The International Cessna^® 170 Association

Just for some fun, let's run up the flag pole some of these hoary old ideas that still seem to be accepted as gospel by many. I'll start it off -

"Always do your run-up into the wind" (or the engine will overheat, or some such nonsense.) This one must date back to the dawn of time...
if you're flying a Curtiss Jenny, it might matter. I still hear this "procedure" taught by the young local CFIs (yes, I was one once...)

Any airplane so marginal in ground cooling that the wind would make any concievable difference shouldn't leave the ground. I started ignoring this one years ago...Russ Farris

Another reason, that hasn't been changed by time, is when you are doing a run-up, down wind you can pick-up rocks that will then return to nick your prop.

Two reasons I run-up into the wind;
1. Facing into the wind anytime you stop helps prevent rock, sand, other debris from being sucked up into the prop. Facing into the wind helps insure anything picked up off the ground by the prop vortex will be blown behind the plane and continue downwind.

Facing downwind, anything picked up by the prop vortex is blown into the wind then it recirculates back into the prop. Same situation applies for run-up and when bringing the plane to a stop and then resuming the taxi. When the throttle is applied a dust cloud will envelop the prop. Not good for long prop life.

2. Facing into the wind for run-up helps airflow thru the cowl. Obviously, it is more pronounced on some airplanes than others but why bet your engine that it doesn't matter? The cooling air flow is assisted by the wind and is not working against it.

Facing downwind forces the cooling airflow into the wind partially negating its effect by lowering its groundspeed. This may or may not cause stagnation of the cooling air inside the cowl. I, for one, am not willing to gamble on whether or not the cooling air flow is enough to keep all of the cylinders cool. I run-up into the wind.

Another reason for facing into the wind is some engine/prop combinations do not have any cooling airflow provided by the prop. A good example is the C-195. The prop shank in front of the cowl opening on the Ham-Std 2B-20 is round! It obviously provides no cooling airflow other than the small amount generated by the venturi effect of the airflow around the outside of the cowl. So, the only effective cooling airflow while on the ground is by the speed of the wind or airflow caused by taxiing. From experience with this type of engine/prop combination it does make a difference if you run-up or taxi into the wind versus downwind. See photo below:

Besides all the reasons listed above, I like facing into the wind because a) the airframe is directionally stable that way, and b) it helps my venturis spool up the gyros.

How about the effect of the advancing and retreating prop blades causing engine roughness in a cross wind run up.

The worst nick I ever got in a prop blade (and with only 50 hours since new) was while throttling up to taxi from a stop with a 15mph cross-wind from the right. I was on asphalt and the impact of the rock was loud enough to hear inside the plane with my headset on.

Sorry guys, I phrased the statement about doing run-ups into the wind poorly. What I was trying to get at was the engine cooling benefit
of doing so. Debris, ground stability and that obnoxious sound of the wind hitting the prop are all good reasons for doing the run-up into the wind, especially high winds.

What I have trouble with is the concept that the wind adds or subtracts to the prop wash velocity, at least in the values I'm discussing here -
for the sake of example, 20 mph.

You're at 1,700 RPM in a no-wind condition. Let's say the prop blast velocity is 60 mph (just a WAG.) Suddenly, the wind picks up to 20 MPH,
directly ahead. The RPM increases as the prop unloads, to say 1,800 RPM, which increases the prop wash to 67 MPH. More air velocity, better cooling. BUT, you throttle back to 1,700 RPM. I'm no expert in Fluid Dynamics (or aviation, for that matter) but how much of an airflow increase would you have now? The prop is now essentially a flat plate area - how does the wind get through that? Only by unloading the prop.

Same situation, 1,700 rpm, dead calm. Then a 20 mph tail wind appears. The RPM drops to 1,600 - you bring it back to 1,700. I'm having trouble understanding how a 20 mph breeze would affect the cooling air velocity from the prop...in a corrected RPM situation. I also don't think a tailwind is also going to lower the "groundspeed" of a 60 mph plus prop blast any significant amount, especially a few feet behind the prop.

Richard (I see from your profile you mess with old cars - I have two Studebakers) ) I appreciate your comments, but I'm talking about your average lightplane, like the 170. I don't think a 30 second run-up will cause a problem, regardless if your pointed into a 10 mph breeze or not. I can see your point about the 195, my radial-engine experience is limited to 900 hours in DC-3s.

This almost sounds like a Mythbusters episode. I'm thinking of experimenting with a bench mounted model airplane engine, a wind guage and my big shop fan! Miles, you're some sort of Rocket Scientist...help me out here! Actually, this might be a good one for Peter Garrison of "Flying" magazine...his column is always very interesting. Russ Farris

I would go flying instead of worrying about wether running up into the wind makes a difference.

Russ-
As you suspected, this has generated a lively discussion! I understand your point. In just about any plane your idea is a valid argument. For short stops on clean pavement you can get away with it without a problem. The length of time is usually not sufficient to cook an engine. But, I have had some long delays at busy airports such as after an airshow and everybody is trying to leave (usually on a hot summer day) where it does make a difference. I use multi-point engine analyzers and can see when a problem is approaching with CHT but most planes do not have them and a single point CHT will not always show the hottest cylinder on the ground. You're taking a chance of damaging the engine. I don't know how much of a chance but turning into the wind reduces the possibility as much as you can so I do it.

I don't know if I buy your concept of wind not affecting prop blast groundspeed. I am of the opinion (but without any proof) that a wind blowing into the prop would be additive to the prop blast speed no matter what the RPM is and increase the airflow thru the cowl and a tailwind would retard the prop blast speed with a resultant decrease in airflow thru the cowl. If the plane was not sitting still on the ground, I might agree with you. But since the plane is not moving along with the air mass, the groundspeed of the wind does matter, in my opinion. It's interesting to consider in any case.

Instead of a bench test, maybe you could convince someone to stand behind your plane and hold an annemometer while you do a run-up into the wind and then downwind. Sounds like a good experiment to do at Petit Jean. We could all bet a dollar on the outcome and the winners could buy everybody a round of drinks.

But the main reason I make it a practice to stop and run-up into the wind is the prop erosion issue and engine injestion of sand, dust, etc. The carb heat check allows unfiltered air into the engine. I want that as clean as possible plus I want to try to keep the air filter clean as long as possible. I cringe every time I have to stop facing downwind then watch that cloud of dust swirl around the prop when I throttle up to proceed. Our 170 prop clearance is very good but it still sucks up a bunch of dust when facing downwind, even on pavement. Every once in a while you have to do it for whatever reason and I accept that but still try to minimize it by thinking ahead about the best way to turn around or park or do the run-up.

So I always turn into the wind, out of habit, because my instructor taught me to do it 38 years ago. I think it's a good practice, whether it is actually necessary or not, it never hurts anything if you do.

Howdy All,

I have heard about this OWT in the past.....and I think it has some merit.....However, I run up downwind....looking for aircraft on base or final that might land on top of me during takeoff..... Just my two centavos worth....and I do the carb heat check at 1000 AGL due to dusty conditions present here in West Texas......

I learned to do the runup into the wind when possible because I flew with turbine equipment... there were times when the intakes facing out of the wind had a significant effect on the engine parameters on a hot day (TOT, ITT, T5, Ng, etc depending on the aircraft).
I don't think it is an absolute, but it seems to be a good habit.
David

My primary instructor in the PT-22 back '43 told me that if I made my run up facing downwind I was gonna die! I've been too scared to try it ever since.

It's not just the actual "run up" to check mags, etc., that counts... it's also the time spent awaiting clearance for takeoff while idling, and it's common courtesy for adjacent aircraft who practice the same technique, because it avoids blasting their flight controls adversely. And it's also just good technique in general because it means less wear and tear on the airframe and flight controls when they are streamlined versus receiving a tailwind (especially if it IS a 20 mph breeze.)

It's not an "OWT" in my view. It's good technique, and it's polite to others.