karolh

Do gas engines unload in the air like their glow counterparts, and if so how much would say a 20cc be expected to gain in rpm over it's static reading at sea level.

CK1

My Feedback: (60)

A lot of variables involved in how much an engine unloads . Aerodynamics of the plane and the prop are significant factors. On average I would say an engine can unload a 1000 rpms plus or minus a few hundred. An engine heavily loaded with a big prop on a high drag airframe will not unload as much as well proped engine on a low drag airframe

Pelle Gris

On my Valach 210 fourstroke I have seen gains of more than 30% compared to static rpm. In this case from 4.600 static to 6.200-6.300 in the air.
So my guess is that you can expect gains of about 1.000 to 2.000 rpm depending on airframe and propeller.

ahicks

My Feedback: (2)

A 20cc Great Planes Revolver is a pretty good example of a clean airflow type plane when it comes to cutting through the air. The 20cc versions are good for well over 100mph (on the level) pretty easily. Imagine a plane like that, and how little resistance there is to building up a LOT of speed in a dive for instance - and what that might do to engine rpm's.

Now imagine that same engine in a WW1 biplane/tri-plane with all of the associated rigging.

The 20cc DLE, RCGF, and VV engines all recommend ground rpm's of about 8500 max.though they are very willing to turn much higher. The issue is they don't do it very long, so setting one up for 9k for instance, not a very good plan.

I would imagine that a Revolver in a dive could be turning up well over 10k. The WW1 planes, well, they may not pick up anything in a dive....

So like CK1 says, it depends....

speedracerntrixie

My Feedback: (29)

karolh

Very good advice and something that I do.

Len Todd

I typically put in telemetry to set the RPM when flying. Then I pull the telemetry back out once I have the prop and mixtures set.