In reference to a recent crash of a full-scale SE5a replica, it was suggested that a high "density altitude" may have been a factor. This was a new term for me so I looked it up on wikipedia:

http://en.wikipedia.org/wiki/Density_altitude

I'm wondering if this explains why it seems harder to fly my Puppeteer lately in the hot summer temps. The engine is running just as it always has but I get the feeling that the model is having to labor a bit more (as compared with crisp winter days) to get into and then stay in the air.

Less O2 molecules in the air means less lift.. pretty simple actually.

I'm trying to reconcile this with my subjective perception that summer time air is "thicker." Cold air seems "thin" to me.

Where I live it is about 250 feet above sea level and it was about 95F with 75% humidity, barometer 30.00in; the air had the same density of a cool day at 3000 ft. Your engine has less power because there is not enough oxygen to make max HP and there is not as much bite with the prop and lift with the wings. Humidity makes the air feel thick, but the water is displacing the oxygen content, so you feel thick air of the water, while you suffocate from lack of O2.

That makes sense! And it really does explain what I've been seeing with the Puppeteer. Since it's a 60" wingspan 4.2kg biplane flying on a Saito 56 it never had a lot of power to spare, which just forced me to really learn to fly it, it's become downright sluggish in the hot weather. It still fun to fly but I feel I have to be more cautious.

Many full scale pilots forget it, and bend something more costly than our stuff.

* The aircraft will accelerate slower on takeoff as a result of reduced power production.
* The aircraft will need to achieve a higher true airspeed to attain the same lift - this implies both a longer takeoff roll and a higher true airspeed which must be maintained when airborne to avoid stalling.
* The aircraft will climb slower as the result of reduced power production and lift.

Yep, yep, and yep.

And yep, this may have been what happened to the Phll Krueger taking off in his SE5a replica. The pilot, luckily escaped with minor injuries, but the replica was pretty much destroyed.

Air viscosity has something to do as well:

http://www.rcuniverse.com/forum/fb.asp?m=9776547

.....and when flying aerobatics on a hot day (high density altitude) give your model more airspace to complete/recover. Start your loops and spins at a higher than normal altitude and begin to recover (from spins) sooner. The model will respond a little slower to your commands. Go a little easy on the elevator imputs. High speed stalls are more common on hot days as well as stalls on take-off or go-arounds.

One of the most common remarks I hear on the flight line, is "wow I can't slow it down". When on your approach to land, pay close attention to your elevator sensitivity/responsiveness and remember the model will have a higher ground speed at touch down than on a colder day, thus requiring a longer rollout. This will be most notable on a (high wing loading) model like a WWII warbird or scale jet.

Always respect density altitude....and Mother nature

It's nice to have a name and explanation for something that was so completely obvious at the field. But I wonder how many of my casual clubmates have noticed this. Maybe the Japanese have their own way to talk about it. It's see if I can get an answer on Sunday.

Anybody with any aviation background will be able to explain density altitude in any country....is one of the first thing you learn.

JG

I think it is important to realize the heat effects everything. The thinner air really effects engine and prop performance. You will notice it can effect your needle settings as well. As the day goes on, you will start to lean out. Maybe not so much with gas, but definately with glow. After competing for the last 18 years in high performance events like CL combat, RC combat and Pylon racing, it is important to note this.
I have found that engine and prop performance suffer even more than my wings performance.

This is also a very important point. Like most guys I tend to just check the engine once before the first flight of the day, but then don't think about for the rest of the day. Of course "the rest of the day" for me is usually no more than about 2-3 hours.

Hot, high (altitude) and humid! Bad things for flying!!!! Use Temp minus Dew Point divided by 4.5 and the answer multplied by 1000 to give the altitude of Cumy clouds' base. Fun stuff!

Hi</p>

</p>

A good example of density altitude was the experience of the Wright Brothers at SIMMS Station in Ohio in the spring of 1904 and the problems they had getting the 1904 Flyer to fly. This is from a interesting book:</p>

"Kill Devil Hill Discovering the secret of the Wright Brothers" by Harry Combs and Martin Caidin. The following is from pages 238 to 240.</p>

1. On December 17, 1903 at Kitty Hawk the temperature was 34 degres F. at first time flight which produced a density altitude of 1800 feet below sea level. THe wind was also high at 27 MPH so they were able to take off with a short track as they did not have to make a long run to acelerate the plane to flying speed. Ideal flying conditions.</p>

2. The following spring at SIMMS Station in Ohio in May 1904, May 23 @ 81 degres F they struggled to take off and were not able toget the 1904 Flyer airborne because the density altitude was 2900 feet above sea level . At SIMMS Station at a stardard day, the density altitide is 815 feet. THey had alsoreduced the wing camber which reduced lift. They were not able to fly, other than short hops, thru the summer.</p>

3. As the temperature dropped later in the fall they had much better success but the main reason was that in September 1904 they constructed a catalput that gave the airplane enough acceleration to get airborne on the track as power was marginal. As the air cooled in the Autumn the flights became longer. THeir longest flight in 1904 was on December 1, which lasted 5 minutes and 8 seconds.</p>

Cheers</p>

Ray</p>

Do you think the Wright Bros (and others of the time) were aware of the effects of temperture/humidity on flight? Or did they just wonder what the heck was going on? BTW, this gives me a good excuse to wait until cooler weather to try fly my Snipe again!

Well they were smart enough to have a wind tunnel and create a very efficient propeller design so, if they wern't aware of it at first I'm sure the soon learned.

Take off from Denver International Airport on a hot summer day with a fully loaded 757 and you feel as if you'll never get off the ground!!