possible think of. Doesn't seem like timing should be the culprit.
[/QUOTE]

advanced timing should not make it easier to start. This usually makes it harder to start with more kick back events. This further points to something else going on.

Larry

[WAM120RV]

Hi,

If you want people to really be able to help you need to supply all the facts and data.

Firstly is your 4 FP or constant speed?

If it's FP and turning the same RPM at the same density altitude and you are sure you are off the back of the drag curve it should do the same speed. If not suspect airframe. If it's constant speed then it may be an engine issue, the prop may be turning at the same speed but will not have coarsened if the power has dropped.

Has you max static dropped?

If pe.rformance has dropped after alterations it is usually something you have done. As you have changed timing have you put it back to where it was to see if it improves.


I have Ei plus a mag, it will drop performance just on the mag set at 25 btdc, compared to an Ei which actually prides a spark when it's needed.

Hope this helps diagnose the problem.

[F1R]

What months of the year did you buy and first fly your 4? At what altitudes?

How many hours have you owned / flown it.

Even with a Rocket there can be a large change of IAS depending on the day in question. Air temp, altitude, weight and where the c of g is for that flight all can make quite a difference. As a rule more weight slows you down, but loading towards the aft Cof G limit normally makes the AC a touch more efficient.

Regarding timing, the simple test is to put it back were it was and go make some flights, in the same air temp and altitudes you were flying when you saw the 177 IAS.

[salto]

My -4 always showd 10knots slower than GPS but was accurate at lower airspeeds. I checked pitot plumbing/ re-caled airspeed indicator to no avail. In the end I just got used to it and was running on GPS anyway. Might be worth seeing if your GPS airspeed using reciprocating headings still represents what you expect your airspeed to be.
Thinking about it I never checked my static ports or lines, there is a chance that I could have got some water in it. Blocked static will probably only cause the instrument to lag so may not be the answer but worth checking.
PS dont forget to unhook your instruments if you are going to blow the lines out!!

[rdrcrmatt]

Retarding the timing is making you less power.

The most power is made with the most timing advance and the lowest octane possible.


That might not be the only thing (I'm not a builder yet), but it is a factor.