Max level speed vs Vertical Dive speed

[izmile]

I found this in some website and was wondering if it is true.

"An 8-4 prop at 10,000 rpm can't pull its model faster than 40 mph. (4 inch pitch times 10K rpm = 40 mph maximum.) The airplane can fly a lot SLOWER -- particularly if it's a high- drag design. But even in a dive, it won't go faster than this "rule of thumb" limit. "


I am not an expert in this but some how I feel that the dive speed of an aircraft should be much more than max level speed. Or is it that this "rule of thumb" is applicable to high drag designs only. Any comments?

-Ismail

[Ollie]

You can fly faster by taking off the prop!
http://www.sloperacing.com/results/ds-speeds.htm
http://www.billpattersonart.com/dszone.swf
http://www.shredair.com/album/dsfest8.html

[vintage1]

Its a lot more complicated than that.

There is a relationship between pitch speed, and maximum speed, but its by no means a direct or simple one, and in general most models aircraft are a lot faster in a dive than inlevel flight. Which is why Stukas were equipped with massive dive brakes...

[izmile]

Its a lot more complicated than that.

There is a relationship between pitch speed, and maximum speed, but its by no means a direct or simple one, and in general most models aircraft are a lot faster in a dive than inlevel flight. Which is why Stukas were equipped with massive dive brakes...


Thanks for the quick reply. You are right to the point. I was thinking of Stukas and other diver bombers and could not corelate it. I was thinking whether the prop acts as some kind of airbrakes during vertical drive :confused: . So, from your post I read that the so called "rule of thumb" isn't valid for all aircrafts.

Ollie - thanks for the DS stuff.

-Ismail

[Sparky Paul]

In a dive, gravity assists the descent, as well as thrust.
A zero-lift dive will have the least amount of drag, and permit the plane to get to its terminal velocity, which it can't do in level flight.

[BMatthews]

Many models that are clean and a bit on the heavy side will easily dive faster than the engine can pull them. In such dives the engine and prop are actually holding back the speed just as the links in Ollie's post suggest. Not only that but the engine is actually being over revved by the air turning the engine faster than it's set for. It's not as bad as a shaft run as the drag of the prop just turning will hold it back but it's faster than it turns in level flight. When you hear the engine speed up some of it is from the doppler shift as it dives towards you and some is from the engine being over driven by the prop.

[vintage1]

Tell me about it.I just beengetting a little IPS powerd vintage thing ready for the skies, and with an 8" prop on a 30" model, I thought I had a real problem when I throttled back. The durned prop was still spinning, and the model came down fast...after a bit I realised it was slipstreaming it. Actually it makes for a very good short landing approach. Use the prop as an airbrake, and come in at a steep angle knowing it won't ever go too fast, and then lift the nose for finals.

Complete contrats to e.g. a 480DD model, which I swear will do 80mph plus in a dive...and floats for ever on landing.

[PGR]

I recently retired my Wing Warrior Straight Edge. It's an EPP 3D plane with a fairly fat symmetric wing and mine was powered by an AXI 2212/34 spinning an 11x4.7 with a 3S pack. AUW was a little shy of 10oz.

I don't have a radar gun but I swear that plane would fly straight up at about the same speed it could fly straight down; neither of which anyone would ever mistake for fast.

Pete