Engine Run-up

[P22]

I suppose that this should really be on the Questions forum, but I thought that the knowledgable gentlemen (and ladies) on this forum might know the answer.

Why do we face into wind to do the engine run-up on a SEP aircraft?

[Charlie Foxtrot India]

Because you are operating an air cooled engine, and because a crosswind on the prop can sound like rough running.
And aeroplanes like pointing into wind!

[A and C]

Run up into wind is a relic from the days of the spitfire with a liquid cooled engine and a small radiator the thing would overheat at the drop of a hat so all the help that it could get in the way of cool air was welcome.

Pointing a modern aircooled engine into wind makes no diference at all in practice and just gives slightly more RPM on the max power check .
for C of A airtests the run up is required to be done cross wind so a true max RPM can be seen.

How ever i think that the run up should not be done with a tail wind as the max RPM would be low due to a high prop angle of attack and for taildraggers pointing into wind will reduce the danger of the aircraft weather cocking in gusting wind conditions.

[P22]

You've got me worried now. My checklist (PA-28) says "RPM......Set 1800". Surely 1800 is 1800 whatever the wind? My checklist doesn't say anything about 'Max Power' during the power check. Am I doing something wrong?

[A and C]

The 1800 RPM check is to check the mags and carb heat you should also have an RPM that is the minimum for take off when the aircraft is static this is to ensure that the engine is producing full power.
most people check this on the end of the runway just befor take off you will find the RPM in the flight manual,2200 RPM is typical but check the RPM for the aircraft you fly.

[Squawk 8888]

I was taught to do the max power check during the T/O roll, unless it was a short field (max power w/brakes) or a density altitude above 3000 (leaning).

[P22]

Many thanks for your replies. I was not talking about the power check on take-off, but the engine run-up which is completed prior to the Pre Take-off check.
I cannot believe that a 10 knot wind makes any difference to the cooling, compared to the gale from a rotating prop. I feel that the real reason is probably because it has always been done that way, so it must be correct.
My question was prompted by watching aircraft at my local airfield executing tight turns in a restricted area, churning up the mud, so as to face into wind, whereas a turn through 90 degrees would have saved the grass and avoided any blast problems for aircraft behind.

['I' in the sky]

As far as I know the cooling thing only applied to radial engines.

What you can't fail to notice though, whatever the reasons, is that if you do the run up not into wind with a wind of any reasonable speed, the plane don't half seem to twist and bounce around a lot as the propwash is wrapped around the airframe.

[AirScream]

What about the idle check, wouldn't that be better carried out with Carb Heat Hot as would be the case in a descent or in fact whenever the throttle is full out? This will indicate whether the engine continues to turn at the lower RPM experienced with Carb Heat. Approach speed Airflow will assist here of course but if it will idle on the ground with Carb Heat Hot then thats a good check.

Most checklists don't call for this.

What do you think?

[Squawk 8888]

Here's what I was taught for a C172 runup:

-1200rpm
-T&P green
-Mixture rich
-1700rpm
-Suction: 4.5-6 inches
-Alternator load: All electrics on, flaps moving, charge meter should stay within 1 needle-width of 0.
-Mags: Loss of max 125rpm on single mag, L&R within 50rpm of each other.
-Carb heat: Drop in rpm
Mixture: Look for rpm to rise then drop with leaning.
-Slow idle: Engine doesn't quit http://www.pprune.org/ubb/NonCGI/eek.gif
-1100rpm
-Carb heat: Off, rpm should rise by 50-100rpm.

[Genghis the Engineer]

Forgive me for being simplistic, but if you take-off into wind the aircraft does not go so far, so fast, as if you take-off downwind.

So, if the aircraft runs away (and it's not unknown) during an engine runup, it shouldn't go so far before you regain control.

I was taught "into wind and into open space".

G

[P22]

So nobody really knows the answer then?

[Squawk 8888]

Well, Ghenghis, it sounds like I've been doing the right thing for the wrong reason :) I was told it was for cooling but control makes more sense.

[Tail Plane]

Into wind means that exhaust gasses will be carried away from the aircraft and not enter the cabin when you throttle back. Into wind is also better for the controls as they are less likeky to flap about, isn't that why we park into wind whenever possible. Idling propellers don't like strong tailwinds.

[Luke SkyToddler]

With most SE trainers, I've always taught to face into wind for runups if the wind is >10 kts. Doesn't really seem to matter below that, but there can be quite a noticeable difference in throttle position when you set your 1800 RPM in a strong head or tail wind.

I've flown various microlights and taildraggers that can be a real handful to control if you go putting large applications of power on with cross- or tail wind, whether you're running up or taxiing.